For the first time in nearly a decade, a case of poliomyelitis has been confirmed in the United States. Health officials in New York’s Rockland County discovered the case last month in an unvaccinated 20-year-old, decades after polio was eliminated in the United States in 1979.
With the country and the public health system already grappling with the brunt of Covid-19 and monkeypox, this news comes as an unpleasant surprise and instantly raises questions. How did it happen? Who else is at risk? What does it mean that the Rockland case was a vaccine-derived strain, and what are the implications for global efforts to completely eradicate polio?
What is polio?
Poliomyelitis, short for poliomyelitis, is caused by poliovirus, an enterovirus that can infect the nervous system. Symptoms can range from those similar to the flu (sore throat, fever and fatigue) to a more serious spinal cord infection causing meningitis and even paralysis. But unlike the flu, the poliovirus multiplies mainly in the intestines, and it spreads when people don’t wash their hands after using the toilet. Polio is highly contagious, at least to unvaccinated people, especially in areas with poor sanitation and water security.
From the first documented epidemic in the United States in 1894 until the development of vaccines in the 1950s, poliomyelitis was one of the most feared childhood diseases. Thousands of children have been left paralyzed with each summer outbreak. The most vulnerable were children under 5 years old.
But these victims were the exception; three-quarters of patients infected with poliovirus show no symptoms. For most of the remaining quarter, the disease never progresses beyond flu-like symptoms. In about one in 25 patients, however, the virus spreads to the nervous system and causes meningitis. About one in eight cases of meningitis – or about 0.5% of all polio cases – will have permanent nerve damage that leaves them paralyzed. There was and is no known cure, only supportive treatments including iron lung – since replaced by more advanced ventilators – and physical therapy.
The threat of poliomyelitis changed permanently when two vaccines were discovered in short succession: a vaccine injected and inactivated by Dr. Jonas Salk in 1955, and a live attenuated vaccine, taken orally, by Dr. Albert Sabin in 1961. Both vaccines are highly effective, granting 99 percent immunity to infection. Sabin’s oral vaccine was eventually widely adopted in the United States, and polio cases dropped drastically in the 1960s and 1970s, until the wild virus was entirely eradicated from the country.
The E. all children on the standard childhood immunization schedule. Worldwide, thanks to ongoing public health efforts, hundreds of millions of children receive the oral vaccine each year, and the original wild virus has been driven out of almost every country.
Where does this case come from?
Since community spread of polio was eliminated from the United States around 1980, all infections have come from other countries that still have the disease. Genetic sequencing shows the recent case was a vaccine-derived strain of poliovirus. This means that the circulating virus is not from one of the few remaining pockets of endemic wild poliovirus, but rather from one of many other countries where outbreaks of polio have mutated from a live attenuated oral vaccine – which is not the vaccine currently used. in the USA.
Polio vaccines fall into one of two types: inactivated or live attenuated. Live-attenuated vaccines, such as the combined measles, mumps, and rubella vaccine recommended for all U.S. children, contain a modified, weakened strain of a pathogen that does not cause disease in humans but still triggers an immune response that protects against the original strain. The oral vaccine used in the countries most at risk is live attenuated. Inactivated vaccines, like the polio vaccine currently used in the United States, contain only dead viral material and may require a longer series of boosters to stimulate the immune system enough to ensure long-lasting and complete immunity.
Although the live attenuated polio vaccine almost never causes polio itself – except in less than one in a million cases when a child is severely immunocompromised – the fact that it contains live virus inevitably carries some risk, unlike inactivated vaccines. When live attenuated polio vaccines are given in a community that contains a high fraction of unvaccinated people, the modified virus can infect other people and, with enough generations of spread, it can – very rarely – turn into a new one. virulent strain. It is essential for public health efforts to ensure that enough people are vaccinated, to protect against both the wild virus and the possibility of new vaccine-derived strains.
Ironically, the fact that most polio cases are asymptomatic or mild – along with an incubation period that can take up to 30 days before symptoms appear – makes polio particularly challenging for contact tracing and public health containment efforts. The only way to keep the virus suppressed is to achieve herd immunity, which for polio requires vaccinating about 80% of the population.
Who is at risk?
For most people in the United States, the newly discovered polio case did not increase risk at all. The Rockland County Public Health Department believes the patient is no longer infectious.
Poliovirus can be detected in stool samples, as well as in sewage monitoring, which looks for evidence of viral genetic material in sewage. On August 1, the New York State Health Department reported that the Rockland polio case was genetically linked to samples of the virus taken from sewage in Jerusalem and London, although the department pointed out that the results did not automatically imply that the patient had traveled to either location. The Rockland Public Health Department was able to use sewage samples collected earlier for Covid-19 surveillance and found poliovirus in samples there of June that are genetically linked to the current case.
Given the frequency of asymptomatic cases and the long incubation period, it is possible that there are additional unrecognized cases in the Rockland area. These can still be contagious, but they are unlikely to spread far. In 2019, more than 90% of American children were fully immunized against polio on time, well above the herd immunity threshold, and this figure has remained stable for decades. Infants 4 months or older will usually have received two doses, which already provides 90% immunity.
Rockland County, however, has a lower vaccination rate than the rest of the country; it was the site of a measles outbreak in 2018-19, and currently only 60% of 2-year-olds there are fully immunized against polio, compared to a national average of 90%. The New York State Department of Health is now urging all unvaccinated people, those who have not completed their series of polio vaccines and pregnant women to get vaccinated. In the month following the discovery of the poliomyelitis case, the Rockland clinic administered nearly 400 doses of the vaccine. People in the Rockland area who were vaccinated as children but fear they may have been exposed should schedule a booster shot.
What does this mean for the global eradication effort?
While the United States remains polio-protected, the same cannot be said for some higher-risk developing countries where the virus is still active.
After his work developing the oral vaccine, Sabin campaigned for a global eradication effort in the 1960s, and in 1972 donated all of his vaccine strains to the World Health Organization in hopes of reduce manufacturing costs. Despite recent efforts to introduce the slightly safer inactivated vaccine globally, most low- and middle-income countries still use the oral vaccine.
The global eradication program has been a huge success overall, with the total number of polio cases worldwide down by more than 99.99% since the program began in 1988. But the closer eradication gets , the more difficult it becomes to reach the finish line. When hundreds of millions of doses of oral vaccine are administered each year, even the very small risk that one dose will spawn a new vaccine-derived strain adds up. Most polio cases that have been detected in African countries like Nigeria and Yemen are of vaccine origin. Interruptions in vaccine coverage due to military conflicts and the Covid-19 pandemic have likely increased the risk of uncontrolled spread of vaccine-derived variants.
Despite the inherent risks of live attenuated vaccines, the oral vaccine has significant advantages, especially for public health campaigns in developing countries. Each dose costs as little as 12 cents, compared to about $2 per dose for the inactivated vaccine, and because it’s given as drops under the tongue, it doesn’t require needles or trained professionals to administer. In general, live attenuated vaccines also confer stronger and longer lasting immunity than inactivated vaccines.
And at first, the infectivity of the oral vaccine strain was actually seen as a plus, because children unreached by health workers could potentially pick up the weakened strain from others, ending up immune. In theory, as long as the vaccination campaign reached enough people in the community, the spread would die down long before the virus had a chance to become virulent again in humans.
Phasing out the oral vaccine, which would eliminate the source of new polio variants, will likely be necessary to achieve complete eradication, but replacing the oral vaccine with the full schedule of booster shots needed to grant immunity will not is not yet possible. Even if funding and personnel were available, the total global supply of inactivated vaccines is far too small to cover the hundreds of millions of children still at risk.