For the first time in nearly a decade, a case of polio has been confirmed in the United States. Health officials in Rockland County, New York discovered the disease in an unvaccinated 20-year-old in 1979, decades after polio was eradicated from the United States.
As the country and public health system grapples with Covid-19 and monkeypox, the news comes as an unpleasant surprise and raises immediate questions. How did this 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 poliomyelitis?
Poliomyelitis is an abbreviation of poliovirus, an enterovirus that infects the nervous system. Symptoms can range from flu-like (sore throat, fever, and fatigue), to meningitis and even a severe infection of the spinal cord leading to paralysis. But unlike the flu, poliovirus reproduces mainly in the gut, where it is mostly spread when people do not wash their hands after using the toilet. Polio is highly contagious, at least to the unvaccinated, especially in areas with poor sanitation and water security.
From the first documented outbreak in the United States in 1894 until the development of vaccines in the 1950s, polio was one of the most feared childhood diseases. Thousands of children became paralyzed every summer. Children under the age of 5 were the most vulnerable.
But those sacrifices were special; About three-quarters of patients infected with poliovirus show no symptoms. For most of the rest of the quarter, the illness is no more than flu-like symptoms. In about one in 25 patients, the virus spreads to the nervous system and causes meningitis. One in eight cases of meningitis, or about 0.5 percent of all polio cases, will cause permanent nerve damage and paralysis. There was and is no known cure, only supportive treatment, including iron lungs – replaced by advanced ventilators – and physical therapy.
The threat of polio changed forever when two vaccines were discovered in a short period of time: Dr. In 1955, Jonas Salk and the oral live attenuated vaccine, Dr. Albert Sabin in 1961. Both vaccines are highly effective, providing 99 percent immunity to infection. Sabin’s oral vaccine eventually became widely accepted in the United States, and in the 1960s and 1970s, the incidence of polio declined dramatically, and the wild virus was completely eradicated from the country.
The US was ahead of the curve—the global vaccination campaign began in earnest in 1988, just a few years after smallpox was declared eradicated in 1980. The US switched to a slightly safer, injectable vaccine in 2000, and vaccination is still recommended. All children are on the standard childhood vaccine schedule. Worldwide, thanks to ongoing public health efforts, hundreds of millions of children receive the oral vaccine each year, and the wild virus has been eliminated from all but a few countries.
Where did this work come from?
Since community transmission of polio was eliminated in the United States in 1980, all infections have come from other countries where the disease still exists. Genetic sequencing showed that the latest case was a strain of poliovirus from the vaccine. This means that the circulating virus is not from one of the few remaining pockets of endemic wild poliovirus, but rather from one of the many other countries with polio epidemics that have mutated from an oral, live-attenuated vaccine – in this case the non-vaccine USA.
Polio vaccines come in one of two types: inactivated or live attenuated. Live attenuated vaccines, such as the measles, mumps, and rubella combination vaccines offered to all American children, contain a modified, weakened strain of the pathogen that does not cause disease in humans but still triggers an immune response that protects against the original infection. strain. The oral vaccine used in the most dangerous countries is live attenuated. Inactivated vaccines, such as the polio vaccine currently used in the US, contain only dead virus material and may require several additional injections to stimulate the immune system sufficiently to provide long-term and complete immunity.
Although the live attenuated polio vaccine will never cause polio – except in at least one in a million cases where the child is immunocompromised – the fact that it contains live virus definitely carries some risk, inactivated vaccines do. When live polio vaccines are given in a community with a large proportion of unvaccinated people, the modified virus can be passed on to others, and as it passes from generation to generation, it can – very rarely – mutate back into a new strain of the virus. Public health efforts are essential to ensure that enough people are vaccinated to protect against the emergence of new strains derived from the wild virus and vaccine.
Ironically, the fact that most cases of polio are asymptomatic or mild—with an incubation period that can last up to 30 days before symptoms appear—makes tracing polio links and public health efforts especially difficult. The only way to suppress the virus is to achieve herd immunity, which requires vaccinating about 80 percent of the population against polio.
Who is at risk?
For most people in the United States, the newly discovered case of polio did not raise the alarm at all. The Rockland County Department of Public Health believes the patient is no longer contagious.
Polio can be detected in stool samples as well as in wastewater monitoring, which looks for evidence of viral genetic material in wastewater. On August 1, the New York State Department of Health said the polio case in Rockland was genetically linked to samples of the virus collected from sewage in Jerusalem and London, but the department emphasized that the results did not automatically mean the patient had visited either. location. The Rockland Public Health Department was able to use previously collected sewage samples to monitor for Covid-19 and found poliovirus there from June is genetically related to the present.
Given how common asymptomatic cases are and the long incubation period, there may be other unrecognized cases in the Rockland area. They can still be contagious, but it can’t spread as far. As of 2019, more than 90 percent of US children were fully vaccinated against polio on schedule, well above the threshold for herd immunity, and this rate has remained stable for decades. Infants 4 months or older usually receive two doses that provide 90 percent immunity.
However, immunization rates in Rockland County are lower than the rest of the country; it was a measles outbreak in 2018-2019, and currently only 60 percent of 2-year-old children there are fully vaccinated against polio, compared to the national average of 90 percent. The New York State Department of Health now encourages people who are unvaccinated, those who have not completed the polio vaccine series, and pregnant women to get vaccinated. In the month since the polio case was diagnosed, the Rockland Clinic has administered nearly 400 doses of the vaccine. People in the Rockland area who were vaccinated as children but are concerned they may have become ill should schedule a follow-up vaccination.
What does this mean for global eradication efforts?
While the US is protected against polio, the same cannot be said for some at-risk developing countries where the virus is still active.
After developing an oral vaccine, Sabin campaigned for a worldwide eradication effort in the 1960s, and in 1972 submitted all of his vaccine strains to the World Health Organization in hopes of reducing production costs. Despite recent efforts worldwide to introduce a slightly safer inactivated vaccine, most low- and middle-income countries still use the oral vaccine.
The Global Eradication Program has generally been a tremendous success, with total polio cases worldwide reduced by 99.99% since the program began in 1988. But the closer the elimination gets, the harder it will be to reach the finish line. When hundreds of millions of doses of oral vaccine are given each year, even the risk of developing a new vaccine strain increases. In African countries such as Nigeria and Yemen, most cases of polio have been reported from the vaccine. Disruptions in vaccination coverage due to military conflict and Covid-19 may increase the risk of uncontrolled spread of vaccine variants.
Despite the inherent risks of live attenuated vaccines, the oral vaccine has significant advantages for public health campaigns, particularly in developing countries. Each dose costs about 12 cents, while a single dose of the inactivated vaccine costs about $2, and because it is given as a sublingual drip, it does not require needles or trained professionals to administer. In general, live attenuated vaccines are more potent and provide longer-lasting immunity than inactivated vaccines.
And early on, the contagiousness of the oral vaccine strain was actually considered a plus, because children who were out of reach of health care workers could catch the weakened strains from others and develop immune systems. In theory, if a vaccination campaign reaches enough people in the community, the virus will spread until it can become virulent in humans.
A phase-out of the oral vaccine, which would eliminate the source of new poliomyelitis variants, would probably be necessary to achieve complete eradication, but replacement of the oral vaccine with the whole-blood vaccine needed to confer immunity is not yet possible. Even if funding and personnel were available, the global supply of inactivated vaccines is still too low to cover the hundreds of millions of children at risk.
With monkeypox recently declared a public health emergency of international concern by the World Health Organization and the threat of a future pandemic on the horizon, the global effort against polio is more important than ever to ensure that polio never happens again. global threat. Maintaining and ideally increasing vaccination rates in the US will protect the US nation and help eradicate polio worldwide by denying its roots.