Understanding Vaccine-Derived Polio

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Administration of an oral polio vaccine to a young child
Photo Credit to the RIBI Image Library, Creative Commons License.

A rare, mutated form of poliovirus has caused paralysis in 2 children in the Ukraine. Several articles discuss what happened but it’s important to also understand the implications.

First, the polio vaccine currently used in the US and most developed
countries is the inactivated polio vaccine (IPV); the use of oral polio vaccine (OPV) was discontinued in the US in 2000. It is not possible for IPV to cause paralytic poliomyelitis (the debilitating disease caused by poliovirus). It is not possible for IPV to shed and infect another person. It is not possible for IPV to mutate into a disease-causing virus strain. OPV, on the other hand, while highly effective in producing immunity to poliovirus, is a live, weakened poliovirus (actually a combo of 3 poliovirus strains, but that is neither here nor there) that can, in rare cases (1 in every 2.7 million first doses), cause vaccine-associated paralytic polio in a recipient (thought to be triggered by immune deficiency), and also very rarely and over time, mutate into a disease-causing virus similar to wild-type poliovirus.

In the recent cases of polio in the Ukraine, while it is true that the strain of poliovirus that caused disease was a mutated form of the OPV strain, it is also true that the cases of polio disease would not have occurred if there was a high enough vaccination rate in the community. Poliovirus is an RNA virus, and RNA is highly prone to errors while replicating, thus causing mutation. As a comparison, when DNA is replicated, there is an error about 1 per billion nucleotides, whereas RNA replication makes a mistake in about 1 per 10,000. This actually corresponds to an average of 1 mutation per whole poliovirus that is replicated. Mutations are random, so sometimes a mutation will change nothing in how the virus acts, sometimes it will make the virus stronger, sometimes it will make the virus weaker. However, the more chances that a virus gets to replicate, the more chances it gets to “win” in the evolutionary game… it will have more of a chance to hit that set of mutations to make it replicate faster and cause disease.

Let’s travel the route of an OPV virus in 2 populations. The first is a community such as the Ukraine, where the vaccination rate is a low 50%. A person receives OPV, the vaccine strain virus replicates slowly in the gut over several weeks while the person produces an immune response, which will then eliminate the vaccine strain virus well before it can do any harm. However, during this time, the vaccine recipient perhaps did not do a great job washing hands after using the bathroom and spreads the vaccine strain virus to 10 people close to them, 5 of whom are unvaccinated. This is actually not a bad thing as now these 5 unvaccinated people receive “passive” immunization and they now produce antibodies to poliovirus, although the vaccine strain virus also slowly replicates in their gut while the person mounts an immune response… which then spreads to other unvaccinated people in close contact, and so on. This can occur silently over many years. However, all this time, the vaccine virus is getting a chance to replicate further, which means more chances of hitting the lottery and mutating into a form that can cause harm just like wild-type poliovirus. Another way to look at it is like playing the game telephone, where you have a long line of kids and one person whispers something to the first person, who then whispers it to the next person, and so on. The longer the message goes down the line, the more chances for the message to get distorted.

Now, let’s travel the route of OPV in a population with a high vaccination rate. In this case, a person receives OPV, the vaccine strain virus replicates slowly in the gut, and even though this person exercised poor hygiene, all the close contacts around the person have been vaccinated, so when they are exposed to the vaccine strain virus, their vaccine-primed immune systems attack the virus before it gets a chance to replicate and be further spread. So in this scenario, rather than allowing a message to get distorted along the way by playing telephone, it’s like everyone got the message first-hand, from the source, so it’s in the original form.

The U.S. and other developed countries can rest easy that it isn’t possible for IPV to mutate into a disease-causing form. However, poliovirus, whether wild-type or OPV-derived, is only a plane ride away, which means it is very important that we continue to vaccinate here in the U.S. In addition, there have been rare cases of long-term shedding of live poliovirus in immune deficient recipients of OPV, and since OPV was still in use up to year 2000, the possibility still exists for exposure to vaccine-derived poliovirus within the U.S. The elimination of wild-type poliovirus from the U.S. has led to a false sense of security where there are now community pockets with low vaccination rates against polio. And since IPV doesn’t have the “passive” immunization benefits of OPV, only those who are vaccinated are protected if exposed to disease-causing poliovirus.

Further Reading:

The WHO Fact Sheet on Polio

Q&A on Vaccine-Derived Polio

CDC Page on Polio