Wastewater testing will occur in places with low polio vaccination coverage as well as counties with possible connections to the at-risk New York communities linked to the Rockland case of paralytic polio. Logistics for the testing are being worked out between federal and state officials, but once it is underway, testing will last at least four months.

The Michigan and Philadelphia health departments are working with the CDC to identify communities that are under-vaccinated for poliovirus and have wastewater sampling locations. Other state and local health departments are also talking to the CDC about potential wastewater testing.

Polio — once one of the most feared diseases in the United States, with annual outbreaks causing thousands of cases of paralysis — was considered to be eliminated in 1979 after widespread vaccination halted routine U.S. spread. But the virus has been brought into the country by travelers.

Evidence of expanding community spread has landed the United States on a list of more than 30 countries with active circulation of a type of polio known as vaccine-derived poliovirus.

One case of paralytic polio potentially indicates that there may be hundreds of other cases, most of whom experience only mild illness. Polio paralyzes about 1 of every 200 people who contract the virus. There is no treatment other than supportive care; once someone catches polio, it is too late to prevent dire complications of the virus.

Polio has been found in the U.S. Here’s what to know.

Despite the renewed threat, a survey released Wednesday by the Annenberg Public Policy Center found that a sizable portion of Americans is unfamiliar with the risks of polio. Only one-third of U.S. adults know there is no cure for polio, and over 1 in 5 don’t know whether they’ve been vaccinated against polio.

Strains of polio virus can be shed in people’s stool without symptoms, putting unvaccinated people at risk. Finding the virus in sewage or wastewater indicates that someone in the community is shedding the virus. But wastewater data cannot be used to determine or identify who is infected or how many people or households are affected.

If the virus is detected, the CDC’s polio lab will conduct genetic sequencing to pinpoint the specific strain. The CDC is also working with state and local officials to make sure they have “the boots on the ground” to implement vaccination and education campaigns, said José R. Romero, director the CDC’s National Center for Immunization and Respiratory Diseases.

“When will we know that we’re out of the woods? When we get our vaccine rates at the national level — 93 or 94 percent — to have herd immunity in the community,” Romero said, referring to when enough individuals have been vaccinated so their collective immunity prevents the virus from circulating in that population.

In the United States, most people were vaccinated during childhood so the risk to the public is low, officials have said. Modern sewage and wastewater systems are separate from access to clean drinking water, which helps prevent viruses like polio from spreading.

But when the virus is found in communities with low vaccination rates, it can spread among unvaccinated people, putting them at risk for becoming infected and developing polio.

“Wastewater testing can be an important tool to help us understand if poliovirus may be circulating in communities in certain circumstances,” Romero said. “Vaccination remains the best way to prevent another case of paralytic polio, and it is critically important that people get vaccinated to protect themselves, their families and their communities against this devastating disease.”

There are two types of polio vaccines. The United States and many other countries use shots made with an inactivated version of the virus. But some countries where polio has been more of a recent threat use a weakened live virus that is given to children as drops in the mouth. Even though the oral vaccine is easier to administer and may give longer-lasting immunity, it has a key disadvantage: It can lead to vaccine-derived polio, a strain of which was identified in the unvaccinated Rockland County patient. Oral polio vaccine has not been used or licensed in the United States since 2000 because of that risk.

Michigan’s Oakland County, the state’s second-most populous county and part of the Detroit metropolitan area, is the state’s first location for the wastewater polio surveillance because of its low vaccination rates and because vaccine-preventable outbreaks have occurred in the past, said Joe Coyle, director of the Bureau of Infectious Disease Prevention at Michigan’s Department of Health and Human Services.

A 2019 measles outbreak in Oakland County that was started by a visitor from New York “certainly shows there’s vaccine-preventable risk in the population,” Coyle said.

In the 2019 measles outbreak, a Brooklyn man who did not know he was infected with the highly contagious respiratory virus spread it to 39 people as he stayed in private homes, attended synagogue and shopped in kosher markets. The outbreak began in ultra-Orthodox Jewish communities in the New York City region that have traditionally had low vaccination rates and have been a source of anti-vaccine misinformation. The Rockland County man diagnosed with paralytic polio in July lives in a community that was an epicenter of the 2019 measles outbreak.

Although most Americans are protected against polio if they have been fully vaccinated, declining childhood vaccination rates in some pockets of the country and the increase in parents seeking childhood immunization waivers for school entry raises the risk for more vaccine-preventable diseases, Coyle said.

Just under 80 percent of children in Oakland County have received three doses of the polio vaccine by the time they were 19 months old. A total of four shots are required for full immunity. The CDC recommends children get their first polio vaccine at two months, with follow-up shots at four months, between six and 18 months, and between ages 4 and 6.

In a few neighborhoods in South Philadelphia, fewer than 60 percent of children under 5 had received four doses of the vaccine, according to the health department.

Only about 60 percent of 2-year-olds in Rockland County have received their first three shots, according to health department data. In some communities in Rockland County, the vaccination rate is lower than 40 percent.

“Since poliovirus is excreted in stool, monitoring wastewater can potentially find chains of transmission earlier, allowing actions to be taken to break the chains before anyone is paralyzed,” said Walter Orenstein, the associate director of Emory University’s Vaccine Center.

Wastewater surveillance has been critical in finding vaccine-derived polioviruses in the United Kingdom and Israel before any paralytic cases have been detected, Orenstein said.