With a single case of paralytic polio detected in Rockland County, New York, and the virus showing up in wastewater samples in two counties in the state, as well as New York City, health officials say polio is likely circulating undetected.
This is a particular danger to unvaccinated communities because polio can occasionally lead to severe symptoms including meningitis, permanent paralysis and even death.
"For every one case of paralytic polio identified, hundreds more may be undetected," New York Health Commissioner Dr. Mary Bassett said in a statement earlier this month. "The best way to keep adults and children polio-free is through safe and effective immunization."
The Centers for Disease Control and Prevention recommends children receive four doses of the polio vaccine with one dose each administered at 2 months old; 4 months old; between 6 months and 18 months old; and between ages 4 and 6.
Currently, every state and the District of Columbia requires children enrolling in kindergarten to have received at least three doses or all four doses of the vaccine.
Although CDC data shows almost 94% of kindergarten-age students in the U.S. received the polio vaccine for the 2020-2021 school year, rates vary by state.
Data shows Mississippi has the highest rate in the country with 98.9% of kindergartners vaccinated 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 just 80.4% of students vaccinated ahead of the school year.
Idaho has the second-lowest rate at 86.6% followed by Wisconsin, Hawaii and Georgia -- all with rates under 90%.
"What you need for outbreaks to start and to persist in our population is where people are not optimally vaccinated, where there are a lot of unvaccinated people," Dr. Adam Ratner, director of pediatric infectious diseases at NYU Langone Health, told ABC News. "And that doesn't have to be at the level of a country or a state or even a county. It can be small pockets of people that can't handle an outbreak and keep it going."
Experts told ABC News there are many factors contributing to low vaccination rates, but one of the main factors is the COVID-19 pandemic.
They explained that because parents weren't taking their children for routine appointments, vaccinations were missed, and because children were not physically in classrooms, enforcement of vaccinations required to attend school became lax.
"Primary health care has been disrupted, overall," Dr. Walter Orenstein, associate director of the Emory Vaccine Center, told ABC News. "Since many children get their immunizations in primary health care, they're not making visits. If it's more telemedicine, then you lose opportunities to vaccinate."
Between 1951 and 1954, an average of 16,000 paralytic polio cases and 1,800 deaths from polio were reported every year, according to the CDC. In 1955, the first polio vaccine became widely available.
Cases slowly fell from fewer than 1,000 per year to fewer than 100 per year. By 1994, polio was declared eliminated in the Americas.
Experts say because the disease is so rarely seen anymore, people don't remember a time when polio ran rampant.
"Many people have never seen a case of polio because we had eliminated, until this recent introduction, polio in the United States and much of the world," Orenstein said. "And these are terrible diseases and vaccines are so good at preventing them."
Exemptions are likely another reason behind the low rates, the experts said.
All 50 states allow children to be exempted from vaccination for medical reasons, such as being allergic to a vaccine component or a weakened immune system that would make getting a vaccine harmful.
However, some states allow for non-medical exemptions including religious beliefs or philosophical and personal beliefs.
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, 5.2% of kindergartners in Wisconsin were exempt from one or more vaccines, CDC data shows.
However, in Mississippi, exemptions for "religious, philosophical, or conscientious reasons" are not allowed under state law, according to the Mississippi State Department of Health.
That means just 0.1% of Mississippi kindergartners had exemptions for one or more vaccines last school year, and solely for medical reasons, according to CDC data.
Similarly, New York state eliminated non-medical exemptions for required vaccinations in schools, including the polio vaccine.
As a result, the Empire State also had just 0.1% of kindergarten-age students in the state who were exempt for medical reasons, CDC data shows.
"The data are clear that the more rigorous school laws are in their enforcement, the higher the immunization coverage," Orenstein said.
To improve these rates, the experts said communities need to be educated on the safety and efficacy of vaccines.
"We need to find people who are trusted by the hesitant and who could be messengers to educate them about the safety and effectiveness of vaccines, the tremendous rigor with which a vaccine is produced and evaluated before it becomes licensed and available, the continuing monitoring system for safety and effectiveness," Orenstein said.
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.
"The things that we vaccinate against are, without exception, dangerous diseases," he said. "They are real, dangerous diseases that are not gone. If you think about polio, where it's been out of people's minds in the U.S. for a long time, because we haven't had polio cases here for a long time and that's great. But the reason that we haven't had polio cases here is that we vaccinate our kids."
Ratner said it's frustrating to hear of polio being detected again.
"And kids who are under-vaccinated, and there are plenty of them, are really, truly at risk of paralytic polio," he added. "And it is sad and frustrating to take care of kids who have things that we can prevent."