The last three years with COVID and the current monkeypox outbreak have dramatically illustrated that viruses are and will remain persistent dangers to human health. In addition to these new threats, the usual panoply of human viruses is ever present and continually spreading among susceptible individuals. The recent polio case in New York (an unvaccinated adult) and the subsequent discovery of poliovirus in wastewater samples make it urgent that Americans don’t neglect their polio vaccines. For some back history, poliovirus is spread via the oral-fecal route. Infected individuals shed copious amounts of virus in their feces. Shed virus can contaminate food, water, hands, and surfaces. Like viruses that cause diarrhea, poliovirus spreads easily among household members. It also spreads readily in daycares, nurseries, and schools where children are in close contact. When it is picked up and ingested by a new victim the virus replicates in the intestines. Most infected people either have no symptoms or have mild gastrointestinal symptoms such as vomiting or diarrhea. Sadly, in a small number of infected people, the virus leaves the digestive tract and enters the nervous system. These individuals develop a more serious illness that can progress to paralysis and death. There are no cures or effective treatments for polio cases, and vaccination to prevent disease is the only protection we have.
There are two types of poliovirus vaccines, the Salk and the Sabin. Jonas Salk developed an inactivated (“killed”) poliovirus vaccine (IPV) in 1955 that is given by injection. A few years later, Albert Sabin developed a live, attenuated (weakened) form of poliovirus that is given orally (OPV for oral polio vaccine). Both IPV and OPV are excellent vaccines although there are significant differences. The killed IPV vaccine prevents serious nervous system infections but does not prevent intestinal infection by poliovirus. People receiving this vaccine can still become infected and shed the virus in their feces where it can go on to infect others. So while IPV-vaccinated individuals are themselves protected, they can still be public health threats by spreading poliovirus. In contrast, the live OPV prevents intestinal infection so recipients of this vaccine don’t get sick or shed virus if they are exposed to poliovirus. Because of its ability to prevent community spread, OPV became the standard in the United States and completely eliminated polio in our country and many other nations around the world. However, at a very low but measurable rate (between 1 in 1,000,000 and 1 in 3,000,000 patients based on several studies conducted in different countries) the live OPV vaccine does revert to wild-type virulence and can cause paralytic disease. Because of this risk, the use of the OPV was discontinued in the United States in the year 2000 and we now only use the IPV.
Polio is strictly a human disease with no animal reservoirs. If we could eliminate all human cases, then poliovirus could be eradicated from the world as was done for smallpox. We are tantalizingly close to that goal because of the success of massive poliovirus vaccine campaigns in every nation. Poliovirus went from a terrifying global killer in the 1950s now to a disease with only a few hundred cases worldwide each year. And these cases are typically confined to a very limited set of distant countries so we rarely hear about them. Unfortunately, the absence of polio in most developed nations, including the United States, can make vaccine compliance difficult. Without the disease around, people forget about it and don’t see the urgency of the vaccination. However, polio can be reintroduced to the US at any time by travelers who contracted it elsewhere, so the risk has never really gone away. As the New York case demonstrates, anyone who isn’t vaccinated for poliovirus is still a potential victim. If you haven’t ever had a poliovirus vaccine, now would be a great time to get one. We can’t let public indifference reintroduce this horrid disease and we have to stay vaccine vigilant until we finally make this virus go extinct.