Several months ago, I wrote a Petri Dish column about the polio vaccine and efforts to eradicate the disease from the globe. I focused on the two types of vaccines: a live but weakened form of the virus developed by Albert Sabin and a killed form of the virus developed by Jonas Salk, both of which have been used to control polio. The live, orally delivered vaccine is the foundation of the eradication effort since it is easy to deliver, induces strong immunity in the gut and prevents virus transmission. While this live virus vaccine is the primary tool for polio eradication, on rare occasions it can revert to a virulent form, causing paralysis and other severe side effects. The killed vaccine is much less potent and less effective, but is far safer since it is not associated with severe side effects. The strategy for polio eradication has been to use the live vaccine as the primary approach to controlling polio in an endemic area and then switch to the killed vaccine to prevent polio reoccurrence.
This strategy has worked well, and polio is close to being the third infectious disease eliminated from the globe. But the recent reappearance of the polio virus in Israel has caused health officials to reconsider this strategy. Polio was essentially eradicated in Israel following the last major outbreak in 1988. To maintain this polio-free status, Israeli health authorities switched from the live to the inactivated polio vaccine in 1995, and vaccination rates with the killed vaccine are now as high as 95 percent of the population. However, routine sewage monitoring late last year revealed the presence of wild polio virus across the country and in the West Bank and Gaza, indicating widespread transmission. No actual cases of paralytic disease have been identified, presumably due to the high vaccination rates. But the presence of the virus in sewage raises the alarming specter of transmission to other countries, especially given the prolonged circulation over a large area.
How could this happen in a country with such a comprehensive vaccination campaign? A clue comes from the fact that there have not been any clinical cases of polio, despite widespread distribution of the virus. This suggests that there are individuals who are actively shedding the virus without succumbing to the disease. Indeed, Israeli medical authorities have now identified many individuals who were shedding poliovirus in their feces, despite having been fully vaccinated with the killed polio virus vaccine. It is becoming apparent that the re-emergence of the virus results from the lower efficacy of this killed vaccine. Although both the live and killed vaccines stop virus from entering the nervous system and thereby prevent the development of the polio disease, only the oral live vaccine is able to generate strong enough immunity to eliminate the virus from the gut. In other words, some individuals who received the killed vaccine and who are subsequently exposed to the polio virus can possess the ability to harbor the virus in their intestines. These “carriers” are actively excreting live virus, thereby explaining the appearance of virus in the Israeli sewage system. This highlights a potential weakness in the polio vaccine strategy; the exclusive use of the inactivated virus vaccine can actually hide transmission of the virus since it potentially facilitates development of asymptomatic carriers of the disease.
The experience of Israeli medical authorities demonstrates that polio continues to be a global threat. While the situation in Israel can be controlled by reimplementation of the live attenuated vaccine, considerable challenges remain in countries where vaccination rates are low. For example, polio has re-appeared in Syria where the disease had been considered eradicated for over a decade. Obviously, the civil war in that country poses considerable difficulties for the polio vaccination campaign, raising fears that there will be a widespread epidemic in the region. Sustained and relentless efforts by the global polio eradication initiative will be essential to keep a lid on this dreadful disease. An important step was Secretary of State John Kerry’s announcement on January 15 that the US will provide $380 million in additional humanitarian assistance to those affected by the war in Syria. Some of this funding is for childhood vaccination campaigns in the region. Let’s all hope that it’s successful.
David L. “Woody” Woodland, Ph.D. is the Chief Scientific Officer of Silverthorne-based Keystone Symposia on Molecular and Cellular Biology, a nonprofit dedicated to accelerating life science discovery by convening internationally renowned research conferences in Summit County and worldwide. Woody can be reached at 970-262-1230 ext. 131 or firstname.lastname@example.org.