Where the Wild Things Are: Polio and Vaccine-Derived Polio are Distinct Issues

One of my major pet peeves with infectious disease reporting is the conflation of polio with vaccine-derived polio. While both conditions can be severe and paralyzing, there's an important distinction that is missing from headlines announcing the "return of polio." 

Vaccine-derived polio is a known and expected risk so long as the Sabin oral polio vaccine is used. The Sabin oral polio vaccine has many advantages that have favored its use: it's given orally (no needles), it's cheaper, and it's "live". This last is important since the vaccine is given orally and replicates in the GI tract -- just like the untamed wild strain of polio -- it more closely mimics natural infection. It is also shed in the stool and others are, in effect, vaccinated upon exposure. On the contrary, the injectable Salk vaccine does not prevent viral spread as the vaccinated are protected against paralytic polio but are still able to be infected with the wild virus, but only in their intestine, and are able to pass the virus along.


However, these advantages are a double-edged sword as the virus, which has been weakened, can mutate its way back to its original virulence level and paralyze someone (vaccine-associated paralytic polio, VAPP) In rare circumstances, the altered vaccine virus can circulate and cause outbreaks as a circulating vaccine-derived poliovirus (cVDPV) -- this last usually requires recombination with a non-polio enterovirus. Usually the impact of cVDPVs is delimited because of population immunity. This paper provides a great overview of the phenomenon. 

The risk of VAPP, in the US, became too much to bear and the US slowly changed recommendations and moved to an all injectable Salk regimen several years ago. The global eradication program will eventually phase out the oral vaccine as well.

What is missed by the recent headlines is the fact that so long as oral polio vaccine is used there will always be a risk of VAPP and the emergence of cVDPVs. I think that the eradication of polio should be restricted to the eradication of the wild virus -- something my mentor and smallpox eradicator DA Henderson insisted upon. cVDPVs and wild polio are distinct problems. Wild polio continues to spread in only two countries: Pakistan and Afghanistan, where a dozen cases have occurred so far this year. cVDPV outbreaks in the DRC, Somalia, and Papua New Guinea are important problems but should not, in my opinion, be considered on the same level as wild polio virus infections.