When Polio is Gone, Should We Still Vaccinate?

The current World Health Organization plan for the global eradication of polio recommends that vaccination be stopped once poliovirus has been eradicated from the wild. While this seems to make sense intuitively, many virologists consider such a step foolhardy. This author and Dr. Vincent Racaniello of Columbia University wrote an article, which appears in the journal Science (8 August 1997), explaining the difficulties raised by stopping vaccination. A response from the World Health Organization (WHO) appears in the same issue. Unfortunately, space limitations prevented us from offering extensive explanations to make the article accessible to a lay audience, so I have summarized our main points here. Where appropriate, I will also explain the WHO responses to our arguments, but of course make no claim to true objectivity on this issue. I urge those who are interested to read the original papers and decide for themselves.
 
 

The Eradication Effort is Working

Using a system which was proven effective in the eradication of smallpox, the WHO, working with national organizations like the Centers for Disease Control (CDC) and Rotary International, has rapidly eliminated poliovirus from the wild. Outbreaks of polio in recent years have been limited to only a few parts of the world, and the wild virus is completely absent from the Western hemisphere. There is little reason to doubt the CDC claim that polio will be "eradicated" by 2003. We applaud the hard work and dedication that have made this effort a success.

 The reason the word "eradicated" appears in quotes above is that there are different definitions of the term. Since the current campaign has relied almost entirely on the live, attenuated Oral Poliovirus Vaccine (OPV), every individual who is vaccinated secretes live virus into the environment. The Sabin vaccine readily mutates back to a wild-type, potentially paralytic virus, so copious amounts of virulent polio are still being dumped into the sewage systems and aquifers of those nations which are "polio-free," including the United States and other developed countries. A stricter definition of "eradication" would be the elimination of all polioviruses from the population and the environment, a goal which cannot be accomplished using the current protocol. True eradication would require use of the Salk Inactivated Poliovirus Vaccine (IPV) exclusively. This vaccine consists of a preparation of wild-type poliovirus which has been killed with chemicals and heat.
 
 

Polio is Not Smallpox

The WHO plan is based on the model used to fight smallpox. Unfortunately, the two viruses differ dramatically in both biology and history. The smallpox vaccine consists of a preparation of Vaccinia virus, a relative of the virus which causes smallpox. Since the vaccine is not directly derived from the wild virus, there is no way it can mutate back into a pathogenic form and cause an outbreak. There are potential complications of smallpox vaccination, but it cannot cause smallpox. OPV, on the other hand, is a preparation of three mutant strains of wild poliovirus, representing the three serotypes of the virus. IPV is the wild-type virus itself, inactivated in the laboratory before injection into the patient. These distinctions are crucial in understanding the difficulties presented by stopping vaccination.

Since Vaccinia is relatively harmless, the biology of smallpox was studied using this virus as a model system - a sort of stand-in. While hundreds of laboratories have stocks of Vaccinia, its more lethal relative was kept closely guarded, and currently exists in only two laboratory freezers in the world. In contrast, scientists who wanted to study poliovirus had to use the wild-type virus - even those labs studying the vaccine strains would routinely isolate mutants that had reverted. As a result, vials of concentrated poliovirus can now be found in hundreds of laboratory freezers, and no inventory of them has ever been kept. In addition, any medical laboratory which keeps pediatric stool samples will likely have the virus in storage without knowing it.
 
 

Problem 1: Persistence in the Environment

As viruses go, polio is relatively durable. It seems likely that it will be able to survive in some concentration for several weeks or months after vaccination has been stopped. If we go from using OPV to using no vaccine at all, babies born during those months would run the risk of contracting the virus from the environment. The WHO argues that the virus's lifetime in water and soil is relatively short, but few studies have been done to test this claim rigorously. Furthermore, in areas where drinking water is recycled, there is the possibility that a chain of infections could cause poliovirus to circulate indefinitely. A more serious problem, mentioned in the WHO article in Science, is that individuals with compromised immune systems may constitute reservoirs for polio. With the increase in the incidence of AIDS around the world, it is not unreasonable to expect that an immunocompromised patient, inadvertently vaccinated with OPV during the eradication campaign, could continue secreting pathogenic poliovirus for years. Such cases have already been documented, and we now know that an individual can shed live virus for at least two years, possibly longer. Once vaccination stops, the number of unvaccinated individuals in the world population will increase every hour of every day. In ten years, there will be a huge number of potential hosts available to support a new outbreak, and virus persistence would represent a major threat to public health.
 
 

Problem 2: Accidental Release

This September, the WHO will hold a conference to discuss strategies for taking an inventory of all existing poliovirus stocks and setting up containment procedures to prevent the accidental release of the virus into the environment. This may prove impossible. As mentioned above, these stocks are globally distributed. Even if we assume that every scientist and every physician in every nation complies perfectly with the WHO recommendations, we cannot be sure of finding all of the stored viruses. The WHO assures us that the number of stocks is finite, but this is hardly helpful; the world's oceans are also finite, but that has not made navigation simple.

Recently, a group of researchers studying Coxsackie B3 (a relatively benign picornavirus related to polio) attempted to purify large quantities of this virus in order to perform structural studies. Only later did they discovery that they had not grown Coxsackievirus at all, but a contaminant that was present in the original stock. The contaminant was the P2/Lansing serotype of polio. Today, with routine vaccination, this story is mildly amusing - the scientists accidentally grew the wrong virus. Twenty years after the end of vaccination, it will not be amusing at all.
 
 

Problem 3: Maintaining Vaccine Stocks

Because the vaccines for polio are derived from (or consist of) the wild-type virus, the maintenance of emergency stocks after vaccination is stopped presents a chicken-and-egg paradox. Workers responsible for preparing and testing these stocks would, ideally, be vaccinated against the virus. Unfortunately, vaccination with OPV would cause them to secrete live poliovirus, possibly causing an epidemic. Using IPV would avoid this problem, but would not provide the same type of immunity as OPV; individuals vaccinated in this way would still be able to act as carriers, negating the benefit of vaccinating them. To date, nobody has proposed a solution to this problem.
 
 

Problem 4: Biological Terrorism

When smallpox was eradicated, the world had two superpowers, and the primary concern was that one of them would use the virus as a biological weapon against the other. As a result, once the Cold War ended, discussion began about destroying the world's only two remaining stocks of smallpox. Poliovirus, distributed among sites from Lichtenstein to Libya in a world where terrorism has supplanted nuclear holocaust as the number one security concern, does not offer such a straightforward solution. Even if it were possible to destroy all existing stocks, the virus could still be synthesized anew using published genome sequences and currently available technology. As a terrorist weapon, poliovirus is nearly ideal: it is highly contagious, easily released into food or water supplies, and virtually impossible to detect until the damage has already been done. Conservatively, we estimate that a release of poliovirus into a city with 10 million unvaccinated individuals would result in approximately 7,000 casualties - enough people to fill sixteen Boeing 747s. Simultaneous release in several locations at once would presumably push that number higher.
 
 

What Should We Do?

At a minimum, vaccination should continue until IPV can be distributed worldwide. This would reduce the danger of a reintroduction from a vaccinee or an environmental source. It would not, however, prevent biological terrorism or accidental release of lab stocks. Ending vaccination has been justified on economic grounds, and switching to IPV would certainly be more expensive than continuing to use OPV. If we are willing to vaccinate indefinitely, a combined IPV/OPV protocol would provide a good balance of cost and benefit, and such a system has already been adopted in the United States. The worldwide economic benefits of polio vaccination to date are reckoned in the trillions of dollars. We should think carefully before risking this gain on a gamble that offers a comparatively small return.

--Alan Dove, New York, August 1997
 

In addition to this author, a group of other scientists has voiced its concern about the WHO plan. The Committee of Concerned Picornavirologists consists of Vadim Agol, Institute of Poliomyelitis, Russia; Jeff Almond, University of Reading, United Kingdom; Florence Colbere-Garapin, Institut Pasteur, France; Bert Flanegan, University of Florida at Gainesville, USA; Jim Hogle, Harvard Medical School, USA; Kathie Kean, Institut Pasteur, France; Akio Nomoto, University of Tokyo, Japan; Vincent Racaniello, Columbia University, USA; Fay Righthand, Wayne State University, USA; Bert Semler, University of California at Irvine, USA; and Eckard Wimmer, State University of New York at Stony Brook, USA). 

This article represents solely the opinions of the author, which are not necessarily shared by Columbia University or any institution or group named in the text.

 If you have comments or questions about this article (or anything else on PICO), please let us know.

 Posted on 8 August 1997