After the first U.S. case of polio in nearly a decade was confirmed last month, the virus is now circulating in New York City wastewater, suggesting there may be "hundreds" of undetected cases. Here's what health officials are saying.

'The tip of the iceberg'

In June, the U.K. Health Security Agency (UKHSA) reported that it had discovered poliovirus in several sewage samples from North and East London in February and April, suggesting that there may have been transmission between individuals. Before this, the last known case of polio in Britain had been in 1984, and the country had been polio-free since 2003.

In response, U.K. health authorities declared the discovery of poliovirus a "national incident" and have expanded surveillance of sewage samples to pinpoint the origin of the outbreak and determine whether it is still ongoing.

According to UKHSA's analysis of the virus samples, "transmission has gone beyond a close network of a few individuals." However, the agency has not yet identified anyone who has been infected with the virus.

Similarly, the United States last month reported its first known case of polio in almost a decade. The case was detected in an unvaccinated adult from Rockland County, New York. Since then, wastewater surveillance has detected poliovirus in six samples in Rockland County and five samples from the nearby Orange County.

"Based on earlier polio outbreaks, New Yorkers should know that for every one case of paralytic polio observed, there may be hundreds of other people infected," said Mary Bassett, New York's health commissioner.

On Friday, polio was also detected in New York City wastewater, suggesting the virus is likely circulating throughout the city. The city's health department did not provide details about exactly where polio had been found, nor did they provide dates for when the virus was detected or say how many samples had tested positive, the New York Times reports.

"Coupled with the latest wastewater findings, the department is treating the single case of polio as just the tip of the iceberg of much greater potential spread," Bassett added.

José Romero, director of the CDC's National Center for Immunization and Respiratory Diseases, also noted that most people with polio don't have symptoms and can therefore spread the virus without knowing it.

"There are a number of individuals in the community that have been infected with poliovirus. They are shedding the virus," he said. "The spread is always a possibility because the spread is going to be silent."

Last week a team of CDC disease detectives traveled to Rockland County out of concern that polio could "could mushroom out of control very quickly and we could have a crisis on our hands," said a community health leader who met with the team.

According to Kathleen O'Reilly, a polio expert at the London School of Hygiene and Tropical Medicine, the poliovirus initially detected in London is "genetically related" to cases in the United States and Israel. Currently, UKHSA is working with health authorities at the World Health Organization and in the United States and Israel to investigate how the detected polioviruses are connected.

Health officials urge vaccination, consider boosters to curtail potential spread

After the poliovirus was first detected in wastewater samples in London, UKHSA recommended that anyone who is not vaccinated do so soon.

Jane Clegg, chief nurse for the National Health Service, said the agency planned to contact parents of children under five who are living in London and not yet vaccinated. In addition, UKHSA also recently authorized booster doses of the polio vaccine for children ages one to nine living in London as a precaution to "ensure a high level of protection and held reduce further spread."

In New York, health authorities have also strongly encouraged polio vaccination. According to the New York Times, both Rockland and Orange Counties where poliovirus was detected have polio vaccination rates of around 60%, much lower than the 80% average in the rest of the state.

"It is concerning that polio, a disease that has been largely eradicated through vaccination, is now circulating in our community, especially given the low rates of vaccination for this debilitating disease in certain areas of our County," said Orange County Health Commissioner Irina Gelman. "I urge all unvaccinated Orange County residents to get vaccinated as soon as medically feasible."

Similarly, Rockland County Health Commissioner Patricia Schnabel Ruppert said, "[a]ny unvaccinated children and adults should receive a first polio immunization immediately."

"The risk to New Yorkers is real but the defense is so simple — get vaccinated against polio," said Ashwin Vasan, the 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," he added.

In addition, CDC said it is considering offering a booster for the polio vaccine for certain New Yorkers, including children in the detected area and certain groups of adults.

"We're looking into all aspects of how to deal with this," said Romero. "At this point, we don't have a definitive answer." (Bean, Becker's Hospital Review, 8/11; Choi, The Hill, 8/11; Shanahan, New York Times, 8/4; Branswell, STAT News, 8/4; Associated Press, 8/4; AP/NPR, 8/10; Cohen, CNN, 8/11; Goldstein/Otterman, New York Times, 8/12)