NEW YORK (NYTIMES) - Polio outbreaks incited regular panics decades ago, until a vaccine was developed, and the disease was largely eradicated.

Then on Friday (Aug 12), 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.

Ms Anabela Borges, a designer who lives in Brooklyn's Crown Heights neighbourhood, said she had friends whose children probably were not vaccinated. After the announcement 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.

Get vaccinated

In New York City, the overall rate of polio vaccination among children 5 and under is 86 per cent, 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 that the discovery of the virus underscored "the urgency of every New York adult and child getting immunised, especially those in the greater New York metropolitan area".

The announcement came three weeks after a man in Rockland County, New York, 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 defence 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 per cent 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 immunised or have put off having them get the shots.

Vaccine less effective vs Transmission

Health officials fear that the detection of polio in New York City's wastewater could precede other cases of paralytic polio.

"Absent a relatively massive vaccine drive, I think it's very likely they'll be one or more cases" in the city, said Dr Jay Varma, a public health researcher and former deputy city health commissioner.

The citywide vaccination rate dipped amid the pandemic, as visits to paediatricians were postponed, and the spread of misinformation about vaccines accelerated.

Even before Covid-19's arrival, vaccination rates for a range of preventable viruses in some neighbourhoods were low enough to worry health officials.

Although effective at preventing paralysis, the vaccine used in the US in recent decades is less effective at limiting transmission. People who have been vaccinated may still carry and shed the virus, even if they do not experience infection or symptoms.

That, public health researchers say, may mean the virus will be difficult to eradicate quickly, further underscoring why vaccination is so critical for protection, a state Health Department spokeswoman said.

Looking out for symptoms

Many people who become infected with polio do not develop symptoms, but some people will have fevers or nausea.

Dr Bernard Camins, an infectious diseases specialist and medical director of infection prevention for the Mount Sinai Health System, urged doctors to be on the lookout for those symptoms and to consider ordering polio tests for patients who are not fully vaccinated.

About 4 per cent of those who contract the virus get viral meningitis, and about 1 in 200 will become paralysed, according to health authorities.

"The problem," Dr Camins said, "is if you have one case of paralysis, there may be hundreds of others that aren't symptomatic or have symptoms that aren't likely to be identified as polio."

The polio virus had previously been found in wastewater samples in Rockland and Orange counties, but the announcement Friday was the first sign of its presence in New York City.

Neither the city nor state health departments provided details about where in the five boroughs the virus had been detected in wastewater.

State officials did say six "positive samples of concern" had been identified in city wastewater: two collected in June and four in July.

The last case of polio to be found in the United States before the one in Rockland County was in 2013.