With the recent report of detection of poliovirus in wastewater samples from the New York City metropolitan area, many people want to understand what is happening, why it is happening, and what it means. However, in order to fully understand what is occurring it is necessary to understand some salient aspects of the history of polio and polio vaccination.
Polio is a fecal-oral spread virus that once was ubiquitous. While the thought of polio may conjure images of iron lungs and leg braces, most individuals infected with the virus never develop any symptoms — it is estimated that roughly one in every 200 infections led to the dreadful paralytic complications that could sometimes be fatal.
The successful efforts of the March of Dimes led to the development of a safe and effective polio vaccine by Jonas Salk, MD, in the 1950s, right in my hometown of Pittsburgh. Salk and his vaccine — which received the offer of a ticker tape parade (imagine a vaccine getting that today!) — were instrumental in reducing the burden of paralytic polio in the U.S. His injectable inactivated vaccine was soon followed by an attenuated oral vaccine developed by Albert Sabin, MD.
Unraveling what is occurring today in New York, Israel, London, and other places requires understanding the differences between these two life-saving vaccines. The Salk vaccine is a standard inactivated viral vaccine that is injected, while the Sabin vaccine contains a replicating altered poliovirus that is given orally, often on a sugar cube. Not only is the Sabin vaccine cheaper and easier to administer — thanks to the lack of need for a syringe, a needle, or a trained person to inject — its very nature was also considered advantageous. Polio is a fecal-oral pathogen and the Sabin vaccine, as an oral vaccine, more naturally mimicked the natural route of infection and therefore triggers different and more potent immune responses than the injectable Salk vaccine. Also, because it is a “live” vaccine, the vaccine virus is shed and can passively immunize close contacts of vaccinated individuals. For these reasons, the Sabin vaccine eventually came to be a more dominant and preferred vaccine than the Salk version.
However, as polio receded as a public health threat, rare complications elicited by the Sabin vaccine became intolerable in many Western countries. The “live” nature of Sabin’s vaccine, in rare instances, could lead to vaccine-associated paralytic polio. In even rarer circumstances, the virus could mutate or recombine with other viruses to become a circulating vaccine-derived poliovirus (cVDPV) that could be capable of infecting others, and in some cases, causing paralysis. This is why 2 decades ago — with polio no longer an issue domestically — the U.S. switched to an all-Salk injectable polio vaccine regimen for children, away from a combination that used the Sabin oral polio vaccine (OPV) and the injectable polio vaccine. But many countries, especially those in which wild polio still circulates, still use OPV.
Currently, wild polio is known only to circulate in Afghanistan, Pakistan, and Mozambique and, in my analysis, should be considered a problem separate from what is occurring in New York and other areas due to cVDPVs. Although many press reports conflate wild and vaccine-derived polio, they are different issues.
Therefore, so long as OPV is being utilized there will be a risk of cVDPVs causing illness in places where there are unvaccinated individuals.
Indeed, the strains that have been uncovered in the New York area bare resemblance to vaccine-derived strains circulating in Israel, pointing to the global nature of this problem. It is crucial to emphasize that cVDPVs only becomes a significant problem if there are non-immune individuals for them to infect. If vaccination rates in Israel, London, or New York City were sufficient, cVDPV would not pose a major risk. As such, the risk of cVDPVs is man-made when people don’t vaccinate sufficiently. Based on the level of circulation estimated, there will likely be more paralytic cases in unvaccinated individuals, though large numbers are unlikely based on high vaccination rates.
In response, healthcare professionals and policymakers need to encourage widespread immunization in areas in which cVDPVs have been found. It is not surprising that this problem has been detected in the same locales in which a very large measles outbreak recently occurred. I suspect other areas likely have cVDPVs circulating as well, and broader wastewater surveillance coupled with targeted catch up immunization will be critical. I can’t stress this point enough: high vaccination rates are required for resiliency. I fear that the anti-vaccine movement’s inroads have had real consequences.
Amesh Adalja, MD, is a senior scholar at the Johns Hopkins Center for Health Security and a practicing infectious disease, critical care, and emergency physician in Pittsburgh.
Adalja is a shareholder of Merck, a manufacturer of certain polio vaccines.