Who would have thought we would need to refresh our knowledge on polio virus in 2022? Fate seems cruel to add this concern on the heels of SARS-CoV-2, monkeypox, abnormal seasons for RSV, acute flaccid myelitis (AFM) linked to enteroviruses, and a summer of parechovirus causing infant meningitis. But confirmation that indeed an adult had polio with paralytic disease raises concerns among public health groups and ordinary citizens alike, particularly those who remember polio in its heyday.
History: In the summer of 1952, polio was among the most feared diseases on the planet. Families were advised to not allow children to congregate in groups or use public swimming pools; little league baseball games were being canceled and there was talk of not opening schools for the fall. Every parent’s nightmare seemed to be the nonspecific febrile summer illness that led to paralytic sequelae. TV news included videos of the iron lung wards in hospitals across the country. Medical providers felt powerless, only able to give nonspecific preventive advice. There was no specific antiviral (there still isn’t) and vaccines seemed a long way off.
Then came the news that Dr. Jonas Salk’s group had gotten an inactivated polio vaccine (IPV) approved for general use in 1955. Families were excited to have their children vaccinated. Paralytic polio cases dropped like a rock from approximately 22,000/year in 1952 to approximately 2,200 in 1956. A surge to near 6,000 cases in 1959 led to Dr. Albert Sabin’s oral polio vaccine (OPV), which supplanted IPV in 1961. OPV had the advantages of: 1) Inducing mucosal as well as serum antibodies, 2) more durable responses, and 3) immunity in unvaccinated persons exposed to vaccine virus that had been shed in stools into wastewater and rivers.
By 1964, polio had nearly disappeared. The last wild-type indigenous U.S. case was in 1979. By 1994, all the Americas were declared polio free. Because the only U.S. paralytic polio cases thereafter were foreign imports or were associated with oral vaccine strains (so-called vaccine-associated paralytic polio [VAPP]), OPV was replaced by an enhanced IPV in 2000 to prevent further VAPP.
Polio facts: Polio is asymptomatic in about 70% of infections. Among the 30% with symptoms, paralysis occurs infrequently, with the overall rate of paralytic infections being 0.5% (rate varies by virus type with type 3 having the highest rate).1 Why then was the world so afraid of polio? If every person in a U.S. birth cohort (about 3.7 million) was unvaccinated and became infected with poliovirus, more than 18,000 would get paralytic polio and almost 1,300 would die. Of note, adults have a higher chance of paralytic polio after infection than children.
Concerns in 2022: Persons vaccinated with at least three doses of either IPV or OPV have historically been protected from paralytic polio (99% protection). But are we sure that the United States remains protected against polio after 2 decades of IPV being the only vaccine? Polio could be reintroduced at any time to the United States from countries with reported cases that likely arose because of low vaccination rates related to war, famine, or political upheavals (Malawi, Mozambique, Nigeria, Pakistan, and Afghanistan).2 The proof? The recent confirmed New York case.
International efforts resulted in global eradication of two polio wild-types viruses (type 2 in 2015 and type 3 in 2019). Nevertheless, vaccine-derived, virulent polio virus (VDPV) type 2 and VDPV-3 still circulate in some areas, particularly Africa (VDPV-2) and Israel (VDPV-3). The above-mentioned U.S. case is an unvaccinated adult traveler who went to an area where VDPV-2 circulates and developed disease after returning home.3 So, it was not an indigenous reappearance in the United States and it was not a breakthrough case in a vaccinated person. But it is sobering to realize that all who are unvaccinated remain at risk for paralytic polio in 2022, particularly because vaccination rates declined nearly everywhere during the initial COVID-19 pandemic. We are still catching up, with vaccination rates under 50% in some ZIP codes.4
Are VDPVs circulating in some parts of the United States? Interestingly, wastewater surveillance programs may be the most economical and practical way to perform polio surveillance. Such a program detected polio virus in London wastewater in June 2022.5 New York has recently detected polio in wastewater during testing begun because of the recent case.6
Good news: For paralytic polio, seropositivity at any titer indicates protection, so U.S. serosurveillance data would also be informative. How durable is polio protection in the IPV era? Available data suggest that even though we have used only IPV these past 20 years, seropositivity rates among vaccinees with at least three doses of either IPV or OPV should persist for decades and likely for life. Even before polio became a concern this year, the Centers for Disease Control and Prevention, being proactive, wanted to ensure that the enhanced IPV was producing durable immunity and that persons of all ages remained seropositive to the three polio virus types over 10 years after discontinuing OPV use in 2012.
The CDC collaborated with investigators in Kansas City, Mo., to evaluate titers and seropositivity to all three types in a 2- to 85-year-old otherwise healthy cohort with demographics that mirrored the 2010 census for the Kansas City region, which in turn mirrored the national 2021 census data.7 There were approximately 100 persons in each age cohort, with 200 below age 11 years (the cohort that had received only IPV). Serology was performed at the CDC.
Overall seropositivity rates were high, but lower for type 3 (83.3%) and type 2 (90.7%) than type 1 (94.4%). Of note, most of those seronegative for one or more types were among 2- to 3-year-olds who had not completed their full IPV series, with most seronegative results being against polio types 1 and 3. Further, five, who were confirmed as having received no polio vaccine, were seronegative for all three types. Two with no available vaccine records (over 18 years old) were also seronegative for all three types.
So, regardless of the era in which one got polio vaccine, vaccine protection appears to persist indefinitely after three doses. Even 80-year-olds were still seropositive if they had three doses. We can confidently reassure our patients that the vaccine still works; the persons who need to fear polio in 2022 are those who are not vaccinated or have had fewer than three doses, particularly if they travel to areas of persistent polio. Wild type 1 virus persists in a few countries as does VDPV type 2 and VDPV type 3. Importantly, wild type 2 and wild type 3 (with the lowest seropositivity in 2012 study) have been eliminated globally so the only circulating type 2 and type 3 polio virus is VDPV in a few countries. Travel to these countries warrants review of polio vaccine records and CDC or WHO current recommendations for travelers to those countries.
Dr. Harrison is a professor, University of Missouri Kansas City School of Medicine, department of medicine, infectious diseases section, Kansas City. Email him at.
2. Franco-Paredes C et al.. doi: 10.1016/S1473-3099(22)00548-5.
3. Link-Gelles R et al. MMWR Morb Mortal Wkly Rep..
6. “NYSDOH and NYCDOHMH wastewater monitoring identifies polio in New York City and urges unvaccinated New Yorkers to get vaccinated now,” nyc.gov. 2022 Aug 12.
7. Wallace GS et al..