Health officials say the polio virus was present in a New York City suburb’s wastewater a month before a case was detected in July,
The case – the first in the US since 2013 – was found in an unnamed patient in Rockland County.
Officials say no new cases have been identified, and it is not yet clear whether it is actively spreading.
Polio was largely eradicated from the US by a vaccination campaign that began in 1955.
According to the US Centres for Disease Control and Prevention (CDC), the presence of polio in the community’s wastewater suggests there may be others who are shedding the virus in their stool.
Officials last month said the Rockland County patient was unvaccinated, and said the man was likely exposed to an individual who received a vaccine that contains the weakened live virus.
Laboratory tests found the strain in the confirmed case was genetically linked to one found in Israel, though that does not mean the patient had travelled there, officials said. It was also linked to samples of the virus in the United Kingdom.
Last month, UK health officials said that the virus that causes polio was detected in a concerning number of sewage samples in London.
There is no cure for polio, but it can be prevented by a vaccine. Mostly affecting children, it causes muscle weakness and paralysis, and in the most serious cases permanent disability and death.
An inactivated polio vaccine is used in both the US and the UK as part of the routine childhood programme. In the US, about 93% of toddlers have received at least three doses of the polio jab, according to vaccination data from the CDC.
Thanks to the robust vaccine programme, annual US cases fell rapidly from fewer than 100 in the 1960s to fewer than 10 in the 1970s – and the US was declared polio-free by 1979.
On Monday, New York’s top health official Mary Bassett urged Americans of all ages to get their polio vaccine.
“Given how quickly polio can spread, now is the time for every adult, parent, and guardian to get themselves and their children vaccinated,” she said.