Though polio has existed throughout history, the first mass outbreaks didn’t occur until the industrial era. Peaking in the Americas and Europe in the 1940’s and 1950’s, the highly contagious virus spread fear as it indiscriminately killed and paralyzed children. Then in 1955 the IPV (Inactive Polio Vaccine) developed by Dr. Salk, and in 1961 Dr. Sabin’s OPV (Oral Polio Vaccine) were first administered. Mass vaccination followed. By 1979 polio had been eradicated in the US. The Global Polio Eradication Initiative launched in 1988. By 2021 there was a 99.9% reduction in wild polio globally. So why are we still vulnerable?
Types of Polio
There is wild polio and vaccine derived polio. Wild polio is naturally occurring and has three types. Types two and three have been globally eradicated. Only wild polio type one is still endemic, and at this point it is only known to be in the border regions of Afghanistan and Pakistan.
Vaccine derived polio occurs in the rare instance when the weakened virus from a live vaccine finds a receptive host and lingers long enough to mutate and regain the characteristics of wild polio.
Types of Vaccines
The OPV is delivered orally and uses a weakened version of the live virus. Since the virus is live, the vaccinated person can experience what is called “viral shed” or “vaccine shed.” Vaccine shed can passively immunize people not directly reached by immunization programs. It can also, although rarely, cause a genetically modified strain of vaccine derived polio. This is of most risk when OPV is administered to immunocompromised individuals, or when there is incomplete vaccination of communities.
The IPV is injected and does not use any live virus so there is no viral shed, but it is less effective at addressing outbreaks. This conundrum has led to evolving guidance on vaccination strategy.
Low Vaccination Rates
As the world approaches total elimination of polio, low instances of infection make the disease seem less dangerous. But it is that perception that can allow polio to be reintroduced into communities where it has been eradicated.
Lack of prioritizing vaccination, vaccine skepticism, and political conflict has led to many countries falling short of the vaccination rate necessary to maintain herd immunity. The Covid-19 pandemic also interrupted vaccination programs, further decreasing coverage.
Just recently, officials detected the virus in UK wastewater samples. It has been so long since polio was considered a threat in much of the world, health experts are concerned doctors might not immediately identify symptoms in patients. Although no paralytic cases have been discovered, the virus’ presence in the UK is cause for concern and emphasizes the need to globally recommit to vaccination efforts to protect against a resurgence of this debilitating disease.