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Given the recent headlines, you may be wondering why polio is even an issue in 2022. For more than 60 years, poliovirus vaccines have protected virtually everyone in the United States from the disease. Polio was considered eradicated in the United States in 1979 due to a tremendously successful polio vaccination campaign that began in the 1950s when the first polio vaccines became available
Unfortunately, even today there are communities in the United States where polio vaccination rates are lower than needed. Because many people have not been vaccinated, there is now a real possibility of a resurgence in polio in the United States
As a Clinical Professor of Pharmacy, I train future pharmacists on how vaccines work, how important they are and how they prevent disease.
Polio cases are rising
Longstanding concerns by public health experts about falling vaccination rates surfaced when a man from Rockland County, New York was diagnosed with polio in July 2022, the first such diagnosis in the United States in almost a decade. The patient – who developed the severe, paralytic form of the disease – had been exposed to an altered live vaccine strain from overseas.
Then, on September 9, 2022, New York declared a state of emergency over continued transmission of the poliovirus. As of that date, officials using wastewater monitoring had identified 57 samples of the poliovirus in wastewater from four New York boroughs. More than half of these were discovered in the same county where the adult patient is from, just outside of New York City.
As a result of the continued presence of poliovirus in wastewater, the Centers for Disease Control and Prevention said the US now meets World Health Organization criteria for “a country with circulating vaccine-derived poliovirus.”
poliovirus vaccines
Two types of polio vaccine are used worldwide today. The inactivated poliovirus vaccine is given as a syringe, and the oral attenuated (or weakened) poliovirus vaccine is given as oral drops, sometimes on a sugar cube.
Since 2000, the US has exclusively used the inactivated poliovirus vaccine, which cannot cause disease because it contains no live virus. But in countries where the poliovirus continues to circulate, such as Pakistan and Afghanistan, the oral attenuated poliovirus vaccine is still used.
In extremely rare cases, the live attenuated vaccine used in other countries can mutate back into its virulent form and cause paralysis. The adult in New York is said to have come into contact with the virus.
How “herd immunity” for polio is determined
The degree to which a community is protected from a pathogen such as poliovirus depends on herd immunity. When a community — known as the herd — reaches a threshold of immunity, it can prevent human-to-human transmission of a pathogen, thereby suppressing the pathogen.
The herd immunity threshold for a given disease is calculated based on the expected number of people that an infected person would spread if they were susceptible. The higher the number of people who would become infected, the higher the percentage of the community or population that needs to be vaccinated to prevent the disease from spreading further.
For the poliovirus, the researchers estimate that between five and seven people would be infected in each case if those people were susceptible. Based on these calculations, experts determined that at least 80% of a community or population should be vaccinated against the poliovirus to prevent its spread.
Polio vaccination rates vary widely in the United States.
The CDC recommends that infants and young children receive a four-dose schedule of inactivated poliovirus vaccine at two months, four months, 6 to 18 months, and 4 to 6 years of age. For those who start vaccination later – as older children, adolescents or adults – three doses are considered a full vaccination. This is because three doses of an inactivated poliovirus vaccine have been shown to provide between 99% and 100% protection against serious illness.
Although all U.S. states are currently above the 80% herd immunity threshold for poliovirus, there are areas within the country that are home to many unvaccinated or undervaccinated individuals – those who have not received a total of three lifetime doses of the inactivated vaccine.
New York state, for example, has one of the highest preschool polio immunization rates in the country, with 97.9% of preschool children vaccinated in the 2020-2021 school year. However, recent estimates from the New York State Department of Public Health suggest that only 79% of 2-year-olds in that state have received three doses of the polio vaccine.
Additionally, in certain pockets and counties of New York, such as Rockland, Orange, and Sullivan, three-dose vaccination rates can be far lower based on the 2-year-old age group, which is the only data available by county: Rockland 60 percent, Orange 59 percent, and Sullivan 62 percent. In fact, 46 percent of New York counties are below the herd immunity threshold for 3-dose poliovirus shots for 2-year-olds.
Who should get the vaccine?
When children are more than a month behind on the recommended polio shot, doctors recommend routine catch-up shots throughout adolescence.
Since most US adults today were vaccinated as children, and the US had eradicated polio starting in 1979, there was little reason for health experts to believe that an adult in the US would be exposed to the poliovirus Adult immunization schedule included.
But in August 2022, the CDC updated its guidance. Because there are communities where poliovirus vaccination rates have fallen below the 80 percent threshold required for herd immunity, in conjunction with the continued spread of poliovirus in New York, the CDC is now recommending that all unvaccinated or undervaccinated adults in those communities get a poliovirus vaccine.
In addition, the CDC suggests that some fully vaccinated adults who are at increased risk of exposure could benefit from a one-time lifetime poliovirus booster dose. This includes healthcare providers caring for people with poliovirus or people traveling to areas where poliovirus has not been eliminated.
If you are unsure whether you need a vaccine or what steps to take, talk to your pharmacist or doctor.
This article was originally published on The conversation by Jennifer Girotto at the University of Connecticut. Read the original article here.
