TWiV 373: The distinguished virology career of Julius S. Youngner

On episode #373 of the science show This Week in Virology, Vincent speaks with Julius about his long career in virology, including his crucial work as part of the team at the University of Pittsburgh that developed the Salk inactivated poliovirus vaccine.

You can find TWiV #373 at Or you can watch the video below.

The switch from trivalent to bivalent oral poliovirus vaccine: Will it lead to polio?

bivalent OPVIn four months, 155 countries will together switch from using trivalent to bivalent oral poliovirus vaccine. Will this change lead to more cases of poliomyelitis?

There are three serotypes of poliovirus, each of which can cause paralytic poliomyelitis. The Sabin oral poliovirus vaccine (OPV), which has been used globally by WHO in the eradication effort, is a trivalent vaccine that contains all three serotypes.

In September 2015 WHO declared that wild poliovirus type 2 has been eradicated from the planet – no cases caused by this serotype had been detected since November 1999. However, in 2015, there were 9 cases of poliomyelitis caused by the type 2 vaccine. For these reasons WHO decided to remove the type 2 Sabin strain from OPV, and switch from trivalent to bivalent vaccine in April 2016.

After OPV is ingested, the viruses replicate in the intestinal tract, providing immunity to subsequent infection. During replication in the intestine, the vaccine viruses lose the mutations that prevent them from causing paralysis. Everyone who receives OPV sheds these revertant viruses in the feces. In rare cases (about one in 1.5 million) the revertant viruses cause poliomyelitis in the vaccine recipient (these cases are called VAPP for vaccine-associated paralytic poliomyelitis). Vaccine-derived polioviruses can also circulate in the human population, and in under-vaccinated populations, they can cause poliomyelitis.

There were 26 reported cases of poliomyelitis caused by the type 1 or type 2 vaccine viruses in 2015. Nine cases of type 2 vaccine-associated polio were detected in four countries: Pakistan, Guinea, Lao People’s Democratic Republic, and Myanmar. Removing the type 2 strain from OPV will eliminate vaccine-associated poliomyelitis in recipients caused by this serotype. When the US switched from OPV to the inactivated poliovaccine (IPV) in 2000, VAPP was eliminated.

The problem with the trivalent to bivalent switch is that vaccine-derived type 2 poliovirus is likely still circulating somewhere on Earth. The last two reported cases of type 2 vaccine-associated polio in 2015 were reported in Myanmar in October. The viruses isolated from these cases were genetically related to strains that had been circulating in the same village in April of the that year. In other words, type 2 vaccine-derived strains have been circulating for an extended period of time in Myanmar; they have been known to persist for years elsewhere. If these viruses continue to circulate past the time that immunization against type 2 virus stops, they could pose a threat to the growing numbers of infants and children who have not been immunized against this serotype.

Eventually as type 3, and then type 1 polioviruses are eradicated, it will also be necessary to stop immunizing with the respective Sabin vaccine strains. The switch from trivalent to bivalent vaccine in April 2016 is essentially an experiment to determine if it is possible to stop immunizing with OPV without placing newborns at risk from circulating vaccine-derived strains.

Over 18 years ago Alan Dove and I argued that the presence of circulating vaccine-derived polioviruses made stopping immunization with OPV a bad idea. We suggested instead a switch from OPV to IPV until circulating vaccine-derived viruses disappeared. At the time, WHO disagreeed, but now they recommend that all countries deliver at least one dose of IPV as part of their immunization program. Instead of simply removing the Sabin type 2 strain from the immunization programs of 155 countries, it should be replaced with the inactivated type 2 vaccine. This change would maintain immunity to this virus in children born after April 2016. Such a synchronized replacement is currently not in the WHO’s polio eradication plans. I hope that their strategy is the right one.

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

TWiV 348: Chicken shift

On episode #348 of the science show This Week in Virology, Vincent and Rich discuss fruit fly viruses, one year without polio in Nigeria, and a permissive Marek’s disease viral vaccine that allows transmission of virulent viruses.

You can find TWiV #348 at

TWiV 331: Why is this outbreak different from all other outbreaks?

On episode #331 of the science show This Week in Virology, the TWiV team discusses the possible association of the respiratory pathogen enterovirus D68 with neurological disease.

You can find TWiV #331 at

An unexpected benefit of inactivated poliovirus vaccine

Poliovirus by Jason Roberts

Poliovirus by Jason Roberts

The polio eradication and endgame strategic plan announced by the World Health Organization in 2014 includes at least one dose of inactivated poliovirus vaccine (IPV). Since 1988, when WHO announced the polio eradication plan, it had relied exclusively on the use of oral poliovirus vaccine (OPV). The rationale for including a dose of IPV was to avoid outbreaks of vaccine-derived type 2 poliovirus. This serotype had been eradicated in 1999 and had consequently been removed from OPV. However IPV, which is injected intramuscularly and induces highly protective humoral immunity, is less effective in producing intestinal immunity than OPV. This property was underscored by the finding that wild poliovirus circulated in Israel during 2013, a country which had high coverage with IPV. Furthermore, in countries that use only IPV, over 90% of immunized children shed poliovirus after oral challenge. I have always viewed this shortcoming of IPV as problematic, in view of the recommendation of the World Health Organization to gradually shift from OPV to IPV. Even if the shift to IPV occurs after eradication of wild type polioviruses, vaccine-derived polioviruses will continue to circulate because they cannot be eradicated by IPV. My concerns are now mitigated by new results from a study in India which indicate that IPV can boost intestinal immunity in individuals who have already received OPV.

To assess the ability of IPV to boost mucosal immunity, 954 children in three age groups (6-11 months, 5 and 10 years) were immunized with IPV, bivalent OPV (bOPV, containing types 1 and 3 only), or no vaccine. Four weeks later all children were challenged with bOPV, and virus shedding in the feces was determined 0, 3, 7, and 14 days later. The results show that 8.8, 9.1, and 13.5% of children in the 6-11 month, 5-year and 10-year old groups shed type 1 poliovirus in feces, compared with 14.4, 24.1, and 52.4% in the control group. Immunization with IPV reduced fecal shedding of poliovirus types 1 (39-74%) and 3 (53-76%). The reduction of shedding was greater after immunization with IPV compared with bOPV.

This study shows that a dose of IPV is more effective than OPV at boosting intestinal immunity in children who have previously been immunized with OPV. Both IPV and OPV should be used together in the polio eradication program. WHO therefore recommends the following vaccine regimens:

  • In all countries using OPV only, at least 1 dose of type 2 IPV should be added to the schedule.
  • In polio-endemic countries and in countries with a high risk for wild poliovirus importation and spread: one OPV birth dose, followed by 3 OPV and at least 1 IPV doses.
  • In countries with high immunization coverage (90-95%) and low wild poliovirus importation risk: an IPV-OPV sequential schedule when VAPP is a concern, comprising 1-2 doses of IPV followed by 2 or mores doses of OPV.
  • In countries with both sustained high immunization coverage and low risk of wild poliovirus importation and transmission: an IPV only schedule.

Type 2 OPV will be gradually removed from the global immunization schedules. There have been no reported cases of type 3 poliovirus since November 2012. If this wild type virus is declared eradicated later this year, presumably WHO will recommend withdrawal of type 3 OPV and replacement with type 3 IPV.

All 342 confirmed cases of poliomyelitis in 2014 were caused by type 1 poliovirus in 9 countries, mainly Pakistan and Afghanistan. Given the social and political barriers to immunization, it will likely take many years to eradicate this serotype.

Oral polio vaccine-associated paralysis in a child despite previous immunization with inactivated virus

Poliovirus by Jason Roberts

Poliovirus by Jason Roberts

Vaccine-associated poliomyelitis caused by the oral poliovirus vaccine is rare, but its occurrence in a healthy, immunocompetent 6-month old child was highly unusual because the child had been previously immunized with two doses of the injected, inactivated poliovirus vaccine (IPV).

The three poliovirus vaccine strains developed by Albert Sabin (OPV, oral poliovirus vaccine) contain mutations which prevent them from causing paralytic disease. When the vaccine is ingested, the viruses replicate in the intestine, and immunity to infection develops. While replicating in the intestinal tract, the vaccine viruses undergo mutation, and 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 the Sabin vaccine.

To prevent vaccine-associated poliomyelitis, in 1997 the US switched to an immunization schedule consisting of two doses of IPV followed by one dose of OPV. The US then switched to using IPV exclusively in 2000. The child in this case essentially had a polio immunization course similar to that utilized in the US from 1997-2000: two doses of IPV, one dose of OPV. Why did the child develop poliomyelitis?

One clue comes from the fact that after the switch to an IPV-OPV schedule in 1997, there were still three cases of VAPP in 1998 and three in 1999. Another hint comes from a study of immune responses in children given multiple doses of IPV. Most of the children receiving two doses of IPV produced antibodies against types 1 and 2 poliovirus (92 and 94%), but only 74% of children produced antibodies against type 3 poliovirus.

The final piece of information needed to solve this puzzle is that the child in this case had vaccine-associated poliovirus caused by the type 3 strain, which was isolated from his feces.

Therefore, the child in this case most likely did not produce sufficient antibodies to type 3 poliovirus after receiving the two doses of IPV. As a consequence, when he was given OPV, he developed type 3 vaccine-associated poliomyelitis.

This case of VAPP could have been prevented: the child was born in Canada, and as customary in that country since 1995, he received two doses of IPV. At 5 months of age the child and his family visited China, where his parents decided to continue his immunizations according to the local schedule. China still uses OPV, so that is what the child received.

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.

TWiV 281: The Salk legacy with Peter L. Salk

On episode #281 of the science show This Week in Virology, Vincent meets up with Peter L. Salk to talk about development of the first poliovaccine, eradication of poliomyelitis, and Jonas Salk’s 100th birth anniversary.

You can find TWiV #281 at