With only one case of paralytic polio found in Rockland County, New York, and the virus found in wastewater samples from two counties in the state as well as New York City, health officials believe polio is likely circulating undetected.

This is especially dangerous in unvaccinated communities because polio can cause severe symptoms such as meningitis, permanent paralysis, and even death.

“For every case of paralytic polio identified, hundreds more may go undetected,” said New York Health Commissioner Dr. Mary Bassett earlier this month in a statement. “Safe and effective immunization is the best way to keep adults and children polio-free.”

The Centers for Disease Control and Prevention recommends that children receive four doses of the polio vaccine, one at two months, four at four months, six at six months, and six at six years old.

Currently, every state and the District of Columbia require children to have received at least three or all four doses of the vaccine before enrolling in kindergarten.

Despite the fact that the CDC reports that nearly 94% of kindergarten-age students in the United States received the polio vaccine for the 2020-2021 school year, rates vary by state.

Mississippi has the highest rate in the country, with 98.9% of kindergarten students immunized against polio. Rounding out the top five are Louisiana; New York, excluding New York City; Nebraska; and Rhode Island.

Meanwhile, Washington, D.C. has the lowest rate, with only 80.4% of students immunized before the start of the school year.

Idaho has the second-lowest rate at 86.6%, followed by Wisconsin, Hawaii, and Georgia, all of which have rates below 90%.

“Where people are not optimally vaccinated, where there are a lot of unvaccinated people, that’s what you need for outbreaks to start and to persist in our population,” Dr. Adam Ratner, director of pediatric infectious diseases at NYU Langone Health, told ABC News. “And it doesn’t have to be at the national, state, or even county level. Small pockets of people may be unable to handle an outbreak and keep it going.”

According to experts, there are many factors contributing to low vaccination rates, but one of the most significant is the COVID-19 pandemic.

They explained that vaccinations were missed because parents did not take their children to routine appointments, and because children were not physically present in classrooms, enforcement of vaccinations required to attend school became lax.

According to the CDC, an average of 16,000 paralytic polio cases and 1,800 polio deaths were reported each year between 1951 and 1954. The first polio vaccine became widely available in 1955.

Cases gradually decreased from less than 1,000 per year to less than 100 per year. Polio had been declared eradicated in the Americas by 1994. People don’t remember a time when polio was common, according to experts, because the disease is so rare now.

According to the experts, exemptions are likely another reason for the low rates.

Children can be exempted from vaccination in all 50 states for medical reasons, such as being allergic to a vaccine component or having a weakened immune system that would make receiving a vaccine harmful.

Non-medical exemptions, such as religious or philosophical or personal beliefs, are permitted in some states.

In Idaho, 8.2% of kindergartners had an exemption from one or more vaccines for the 2020-2021 school year, mostly for non-medical reasons. Similarly, according to CDC data, 5.2% of Wisconsin kindergarteners were exempt from one or more vaccines.

Exemptions for “religious, philosophical, or conscientious reasons” are not permitted in Mississippi, according to the Mississippi State Department of Health.

According to CDC data, only 0.1% of Mississippi kindergartners had exemptions for one or more vaccines last school year, and only for medical reasons.

Similarly, non-medical exemptions for required school vaccinations, including the polio vaccine, were eliminated in New York state.

As a result, according to CDC data, the Empire State had only 0.1% of kindergarten-age students who were exempt for medical reasons.

Experts believe that communities must be educated on the safety and efficacy of vaccines in order to improve these rates.

Ratner said public health departments can also have targeted outreach to communities with lower vaccination rates to combat misinformation and understand why people are hesitant about vaccination.