The polio virus was discovered in New York City’s wastewater, indicating that the disease, which had not been seen in the United States in a decade, is quietly spreading among unvaccinated people, according to health officials on Friday.

The presence of the poliovirus in the city’s wastewater indicates that the virus is most likely being circulated locally, according to the city and state health departments.

The discovery of poliovirus in wastewater samples in New York City, according to State Health Commissioner Dr. Mary T. Bassett, is concerning but not surprising.

“The risk to New Yorkers is real, but the defense is so simple — get vaccinated against polio,” said Dr. Ashwin Vasan, New York City Health Commissioner. “With polio circulating in our communities there is simply nothing more essential than vaccinating our children to protect them from this virus, and if you’re an unvaccinated or incompletely vaccinated adult, please choose now to get the vaccine. Polio is entirely preventable and its reappearance should be a call to action for all of us.”

As city health officials struggle to vaccinate vulnerable populations against monkeypox and adjust to changing COVID-19 guidelines, New York City is forced to confront polio.

“We’re dealing with a trifecta,” said Mayor Eric Adams on CNN on Friday. “COVID is still very much alive and well. We have polio in our sewage, and we are still dealing with the monkeypox crisis. But the team is present. And we’re coordinating and addressing threats as they arise, and we’re ready to deal with them with the help of Washington, D.C.”

The discovery of the polio virus in New York City comes just days after British health officials reported finding evidence of the virus spreading in London but no cases in people. On Wednesday, children aged 1 to 9 in London were made eligible for booster doses of polio vaccine.

In New York, a polio infection in Rockland County, north of the city, resulted in paralysis several weeks ago. The virus was discovered in wastewater samples collected in June in both Rockland and neighboring Orange County.

Most polio patients have no symptoms but can still spread the virus for days or weeks. Vaccination provides strong protection, and authorities urged people who haven’t received the shots to do so right away.

According to officials, based on previous outbreaks, hundreds of people in the state may have contracted polio and are unaware of it. Polio was once one of the country’s most feared diseases, with annual outbreaks resulting in thousands of paralysis cases. Children are disproportionately affected by the disease.

According to the Centers for Disease Control and Prevention, vaccines became available in 1955, and a national vaccination campaign reduced the annual number of cases in the United States to less than 100 in the 1960s and less than 10 in the 1970s.

Polio causes paralysis in a small percentage of people. The disease kills between 5 and 10% of those who are paralyzed.

All New York schoolchildren are required to have a polio vaccine, but Rockland and Orange counties are both known for vaccine resistance.