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. 

Polio Re-enters Equatorial Guinea

The eradication of the poliovirus suffered another setback. In the past, I have discussed the number of countries still harboring the virus and the threat that spill over events pose to neighboring nations. 

So in recent weeks, Equatorial Guinea, a country that hasn't had a polio case in over a decade has reported one.

The sequence of the virus isolated indicates it came from Cameroon (where 3 cases have occurred in 2014). The threat of these spillovers is reinforced because the Cameroon cases which began in 2013 were the result of spillover from cases in Chad which were themselves spillovers from Nigeria, where the disease remains. 

So, the current scoreboard is 37 cases--a case count higher versus this time last year--in 5 countries. The Big Three: Afghanistan, Pakistan, and Nigeria remain the heads of the polio hydra that urgently need decapitation.

Polio Wars: Dying for Vaccination in Pakistan

I discussed, in a prior post, the progress of the global polio eradication effort. Last year marked a major setback in the eradication effort as 9 countries reported 389 cases. Though not even a month old, 2014 has already seen 4 cases occur--all of which are located in the North Waziristan region of Pakistan. 

It is no accident that Pakistan remains the biggest obstacle to polio eradication given the continued murderous violence Taliban Islamists direct at polio vaccinators. Since July of 2012, 33 individuals associated with the vaccination effort have been killed. 

The contrast between those who are working to improve human life by working to eradicate one of mankind's scourges from the planet and those who seek to relegate mankind to a state in which polio is allowed to spread with abandon couldn't be starker. 

2013 Eradication Final Score: Polio 8 countries, Guinea Worm 4

Since I work on a daily basis with DA Henderson, the man who led the only successful effort to eradicate smallpox from the planet (see his excellent book), the eradication of other infectious diseases are always something I track. 

Since smallpox, only the cattle disease rinderpest has been eradicated. 

Two major eradication efforts are currently underway with varying degrees of success. One is focused on polio, the other on guinea worm disease. Polio remains in 8 countries; guinea worm disease (dracunculiasis) in 4.

To eradicate polio is actually a tripartite task,  as the disease is caused by three types of poliovirus. Poliovirus type 2 was eliminated in 1999 and type 3 is likely on the verge of eradication. Type I is a different story, however, and has proven difficult to extinguish and has been abetted by social and political developments conducive to its spread. 

In 2013, polio cases increased by approximately 62% (from 2012) largely as a result of importation of cases to countries from which it had been previously driven out (Cameroon, Kenya, Somalia, Ethiopia, and Syria).

Polio remains endemic in 3 countries: Afghanistan, Pakistan, and Nigeria. In Afghanistan and Pakistan there have been several reports of violence directed against vaccinators by the Taliban. There is also an excellent book on the topic of polio in Pakistan detailing the structural problems involved there.

Guinea worm disease, on the other hand, experienced a 73% decline in cases from 2012 and remains in South Sudan, Ethiopia, Mali, and Chad.

As the new year unfolds, it will be fascinating to track the progress--and setbacks--of these two programs. For a good overview of eradication in general, see this book