The detection of the virus in sewage suggests it is circulating in the city, Health Department officials said.
Polio outbreaks incited regular panics decades ago, until a vaccine was developed and the disease was largely eradicated. Then on Friday, New York City health authorities announced that they had found the virus in wastewater samples, suggesting polio was probably circulating in the city again.
Parents of young children found themselves wondering — perhaps for the first time in their lives, and, collectively, for the first time in generations — just how much they should worry about polio.
Anabela Borges, a designer who lives in Brooklyn’s Crown Heights neighborhood, said she had friends whose children probably were not vaccinated. After the announcement on Friday, she said she planned to “make her friends aware.”
Ms. Borges said she hoped her 7-month-old daughter, Ava, who is old enough to have had three of the four shots recommended for children, was far along enough in the regimen to be protected. “Polio is really dangerous for babies like her,” Ms. Borges said as she and her daughter’s nanny took Ava for a walk in her stroller.
In New York City, the overall rate of polio vaccination among children 5 and under is 86 percent, and most adults in the United States were vaccinated against polio as children. Still, in some city ZIP codes, fewer than two-thirds of children 5 and under have received at least three doses, a figure that worries health officials.
The state Health Department said in a statement the discovery of the virus underscored “the urgency of every New York adult and child getting immunized, especially those in the greater New York metropolitan area.”
The announcement came three weeks after a man in Rockland County, N.Y., north of the city, was diagnosed with a case of polio that left him with paralysis. Officials now say polio has been circulating in the county’s wastewater since May.
“The risk to New Yorkers is real but the defense is so simple — get vaccinated against polio,” Dr. Ashwin Vasan, the New York City health commissioner, said in a statement. “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.”
The spread of the virus poses a risk to unvaccinated people, but three doses of the current vaccine provide at least 99 percent protection against severe disease. Children who are too young to be fully vaccinated are also vulnerable, as are children whose parents have declined to have them immunized or have put off having them get the shots.
Health officials fear that the detection of polio in New York City’s wastewater could precede other cases of paralytic polio.