Why do we still use Sabin poliovirus vaccine?

VAPPThe Sabin infectious, attenuated poliovirus vaccines are known to cause vaccine-associated paralysis in a small number of recipients. In contrast, the Salk inactivated vaccine does not cause poliomyelitis. Why are the Sabin vaccines still used globally? The answer to this question requires a brief visit to the history of poliovirus vaccines.

The inactivated poliovirus vaccine (IPV) developed by Jonas Salk was licensed for use in 1955. This vaccine consists of the three serotypes of poliovirus whose infectivity, but not immunogenicity, is destroyed by treatment with formalin. When prepared properly, IPV does not cause poliomyelitis (early batches of IPV were not sufficiently inactivated, leading to vaccine-associated outbreaks of polio, the so-called Cutter incident). From 1955 to 1960 cases of paralytic poliomyelitis in the United States dropped from 20,000 per year to 2,500.

While Salk’s vaccine was under development, several investigators pursued the production of infectious, attenuated vaccines as an alternative. This approach was shown to be effective by Max Theiler, who in 1937 had made an attenuated vaccine against yellow fever virus by passage of the virulent virus in laboratory mice. After many passages, the virus no longer caused disease in humans, but replicated sufficiently to induce protective immunity. Albert Sabin capitalized on these observations and developed attenuated versions of the three serotypes of poliovirus by passage of virulent viruses in different animals and cells. In contrast to Theiler’s yellow fever vaccine, which was injected, Sabin’s poliovirus vaccines were designed to be taken orally – hence the name oral poliovirus vaccine (OPV). As in a natural poliovirus infection, Sabin’s vaccines would replicate in the intestinal tract and induce protective immunity there and in the bloodstream.

Sabin began testing his attenuated vaccines in humans in 1954. By 1957 there was evidence that the virus that was fed to volunteers was not the same as the virus excreted in the feces. As Sabin writes:

It was evident, however, that as in the young adult volunteers, the virus in some of the stool specimens had a greater neurovirulence than the virus originally swallowed in tests in monkeys.

What Sabin did not know was whether the change in neurovirulence of his vaccine strains constituted a threat to the vaccine recipients and their contacts, a question that could only be answered by carrying out larger clinical trials. Many felt that such studies were not warranted, especially considering the success of IPV in reducing the number of paralytic cases. Sabin notes that his friend Tom Rivers, often called the father of American virology, told him to ‘discard the large lots of OPV that I had prepared into a suitable sewer’.

Despite the opposition to further testing of OPV in the US, others had different views. An international committee of the World Health Organization recommended in 1957 that larger trials of OPV should be carried out in different countries. Sabin’s type 2 vaccine was given to 200,000 children during an outbreak of polio in Singapore in 1958, and follow-up studies revealed no safety problems. In Czechoslovakia 140,000 children were given OPV and subsequent studies revealed that the virus spread to unimminized contacts but did not cause disease.

Perhaps the most important numbers came from trials of OPV in the Soviet Union. Sabin had been born in Russia and had close contacts with Soviet virologists, including Mikhail Chumakov, director of the Poliomyelitis Research Institute in Moscow. Chumakov was not satisfied with the results of IPV trials in his country and asked Sabin to send him OPV for testing. By the end of 1959 nearly 15,000,000 people had been given OPV in different parts of the Soviet Union with no apparent side effects. Dorothy Horstmann, a well known virologist at Yale University, was sent to the Soviet Union to evaluate the outcome of the trials. Horstmann writes:

It was clear that the trials had been carefully carried out, and the results were monitored meticulously in the laboratory and in the field. By mid-1960 approximately 100 million persons in the Soviet Union, Czechoslovakia, and East Germany had received the Sabin strains. Of great importance was the demonstration that the vaccine was safe, not only for the recipients, but for the large numbers of unvaccinated susceptible who must have been exposed as contacts of vaccines.

The results obtained from these trials in the Soviet Union convinced officials in the US and other countries to carry out clinical trials of OPV. In Japan, Israel, Chile, and other countries, OPV was shown to be highly effective in terminating epidemics of poliomyelitis. In light of these findings, all three of Sabin’s OPV strains were approved for use in the US, and in 1961-62 they replaced IPV for routine immunization against poliomyelitis.

As soon as OPV was used in mass immunizations in the US, cases of vaccine-associated paralysis were described. Initially Sabin decried these findings, arguing that temporal association of paralysis with vaccine administration was not sufficient to implicate OPV. He suggested that the observed paralysis was caused by wild-type viruses, not his vaccine strains.

A breakthrough in our understanding of vaccine-associated paralysis came in the early 1980s when the recently developed DNA sequencing methods were used to determine the nucleotide sequences of the genomes of the Sabin type 3 vaccine, the neurovirulent virus from which it was derived, and a virus isolated from a child who had developed paralysis after administration of OPV. The results enumerated for the first time the mutations that distinguish the Sabin vaccine from its neurovirulent parent. More importantly, the genome sequence of the vaccine-associated isolate proved that it was derived from the Sabin vaccine and was not a wild-type poliovirus.

We now understand that every recipient of OPV excretes, within a few days, viruses that are more neurovirulent that the vaccine strains. This evolution occurs because during replication of the OPV strains in the human intestine, the viral genome undergoes mutation and recombination that eliminate the attenuating mutations that Sabin so carefully selected by passage in different hosts.

From 1961 to 1989 there were an average of 9 cases (range, 1-25 cases) of vaccine-associated paralytic poliomyelitis (VAPP) in the United States, in vaccine recipients or their contacts, or 1 VAPP case per 2.9 million doses of OPV distributed (illustrated). Given this serious side effect, the use of OPV was evaluated several times by the Institute of Medicine, the Centers for Disease Control and Prevention, and the Advisory Committee on Immunization Practices. Each time it was decided that the risks associated with the use of OPV justified the cases of VAPP. It was believed that a switch to IPV would lead to outbreaks of poliomyelitis, because: OPV was better than IPV at protecting non-immunized recipients; the need to inject IPV would lead to reduced compliance; and IPV was known to induce less protective mucosal immunity than OPV.

After the WHO began its poliovirus eradication initiative in 1988, the risk of poliovirus importation into the US slowly decreased until it became very difficult to justify routine use of OPV. In 1996 the Advisory Committee on Immunization Practices decided that the US would transition to IPV and by 2000 IPV had replaced OPV for the routine prevention of poliomyelitis. As a consequence VAPP has been eliminated from the US.

OPV continues to be used in mass immunization campaigns for the WHO poliovirus eradication program, because it is effective at eliminating wild polioviruses, and is easy to administer. A consequence is that neurovirulent vaccine-derived polioviruses (VDPV) are excreted by immunized children. These VDPVs have caused outbreaks of poliomyelitis in areas where immunization coverage has dropped. Because VDPVs constitute a threat to the eradication campaign, WHO has recommended a global transition to IPV. Once OPV use is eliminated, careful environmental surveillance must be continued to ensure that VDPVs are no longer present before immunization ceases, a goal after eradication of poliomyelitis.

As a virologist working on poliovirus neurovirulence, I have followed the vaccine story since I joined the field in 1979. I have never understood why no cases of VAPP were observed in the huge OPV trials carried out in the Soviet Union. Had VAPP been identified in these trials, OPV might not have been licensed in the US. Global use of OPV has led to near global elimination of paralytic poliomyelitis. Would the exclusive use of IPV have brought us to the same point, without the unfortunate cases of vaccine-associated paralysis? I’m not sure we will ever know the answer.

Update: As recently as 1997 DA Henderson, architect of smallpox eradication, argued that developed countries should not use IPV, because it ‘implies accepting the potential of substantial penalties while reducing but not eliminating, an already extremely small risk of vaccine-associated paralytic illness’.

Shedding poliovirus for 28 years

Glass PoliovirusAn immunodeficient individual has been excreting poliovirus in his stool for 28 years. Such chronic excreters pose a threat to the poliovirus eradication program.

Since its inception in 1988 by the World Health Organization, the poliovirus eradication program has relied on the use of the infectious, attenuated vaccine strains produced by Albert Sabin. These viruses are taken orally, replicate in the intestine, and induce protective immunity. During replication in the gut, the Sabin strains lose the mutations that prevent them from causing paralysis. Nearly every individual who receives the Sabin vaccine strains excretes so-called vaccine-derived polioviruses (VDPVs) which are known to have caused outbreaks of poliomyelitis in under-immunized populations.

Immunocompromised individuals who produce very low levels of antibodies (a condition called agammaglobulinemia) are known to excrete VDPVs for very long periods of time – years, compared with months in healthy individuals. Seventy-three such cases have been described since 1962. These individuals receive the Sabin vaccine in the first year of life, before they are known to have an immunodeficiency, at which time they must receive antibodies to prevent them from acquiring fatal infections.

The most recently described immunocompromised patient was found to excrete poliovirus type 2 vaccine for 28 years (the time period is determined by combining the known rate of change in the poliovirus genome with sequence data on viruses obtained from the patient).  The VDPV is neurovirulent (causes paralysis in a mouse model), antigenically drifted, and excreted in the stool at high levels.

Because the polio eradication plan calls for cessation of vaccination at some future time, these immunocompromised poliovirus shedders pose a threat to future unimmunized individuals. The global number of such patients is unknown, and there is no available therapy to treat them – administration of antibodies does not clear the infection. The development of antivirals that could eliminate the chronic poliovirus infection is clearly needed (and ongoing). It will also be necessary to conduct environmental surveillance for the presence of VDPVs – they can be identified by properties that distinguish them from VDPVs produced by immunocompetent vaccine recipients.

While the WHO eradication plan now includes a shift to using inactivated (Salk) poliovaccine, this strategy would not impact the existing immunocompromised poliovirus shedders. Should a VDPV from these individuals cause an outbreak of polio in the post-vaccine era, it will be necessary to control the outbreak with Salk vaccine, or an infectious poliovirus vaccine that cannot revert to virulence during replication in the intestine. Polioviruses with a recoded genome are candidates for the latter type of vaccine.

Image credit: Jason Roberts

A collection of polioviruses

PoliovirusesIn midsummer 1986, five years after starting my poliovirus laboratory at Columbia University, I received a letter from Frederick L. Schaffer, a virologist at the University of California, Berkeley, asking if I would like to have his collection of poliovirus stocks. He was retiring and the samples needed a home, otherwise they would be destroyed. Of course I jumped at the opportunity to have a bit of virology history.

Three boxes full of dry ice arrived in the laboratory in August 1986. In them were sixty-six containers of different polioviruses that Dr. Schaffer had collected over the years. All three poliovirus serotypes were represented, both wild type and vaccine strains. The tubes were marked with dates ranging from 1948 to 1965. Most had come into Dr. Schaffer’s hands from well known poliovirologists, including Jonas Salk, Albert Sabin, Igor Tamm, Renato Dulbecco, and Charles Armstrong.

All of the samples were in glass containers, either tubes with a screw cap or a rubber stopper held in place with tape (pictured). A few were in glass bottles of the type that were used to grow cells. The collection held not a single plastic tube: these were the days before plastics entered the virology world. All were identified by hand-written labels on white cloth tape. Some of the labels in the photo read: Polio 1 Brunhilde (1963), Polio 2 MEF1, Polio 2 P712, HeLa P3, 10-21-62. Nearly all the samples were virus-containing cell culture supernatants.

Lansing poliovirusPerhaps the most amazing sample was a specimen labeled ‘Lansing poliomyelitis virus, 9/27/50, passage 379, C. Armstrong, NIH’. Within the glass vial was a intact mouse brain still attached to the spinal cord (there were no cell phones in 1986, hence no photo). The Lansing type 2 strain of poliovirus had been adapted to grow in mice by Charles Armstrong, and this was apparently the 379th intracerebral mouse-to-mouse passage! It was especially exciting for me to receive this sample, because my laboratory had been studying the ability of the Lansing strain of poliovirus to infect mice. I believe that the note accompanying the tube is in Dr. Armstrong’s writing.

I have since transferred most of the samples to plastic tubes which are stored in a -70C freezer. There is a trove of information to be obtained by studying these samples, but there are few poliovirologists left who are interested. Once poliomyelitis is eradicated – perhaps within the next 10 years – these samples, and similar ones throughout the world, will have to be destroyed.

Poliovirus escapes antibodies

Antibodies bound to poliovirus.

Antibodies (purple) bound to poliovirus. Image credit: Jason Roberts

Antigenic variation is a hallmark of influenza virus that allows the virus to evade host defenses. Consequently influenza vaccines need to be reformulated frequently to keep up with changing viruses. In contrast, antigenic variation is not a hallmark of poliovirus – the same poliovirus vaccines have been used for nearly 60 years to control infections by this virus. An exception is a poliovirus type 1 that caused a 2010 outbreak in the Republic of Congo.

The 2010 outbreak (445 paralytic cases) was unusual because the case fatality ratio of 47% was higher than typically observed (usually less than 10% of patients with confirmed disease die). The first clue that something was different in this outbreak was the finding that sera from some of the fatal cases failed to effectively block (neutralize) infection of cells by the strain of poliovirus isolated during this outbreak (the strain is called PV-RC2010). The same sera effectively neutralized the three Sabin vaccine viruses as well as wild type 1 polioviruses isolated from previous outbreaks. Therefore gaps in vaccination coverage were solely not responsible for this outbreak.

Examination of the nucleotide sequence of the genome of type I polioviruses isolated from 12 fatal cases revealed two amino acid changes within a site on surface of the viral capsid that is bound by neutralizing antibodies (illustration). The sequence of this site, called 2a, was changed from ser-ala-ala-leu to pro-ala-asp-leu. This particular combination of amino acid substitutions has never been seen before in poliovirus. Virus PV-RC2010, which also contains these two amino acid mutations, is completely resistant to neutralization with monoclonal antibodies that recognize antigenic site 2 (monoclonal antibodies recognize a single epitope, as opposed polyclonal antibodies which is a mixture of antibodies that recognize many epitopes. The antibodies in serum are typically polyclonal).

Poliovirus neutralization titers were determined using sera from Gabonese and German individuals who had been immunized with Sabin vaccine. These sera effectively neutralized the type I strain of Sabin poliovirus, as well as type 1 polioviruses isolated from recent outbreaks. However the sera had substantially lower neutralization activity against PV-RC2010. From 15-29% of these individuals would be considered not to be protected from infection with this strain.

Nucleotide sequence analysis of PV-RC2010 reveals that it is related to a poliovirus strain isolated in Angola in 2009, the year before the Republic of Congo outbreak. The Angolan virus had just one of the two amino acid changes in antigenic site 2a found in PV-RC2010.

It is possible that the relative resistance of the polioviruses to antibody neutralization might have been an important contributor to the high virulence observed during the Republic of Congo outbreak. The reduced ability of serum antibodies to neutralize virus would have lead to higher virus in the blood and a greater chance of entering the central nervous system. Another factor could also be that many of the cases of poliomyelitis were in adults, in which the disease is known to be more severe.

An important question is whether poliovirus strains such as PV-RC2010 pose a global threat. Typically the fitness of antigenically variant viruses is not the same as wild type, and therefore such viruses are not likely to spread in well immunized populations. Today some parts of the world have incomplete poliovirus immunization coverage, which together with the reduced circulation of wild type polioviruses leads to reduced population immunity. Such a situation could lead to the evolution of antigenic variants. This situation occurred in Finland in 1984, when an outbreak caused by type 3 poliovirus took place. The responsible strains were antigenic variants that evolved due to use of a sub-optimal poliovirus vaccine in that country.

The poliovirus outbreaks in the Republic of Congo and Finland were stopped by immunization with poliovirus vaccines, which boosted the population immunity. These experiences show that poliovirus antigenic variants such as PV-RC2010 will not cause outbreaks as long as we continue extensive immunization with poliovirus vaccines, coupled with environmental and clinical testing for the presence of such viruses.

World Polio Day

gold poliovirus

Image credit: Jason Roberts

As a virologist who has worked on poliovirus since 1979, I would be remiss if I did not note that today, 24 October, is World Polio Day. World Polio Day was established by Rotary International over a decade ago to commemorate the birth of Jonas Salk, who led the first team to develop a vaccine against poliomyelitis.

The polio eradication effort has made impressive progress towards eliminating polio from the planet. In 1988 it was estimated that there were a total of 350,000 cases of poliomyelitis (probably an underestimate); as of this writing there have been 301 cases in 2013, which is unfortunately already more than in all of 2012 (223). Some setbacks to the program include an outbreak in the Horn of Africa, the finding of wild poliovirus (but no paralytic cases) in Israel, and two suspected cases in Syria. Transmission of wild poliovirus has never been interrupted in three countries: Afghanistan, Nigeria, and Pakistan. The good news is that India remains polio-free, a remarkable achievement.

Currently the eradication effort mainly utilizes the Sabin oral poliovirus vaccine strains (OPV). These vaccines are taken orally and replicate in the intestine, followed by entry into the bloodstream. They induce antiviral immunity in both the intestine and the blood. However, a drawback to using the Sabin vaccines is that the viruses revert to neurovirulence during replication in the intestine. As a consequence, virulent polioviruses are shed in the feces. These can cause poliomyelitis, either in the vaccine recipient or in unimmunized contacts. As wild polioviruses are eliminated, vaccine-derived polioviruses will continue to circulate, necessitating ‘vaccinating against the vaccine’. As a consequence, WHO has proposed a switch to the inactivated poliovirus vaccine, IPV, which if prepared properly cannot cause poliomyelitis.

A very good question is whether the use of IPV can lead to elimination of poliovirus from the planet. Consider the following scenario: at some point in the future the use of Sabin vaccines is discontinued, and all polio immunizations are done with IPV. Vaccine-derived polioviruses will still be present, and possibly also wild polioviruses. As shown by the recent detection of poliovirus in Israel, poliovirus can replicate in the intestines of individuals who have been immunized with IPV. Therefore, in a post-OPV world, immunization with IPV will still allow circulation of vaccine-derived polioviruses. As long as immunization continues at a high rate, there should be no cases of paralytic disease – but we already know that high immunization coverage is difficult to maintain. How long will we need to immunize with IPV before circulation of vaccine-derived polioviruses will stop?

Below are links to resources on polio, provided by David Gold at Global Health Strategies:

  • An expert panel including Dr. Bruce Aylward, WHO’s Assistant Director-General for Polio, will discuss the status of eradication today at Rotary International’s ‘Making History‘ event. Help share and watch live at 6:30 PM ET.
  • Look out for A Shot to Save the World, a documentary about Jonas Salk’s vaccine discovery, airing on the Smithsonian Channel today at 8:00 pm ET/PT.
  • President-elect of the Asia Pacific Pediatric Association Naveen Thacker wrote an opinion piece on India’s incredible achievements against polio, and the benefits and lessons India’s experience offers. Help share his piece.
  • Check out a video by footballer Leo Messi (tweet), a blog post by Paralympian polio-survivor Dennis Ogbe (tweet), a Vaccines Today blog post by Ramesh Ferris (tweet) and an Impatient Optimists post on other ways to get involved today.
  • Pakistan: Thanks to the work of heroic vaccinators, Pakistan has eliminated polio from much of the country. This year, 74% of cases, and 93% during the high season, have occurred in one region: the Federally Administered Tribal Areas (FATA) of northern Pakistan. North Waziristan, in FATA, has been inaccessible since June 2012, and has reported 14 wild polio cases this year in an increasingly severe outbreak. The program is intensifying immunizations in neighboring areas to prevent spread, but continued inaccessibility in this region poses a serious risk to the global effort.
  • Nigeria: Challenges persist in northern Nigeria, particularly in Borno and Kano, but other traditional reservoir areas appear to be largely polio-free — reminders that success is possible. Of particular importance, the northwest of the country, from which polio has historically spread into West Africa, has not had any cases this year. Read and help share a recent Science article (available with free registration) that takes an in-depth look at Nigeria’s eradication efforts.
  • Afghanistan: Afghanistan’s traditionally endemic Southern Region remains polio-free, with all cases this year linked to cross-border transmission with Pakistan. Next month will mark one year since the last case was recorded in the Southern Region.
  • Horn of Africa: GPEI partners responded rapidly to the outbreak, and we’re seeing signs of progress: there have been no confirmed cases in the Banadir region of Somalia, the epicenter of the outbreak, or in Kenya, since August. The number of unimmunized individuals in the region still poses a major risk for further spread. Outbreak response will continue aggressively into 2014.
  • Possible Polio Cases Detected in Syria: Syria reported a cluster of possible polio cases on 17 October that is currently being investigated. The country has been polio-free since 1999, but is considered at high risk for polio due to declining immunization rates. Syria’s Ministry of Health is preparing an urgent response across the country, aiming to conduct the first campaign by the end of October. Supplementary immunization activities are being planned in neighboring countries, including Lebanon, Jordan, Egypt, southern Turkey and western Iraq. The GPEI has a history of eliminating polio in areas of insecurity. Drawing from past successful efforts in insecure areas, including El Salvador and Angola, the Strategic Plan outlines approaches to eliminating polio in areas of conflict that are informing Syria’s response.
  • IMB Report: The International Monitoring Board (IMB), tasked with assessing the GPEI effort each quarter, met earlier this month to review the program’s progress, challenges and risks in endemic countries, the Horn of Africa and Israel. The IMB’s report from this meeting will be available here on Friday, 25 October

 

WHO will switch to type 2 inactivated poliovirus vaccine

Poliovirus by Jason RobertsThe World Health Organization’s campaign to eradicate poliomyelitis made impressive inroads in 2012: only 212 cases were reported, compared with 620 the previous year; moreover, India remained polio-free. The dark side of this story is that as wild polio is eliminated, vaccine-associated poliomyelitis moves in to take its place. The landmark decision by WHO to replace the infectious, type 2 Sabin poliovaccine with inactivated vaccine is an important step towards eliminating vaccine-associated polio.

A known side effect of the Sabin poliovirus vaccines, which are taken orally and replicate in the intestine, is vaccine-associated poliomyelitis. During the years that the Sabin poliovirus vaccines (also called oral poliovirus vaccine, or OPV) were used in the US, cases of poliomyelitis caused by vaccine-derived polioviruses (VDPV) occurred at a rate of about 1 per 1.4 million vaccine doses, or 7-8 per year. Once the disease was eradicated from the US in 1979, the only cases of polio were caused by VDPVs. For this reason the US switched to the Salk inactivated poliovirus vaccine (IPV) in 2000.

The main vaccine used by WHO in the global eradication effort has been a trivalent preparation comprising all three serotypes. When type 2 poliovirus was eradicated in 1999, many countries began immunizing only against types 1 and 3 poliovirus. As a consequence of this immunization strategy, population immunity to type 2 poliovirus declined. This switch, together with poor routine immunization coverage in some areas, has lead to polio outbreaks caused by cVDPV2 in countries such as Pakistan.

Alan Dove and I suggested in 1997 that it would be necessary to switch from OPV to IPV to achieve polio eradication. However, WHO did not agree with our position:

Dove and Racaniello believe that the reliance of the WHO on the live Sabin oral poliovirus vaccine (OPV) means that there will be a continuing threat of release of potentially pathogenic virus into the environment. They therefore recommend a switch to the inactivated polio vaccine (IPV). In response, Hull and Aylward explain why a switch from OPV is not necessary and describe the studies being sponsored by the WHO to determine how and when immunization can safely be ended.

I remember well the words of DA Henderson, the architect of smallpox eradication, when I proposed a switch to IPV at a conference in 2001:

There is no way it is going to come about and as an end-game strategy it is dreaming to believe that this is reasonable. So, it is just not on.

Apparently I was not dreaming: in May 2012 the 65th World Health Assembly requested that the Director-General “coordinate with all relevant partners, including vaccine manufacturers, to promote the research, production and supply of vaccines, in particular inactivated polio vaccines, in order to enhance their affordability, effectiveness and accessibility”. Later last year the Strategic Advisory Group of Experts on immunization (SAGE) called for a global switch from trivalent to bivalent OPV, eliminating the type 2 component. To ensure that circulating type 2 VDPVs do not pose a threat, SAGE also recommended that all countries introduce at least one dose of inactivated poliovaccine. This decision was announced in the 4 January 2013 Weekly Epidemiological Record (pdf).

The fact that WHO believes it is necessary to switch from type 2 OPV to IPV surely means that in the future, when types 1 and 3 polioviruses are eradicated, types 1 and 3 OPV will be replaced with IPV. This is the correct endgame strategy for eradicating polio. Once circulating VDPVs are no longer detectable on the planet – something that will probably not happen before 2020 – then we may safely stop immunization with IPV.

Poliovirus image courtesy of Jason Roberts.

Vaccine-associated poliomyelitis in Pakistan

Poliovirus by Jason RobertsAn outbreak of ten cases of poliomyelitis caused by circulating vaccine-derivied poliovirus type 2 (cVDPV2) is ongoing in Pakistan, centered in the Kila Abdulla/Pishin area of Baluchistan. The same virus strain has spread to the neighboring Kandahar province in Afghanistan, where two paralytic cases have been reported. Vaccine-derived poliomyelitis is a well-known consequence of immunization with the Sabin poliovirus vaccine.

There are three serotypes of poliovirus, each of which causes poliomyelitis. The three vaccine strains developed by Albert Sabin (OPV, oral poliovirus vaccine) contain mutations which prevent them from causing paralytic disease. When the vaccine is taken orally, the viruses replicate in the intestine, and immunity to infection develops. While replicating in the intestinal tract, the vaccine viruses undergo genetic changes. As a consequence, the OPV recipients excrete neurovirulent polioviruses. These so-called vaccine-derived polioviruses (VDPV) can cause poliomyelitis in the recipient of the vaccine or in a contact. During the years that the Sabin poliovirus vaccines were used in the US, cases of poliomyelitis caused by VDPV occurred at a rate of about 1 per 1.4 million vaccine doses, or 7-8 per year. Once the disease was eradicated from the US in 1979, the only cases of polio were caused by VDPVs. For this reason the US switched to the Salk (inactivated) poliovirus vaccine in 2000.

Because VDPVs are excreted in the feces, they can spread in communities. These circulating VDPVs, or cVDPVs, can cause outbreaks of poliomyelitis in under-immunized populations. Examples include outbreaks of poliomyelitis in an Amish community and in Nigeria in 2009 caused by cVDPV2. Nigeria employed trivalent OPV before 2003, the year that this country began a boycott of polio immunization. Because type 2 poliovirus had been eradicated from the globe in 1999, when immunization in Nigeria resumed in 2004, monovalent types 1 and 3 vaccine were used. The source of the VDPV type 2 in Nigeria was the trivalent vaccine used before 2003.

For many years the vaccine used by WHO in the global eradication effort was a trivalent preparation comprising all three serotypes. When type 2 poliovirus was eliminated, many countries began immunizing only against types 1 and 3 poliovirus. As a consequence of this immunization strategy, population immunity to type 2 poliovirus declined. This has likely lead to the emergence of cVDPV2 in Pakistan, together with poor routine immunization coverage.

The resurrection of poliovirus type 2 highlights the difficulties in eradicating a pathogen using a vaccine that can readily mutate to cause the disease that it is designed to prevent. As wild type polioviruses are eliminated, the only remaining polio will be caused by the vaccine. If immunization is then stopped, as planned by WHO, there will likely be outbreaks of polio caused by cVDPV of all three serotypes. The solution to this conundrum is to switch to the inactivated vaccine until cVDPVs disappear from the planet.

Exacerbating the polio situation in Pakistan was the murder in the past week of nine immunization workers in several provinces. The Taliban, which carried out the executions, accused them of being spies. This accusation originates from the CIA operation in 2011 in which a Pakistani doctor ran an immunization program in Abbottabad in an attempt to obtain DNA samples from the Bin Laden family. As a result of this violence, immunization campaigns in Balochistan have been suspended. Coupled with the previous refusal of many parents to have their children immunized, this action makes it likely that poliovirus will spread more extensively in the country, making eradication even more difficult.

Poliovirus image courtesy of Jason Roberts.

Poliovirus vaccine, SV40, and human cancer

SV40Deep sequencing – which identified a viral contaminant of the rotavirus vaccine Rotarix – could have revealed the presence of simian virus 40 (SV40) in the poliovirus vaccine, had the technique been available in the 1950s. Exposure of over 100 million Americans to SV40, and many more worldwide, could have been avoided, as well as the debate about the role of this monkey virus in human cancer.

SV40 was discovered by Maurice Hilleman in 1960 as a contaminant of poliovirus vaccine. It was present in batches of both the Salk and Sabin poliovirus vaccines produced and distributed from 1954 to 1963. The source was the rhesus and cynomolgous monkey kidney cells used to produce the vaccine. Even more troubling was the observation that SV40 could cause tumors in hamsters. By 1963 screening procedures were instituted to ensure the absence of SV40 in poliovirus vaccines. Ironically, monkey cells were used for poliovirus vaccine production because it was feared that human cells might contain unknown human cancer viruses.

SV40 does not cause tumors in its natural host – monkeys – because it kills infected cells. However, in the wrong host- such as a hamster – the viral replication cycle is incomplete and virions are not produced. At a very low frequency, pieces of the viral DNA become integrated into the host chromosomal DNA. Problems arise if these viral DNA fragments encode the viral T (tumor) antigen. This protein is essential for lytic replication (which takes place in monkey cells) because it kick-starts cellular DNA synthesis. The cellular DNA synthetic machinery is then co-opted for replication of the viral DNA. When only T antigen is present, the cells divide without stopping – they are transformed, and on the way to becoming a tumor. SV40 does not need to cause tumors as part of its life cycle; they are an aberrant result of having T antigen push the cells to divide. SV40 T antigen can transform human cells, and therefore in theory the virus could cause human tumors.

The results of epidemiological studies initiated in the 1960s through the 1970s, in which thousands of poliovirus vaccine recipients were studied, indicated that this population did not have an increased risk of developing cancer. More recent reports that SV40 viral DNA is present in human tumors have led to a debate on the contribution of this virus to human cancer. Some of the arguments for and against presence of SV40 in human cancers are presented below.

Evidence that SV40 is present in human tumors

  • SV40 DNA has been detected in several human tumors, including osteosarcoma, mesothelioma, and non-Hodgkin’s lymphoma. Similar tumors are induced by the virus in hamsters.
  • Poliovirus vaccine produced in 1954 contained a variant of SV40 that can be distinguished from common laboratory strains. This viral variant has been found in three non-Hodgkin’s lymphoma patients

Evidence that SV40 is not present in human tumors

  • SV40 DNA is not present in all samples of a cancer, and in some studies of mesotheliomas, it has not been detected in any.
  • SV40 viral DNA has been detected in tumors of those who could not have received contaminated poliovirus vaccine.
  • In a comparison of mesotheliomas and normal tissues, SV40 DNA has been detected as frequently in both.
  • Analysis of the SV40 sequences in mesotheliomas showed that the viral DNA was derived from a laboratory strain which contains a gap that is not present in the wild type viral genome.

Even if SV40 DNA were definitively shown to be present in human tumors, this would not answer the question of whether the virus caused the cancer. The debate on the role of SV40 in human malignancy illustrates the difficulty in establishing cause and effect, and provides ample impetus for using genomic technologies to ensure that vaccines and other biological products are free of adventitious agents.

Garcea, R., & Imperiale, M. (2003). Simian Virus 40 Infection of Humans Journal of Virology, 77 (9), 5039-5045 DOI: 10.1128/JVI.77.9.5039-5045.2003

López-Ríos F, Illei PB, Rusch V, & Ladanyi M (2004). Evidence against a role for SV40 infection in human mesotheliomas and high risk of false-positive PCR results owing to presence of SV40 sequences in common laboratory plasmids. Lancet, 364 (9440), 1157-66 PMID: 15451223

PEDEN, K. (2008). Recovery of strains of the polyomavirus SV40 from rhesus monkey kidney cells dating from the 1950s to the early 1960s Virology, 370 (1), 63-76 DOI: 10.1016/j.virol.2007.06.045

Learning vaccinology from an immunization record

vrr-immunization-recordLast week on TWiV #26 Rich Condit and I were reminiscing about the possibility that we had received the virus SV40 along with our polio immunizations. That discussion prompted me to examine my immunization record for poliovaccine that my Mother had given to me in the early 1990s. She was cleaning out her house and decided that I should have it – a little memento for someone who has studied poliovirus for most of his career. Can we learn anything of value from this old immunization record?

The first lesson comes from the earliest date on the card, September 1963. I was ten years old – why was I receiving poliovaccine at that advanced age? A child born today in the US is given poliovaccine at 2, 4, and 6-18 months, and then at 4-6 years of age. I was immunized in 1955-56 with the first batches of inactivated poliovirus vaccine, IPV, created by Jonas Salk. Although IPV was used in mass immunization campaigns in the US and reduced the incidence of polio from a peak of 21,000 cases in 1952 to 2,500 in 1960, many felt that it did not confer lifelong immunity and would not lead to eradication of the disease. Hilary Koprowski, Herald Cox, and Albert Sabin all believed that an infectious poliovirus vaccine, which mimicked a natural infection, was needed. Sabin’s oral poliovirus vaccine (OPV) was licensed in the US in 1961, and once again American children stood in line for mass immunizations. OPV is taken by mouth, and I remember clearly waiting in the school auditorium in front of rows of tables covered with small paper cups. Inside each one was a pink sugar cube – the color a consequence of the vaccine which had been poured on it. I ate the sugar cube three times – September 1963, and then April and May of 1964.

If I had been immunized with IPV in the 1950s, wouldn’t the immunological memory block infection with OPV? No, because immunization with IPV did not confer protection of the alimentary tract, which is the initial site of replication of the OPV strains.

There are three columns on the card labeled type I, type II, and type III, which refer to the serotypes of poliovirus that had been poured on the sugar cubes. These monovalent poliovirus vaccines were stopgap measures, used only until approval came in 1964 for a single vaccine containing all three viral serotypes – trivalent OPV. The use of monovalent type 1 OPV resumed a few years ago in Egypt, because of the rarity of polio caused by the other serotypes.

Notice the order in which I received the three monovalent vaccines – type I first, followed by type III, and then type II. This sequence was initially used to prevent the type II virus from interfering with the subsequent replication of the type III strain. Later in 1964, the Advisory Committee on OPV recommended that the order of administration be changed to type II, I, and III. It was believed that giving type II vaccine first would diminish the risk of paralysis associated with types I and III.

Finally, note where the vaccine was administered – Edith A. Bogert School. Why didn’t I receive it in a doctor’s office? Because in mass immunization campaigns, vaccine is most efficiently given in locations where millions of children can be inoculated in a short time. We no longer use this approach – in developed countries new vaccines are incorporated into routine immunization schedules and dispensed in doctors’ offices. Mass immunization campaigns are deployed when it is necessary to quickly stop the chain of transmission. They are still used countries where polio is endemic, such as India, Pakistan, and Afghanistan.

That’s quite a bit to learn from a very small piece of paper. Maybe that’s why my Mother saved it for me.

Poliovirus

Poliovirus by Jason Roberts

Poliovirus is the etiologic agent of the paralytic disease known as poliomyelitis. It’s also the virus I’ve worked on for most of my career. The World Health Organization is in the midst of a massive effort to eradicate the disease, an undertaking that has encountered a number of obstacles. In coming posts I’d like to discuss the polio eradication effort, but first we need some background on poliovirus.

Poliovirus is a member of a family of viruses called the Picornaviridae. Viruses are classified into families, and then genera, based on many criteria, including physical and biological properties. The Picornaviridae includes many other human pathogens, such as Coxsackieviruses, echoviruses, enteroviruses, hepatitis A virus, and rhinoviruses (which cause the common cold). The International Committee for the Taxonomy of Viruses is responsible for virus classification and maintains the Universal Virus Database, where you can find information about most known viruses.

Poliovirus is a rather small and simple virus. It is composed of a shell, or capsid, made of protein, as shown.

The poliovirus capsid is about 30 nanometers in diameter. Within the capsid is the information to make new virus particles – a single molecule of ribonucleic acid, or RNA. In the image, part of the capsid has been cut away to reveal the viral RNA. When the virus infects a cell, the RNA genome enters the cell and programs it to make new virus particles. These virus particles are released from the cell and go on to infect new cells.

In humans, poliovirus is ingested, and replicates in cells of the gastrointestinal tract. Newly synthesized virus particles are released into the intestine and shed in the feces. Transmission of poliovirus to another human occurs through contact with virus-containing feces or contaminated water. After multiplying in the gastrointestinal tract, poliovirus may enter the spinal cord and brain. Destruction of motor neurons by the virus leads to limb paralysis.

Poliomyelitis became a common disease at the turn of the 20th century. Two vaccines were developed in the 1950s that can effectively prevent the disease – inactivated poliovaccine (IPV, developed by Jonas Salk) and live, oral poliovaccine (OPV, developed by Albert Sabin). In 1988 the World Health Organization announced that it would eradicate poliomyelitis from the globe by the year 2000. We’ll discuss that goal, and obstacles that might prevent it from being attained, in subsequent posts.