virology blog
About viruses and viral disease
On episode #371 of the science show This Week in Virology, the TWiVologists discuss the finding of a second transmissible cancer in Tasmanian devils, and development of new poliovirus strains for the production of inactivated vaccine in the post-eradication era.
You can find TWiV #371 at www.microbe.tv/twiv.
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. 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’.
On episode #351 of the science show This Week in Virology, the Masters of the ScienTWIVic Universe discuss a novel poxvirus isolate from an immunosuppressed patient, H1N1 and the gain-of-function debate, and attenuation of dengue virus by recoding the genome.
You can find TWiV #351 at www.microbe.tv/twiv.
The oral poliovirus vaccine strains (OPV) developed by Albert Sabin were licensed in the United States in 1962, and over the next 37 years immunization with these vaccines lead to the eradication of poliomyelitis in this country. During that period, the vaccine was responsible for 5-10 cases of poliomyelitis each year, either in recipients of the vaccine or in their contacts. Some of these individuals have sued the manufacturers of the vaccine, claiming that they made a “defective product.” My experience as an expert witness in a recent poliovirus vaccine litigation illustrates how difficult it can be for a jury to understand complex scientific issues in cases where there typically is no dispute that the product caused the injury.
My most recent experience involved a case in New York that began early in May 1979, when the plaintiff’s daughter received her second dose of Orimune (the trade name for the live, oral trivalent poliovirus vaccine previously produced by Lederle Laboratories). Approximately 6 weeks later, the plaintiff developed severe back pain and then permanent paralysis of both legs. I will focus on one of the many claims of his lawsuit against Lederle Laboratories: that the vaccine failed to comply with the release standards previously set by the Food and Drug Administration (FDA) for monkey neurovirulence testing, which must be conducted on each monovalent lot, or “monopool,” of vaccine. (OPV is no longer used in the United States for routine immunizations, and the FDA’s regulations on testing the vaccine have been repealed.)
The Sabin poliovirus vaccine strains are prepared by separately growing each of the three viral serotypes in primary monkey kidney cells. Before the vaccines can be released, they are tested by both the manufacturer and the FDA for neurovirulence by injecting samples of the monopools directly into the brains and spinal cords of monkeys. Three weeks after inoculation, the animals are sacrificed, and histological sections are prepared and examined by a pathologist for evidence of cell damage caused by poliovirus. The lesions are scored on a scale of 0 to 4, depending on the extent of cell destruction caused by poliovirus. Each monkey is then given a neurovirulence score, which is expressed as ‘severity’ (the lesion score at the site of inoculation) and ‘spread’ (the lesion score distal to the site of inoculation). These scores are then compared with all the values historically obtained in monkey neurovirulence testing conducted using a reference strain of poliovirus. The FDA’s regulations allowed the monopools to be used in the further manufacture of vaccine only if their neurovirulence in monkeys did not exceed the neurovirulence of the reference preparation.
The plaintiff claimed that one of the two type 3 monopools that were likely part of his daughter’s vaccine was excessively neurovirulent. In particular, the plaintiff argued that the FDA’s intraspinal neurovirulence test on one of these type 3 pools revealed a single monkey with a severity-spread score of 3-3. No monkeys with a 3-3 score had been observed on Lederle’s intraspinal test of this vaccine monopool. Plaintiff claimed that this monovalent pool should not have been released under the FDA’s standards, because a monkey with a severity-spread score of 3-3 had never been observed previously on intraspinal testing of the reference strain.
From a scientific point of view, the plaintiff’s claims are, in my view, insupportable.
First, claims about the type component of the vaccine do not appear to be scientifically relevant unless the plaintiff’s polio was caused by type 3 poliovirus. In this case, type 3 poliovirus was isolated in the plaintiff’s stool shortly after the onset of paralysis, demonstrating that type 3 virus was replicating in his digestive tract. But complement fixation tests conducted on blood samples also demonstrated a vigorous antibody response to type 2 poliovirus before any response was detected to type 3 poliovirus. From this evidence, I do not think that it can be said that the plaintiff’s infection of his spinal cord was caused by type 3 virus, as opposed to type 2 virus. The jury reached a different conclusion which in my view is not supported by the scientific evidence.
Second, a single monkey with a 3-3 neurovirulence score appears to have played an important role in this case, but that ignores the fact that there are almost always wide variations that result in “outliers” whenever biological assays are involved. We have studied poliovirus infection of mice in my laboratory, for example, and outliers are common – the one mouse in twenty that becomes ill or dies, while the others remain well. Other investigators have shown that poliovirus recovered from the spinal cord of an outlier monkey – one with a 3-3 neurovirulence score – does not produce these results when re-injected in a different group of monkeys. The monkey with a 3-3 score observed in the government’s test of one monopool of type 3 poliovirus vaccine is clearly an outlier. The other 14 monkeys in the intraspinal test had very low scores, and Lederle’s neurovirulence test did not produce a monkey with a 3-3 score. I conclude that the neurovirulence test on this type 3 monopool clearly did not exceed that of the reference virus – there is no scientifically valid justification for arguing otherwise.
If you don’t believe in outliers, there is another way to look at this issue: does a monkey with a 3-3 score in the neurovirulence test mean that the lot of vaccine is more likely to cause paralysis in humans? We cannot carry out such an experiment prospectively, but we can do the next best thing – compare the results of the monkey neurovirulence test with the rate of vaccine-associated poliomyelitis. In the entire history of the monkey neurovirulence test – from 1962-1999 – some monopools of vaccine periodically had one or two monkeys with a 3-3 neurovirulence score, and others did not. Nevertheless, the rate of vaccine-associated paralytic disease remained remarkably constant over this time: 5-10 cases per year. There were no spikes in the years when the “3-3” vaccines were in distribution. The conclusion is clear: no lot of vaccine is associated with an increase in the number of paralytic cases in any year.
Why do the Sabin vaccine strains cause paralytic disease in some recipients and contacts? Albert Sabin derived these vaccine strains by serially passing neurovirulent isolates in different cell types, empirically identifying viral mutants with a reduced capacity to cause disease. There are few mutations responsible for the reduced neurovirulence of the Sabin strains – 5 for type 1, and 2 each for the type 1 and type 2 strains. These mutations rapidly revert during multiplication of the vaccine viruses in the human gut, and that occurs in every recipient of the vaccine. Within several days, the recipient sheds viruses that no longer bear the mutations that Sabin so painstakingly selected. These excreted revertants, when tested in monkeys, are more neurovirulent than the vaccine that was fed to the recipient.
Given the high reversion rate of the poliovirus vaccine in the human intestine, it is hard to understand why the vaccine is so safe in practice. Put another way, why do some individuals contract poliomyelitis after exposure to the vaccine, while the vast majority to not? Their paralytic disease is caused by neurovirulent revertant viruses, but why these individuals are more susceptible than the general population is unknown. One hypothesis is that they have a sub-optimal, innate immune response to infection, which allows unchecked multiplication of the revertant viruses and eventual invasion of the brain and spinal cord.
The timing of the plaintiff’s paralysis in the New York case is highly revealing. He reported the first symptoms of paralysis 6 weeks after his daughter received the vaccine. There is a 12-15 day incubation period between the time when poliovirus is ingested and the first appearance of paralytic symptoms. Working backwards, it is likely that the plaintiff was infected by his daughter’s vaccine during the first week of June – nearly four weeks after administration of Orimune to the infant. By this time, all the type 3 viruses that the baby was excreting had reverted to greater neurovirulence. The results of the monkey neurovirulence tests on the vaccine that was fed, therefore, have no relevance to the virus that infected the plaintiff. No matter what scores had been obtained on the monpools by the FDA or by the manufacturer, the excreted virus would have all reverted to neurovirulence long before the plaintiff’s infection.
In my view, the scientific evidence overwhelmingly indicates that the neurovirulence of the vaccine in this case was acceptable. I believe that I explained the science clearly to the jury; nevertheless, they found the defendant released vaccine that violated the FDA’s monkey neurovirulence standards and that posed a greater risk than the risk that is inherent in all OPV. This case illustrates that complex scientific issues that we struggle with in the laboratory are most difficult to grasp by jurors. Indeed, one could argue that using a lay jury to decide scientific issues is an imperfect solution. Perhaps cases like this should be resolved by truly expert panels of scientists who are in a better position to evaluate the evidence presented.
Oral polio vaccine has an inherent risk that public health authorities deemed to be acceptable, given the extraordinary benefits of the vaccine. However, in a sense, the ~400 individuals who contracted vaccine-associated polio from 1962-1999 paid a price for the greater good of the population. For this reason alone they deserved compensation, which is what they now receive under the National Childhood Vaccine Injury Act. Compensation for these individuals is given without the need to castigate life-saving vaccines likes OPV.