The global withdrawal of the Sabin type 2 poliovirus vaccine is a test of the feasibility of the plan, declared by the World Health Assembly in 1988, to eradicate all polioviruses.
I cannot let September pass without noting that 34 years ago this month, I arrived at Columbia University to start my laboratory toÂ do research on poliovirus (pictured). That virus is no longer the sole object of our attention – we are wrapping up some work on poliovirus andÂ our attention has shifted elsewhere. But this is a good month to think about the status of the poliovirus eradication effort.
So far this year 26Â cases of poliomyelitis have been recorded – 23 caused by wild type virus, and three caused by vaccine-derived virus. At the same time in 2015Â there wereÂ 44 reported cases of polio – smallÂ progress, but, in the words of Bill Gates, the last one percent isÂ the hardest.
One of the disappointmentsÂ this year is Nigeria. It was on the verge of being polio-free for one year – the last case of type 1 poliovirus in Nigeria had beenÂ recorded in July of 2014. In AugustÂ the government reported that 2 children developed polio in the Borno State. The genome sequence of the virus revealed that it had been circulating undetected in this region since 2011.Â Due to threats from militant extremists, it has not been possible for vaccination teams to properly cover this area, and surveillance for polioviruses has also been inefficient. The virus can circulate freely in a poorly immunized population, and as only 1% of infections lead to paralysis, cases of polio might have been missed.
The conclusion from this incident is thatÂ the declaration that poliovirus is no longer present in any region is only as good as the surveillance for the virus, which can never be perfect as all sources of infection cannot be covered.
Of the 26 cases of polio recorded so far in 2016, most have been in Afghanistan and Pakistan (9 and 14, respectively). It is quite clear that conflict has prevented vaccination teams from immunizing the population: in Pakistan, militants haveÂ attacked polio teams during vaccinationÂ campaigns.
Recently 5 of 27 sewageÂ samples taken from different parts of the province of Balochistan in Pakistan have tested positive for poliovirus. Nucleotide sequence analysis revealed that the viruses originated in Afghanistan. The fact that such viruses are present in sewage means that there are still individuals without intestinal immunity to poliovirus in these regions. In response to this finding, a massive polio immunization campaign was planned for the end of September in Pakistan. This effort would involve 6000 teams to reach 2.4 million children. Apparently police will be deployed to protect immunization teams (source: ProMedMail).
The success of the polio eradication program so far has made it clear that if vaccines can be deployed, circulation of the virus can be curtailed. If immunization could proceed unfettered, I suspect the virus would be gone in five years. But can anyone predictÂ whether it will be possible to curtail the violence in Pakistan, Afghanistan, and Nigeria that has limited polio vaccination efforts?
The 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.Â Why areÂ the Sabin vaccines still used globally? The answer to this questionÂ 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 madeÂ 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 ofÂ 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 inÂ a natural poliovirus infection, Sabin’sÂ vaccinesÂ 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 theÂ 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 inÂ 1961-62Â 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 suggestedÂ 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 1980sÂ 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 enumeratedÂ for the first time the mutations that distinguish the Sabin vaccine from its neurovirulent parent. More importantly, the genome sequence of the vaccine-associatedÂ 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,Â 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 aÂ 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; andÂ 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 ofÂ OPV. In 1996 theÂ 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) areÂ 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,Â WHO has recommended aÂ globalÂ 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. WouldÂ the exclusiveÂ 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’.
An 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 produceÂ very low levels of antibodies (a conditionÂ called agammaglobulinemia) are known to excrete VDPVsÂ 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). Â 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 vaccinationÂ at some future time, these immunocompromised poliovirus shedders pose a threat to future unimmunizedÂ 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 fromÂ 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 theseÂ 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
In 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 wereÂ 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 glassÂ 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.
Perhaps 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 intactÂ 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 aÂ -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.
Antigenic variation is a hallmark of influenza virus that allows the virus to evadeÂ 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 protectedÂ fromÂ infection with thisÂ 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, whichÂ 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.