Some of New York City's area have less than 60% of kids fully vaccinated against polio, according to data from the city's Department of Health.
Just last week, officials found traces of the poliovirus in the city's wastewater system. Officials believe the virus has been in the system since April.
This comes after an unvaccinated Rockland County man tested positive for polio and displayed symptoms of paralysis.
According to the DOH data, the Williamsburg, Battery Park City and Bedford-Stuyvesant/Ocean Hill/Brownsville zip codes all report a vaccination level under 60% for children 6 months to 5 years of age.
The Brooklyn neighborhoods that show low vaccination rates in young kids are linked to the Hasidic Jewish populations that have become increasingly adamant against vaccinations, experts say.
The overall New York City polio vaccination rate for that age group is 82.6%, according to the DOH.
Experts say the rates in low-vaccinated neighborhoods are alarming because unvaccinated populations will allow a greater spread, opening the way for more severe cases.
Adam J. Ratner, director of pediatric infectious diseases at NYU Langone Health, told ABC News that 75% of those infected with polio have no symptoms. Another 20-25%, he said, will experience mild symptoms such as fatigue or low fevers. Less than 1% of polio cases are paralytic, according to the CDC.
Because of the rarity of the Rockland County case, experts believe many individuals must have been infected in the area to lead to the paralytic case.
"When we see one case of paralysis, that very likely means that several hundred people have actually been exposed and infected with the polio virus," Roy M. Gulick, chief of the Weill Cornell Medicine Division of Infectious Diseases, told ABC News.
New York state has some of the strictest vaccination requirements for schools across the nation. In 2019, the state eliminated all nonmedical exemptions for required vaccinations in schools, including the three-dose vaccination for polio.
So, how are so many young children in New York City unvaccinated?
Experts say there are two main factors: the COVID-19 pandemic and the spread of misinformation.
The pandemic had multiple effects on vaccination status, according to experts, including the fact that kids weren't physically in school, so vaccination enforcement was essentially paused, and that parents weren't taking their kids to the doctor for routine checkups and immunizations due to lockdowns.
Ratner said that despite the harsher requirements established in 2019, many communities are still playing "catch-up," particularly because parents faced less pressure to take their kids to the doctor for routine vaccinations during remote learning.
Part of why today's parents are not so worried about getting their kids vaccinated is because people have forgotten how terrible the virus actually is, Ratner and Gulick both said.
Practically all adults in the U.S. are vaccinated against polio because their parents watched the destruction of rampant polio in the 1950s, Gulick said.
"People may have heard of it, but others will have not," Gulick said. "It's the older people who remember it well, about how transmissible it was, the fear of getting polio and the dreaded complications in terms of paralysis."
Parents today take the vaccine for granted, Gulick said. The other part, though, is misinformation.
"I'm thinking specifically of Orthodox Jewish communities in in Brooklyn, where there are some Orthodox Jewish groups that have very high levels of vaccine hesitancy," Ratner said. "I think that the Orthodox Jewish community has unfortunately been directly targeted by anti-vaccine groups that have spread misinformation in the community, which is unfortunate."
Walter Orenstein, a polio expert at Emory University and former director of the United States' National Immunization Program, told ABC News that these communities have been plagued by anti-vaccination groups spreading allegations of danger with vaccines.
Orenstein said it's all about finding "the right messenger to give the right message" when it comes to battling misinformation.
"We need to work with them, to better understand what vaccines are, how they work and all the efforts that are made to assure they're not only effective, but safe before they're made available," Orenstein said.
Ratner said it's essential for parents to understand that it is a dangerous choice to not vaccinate their kids against polio, both for those children and for their communities.
If these low-vaccinated communities do not decide to start vaccinating their children, paralytic cases will increase, Ratner said.