The United States confirmed its first case of polio in almost ten years in New York last month, but a new report from CDC suggests the virus has been circulating in the state as early as April—and with community transmission still ongoing, those who are unvaccinated are at risk of paralysis.

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Polio may have been spreading earlier than previously thought

In July, New York health officials announced that a case of polio had been detected in the state after a young adult from Rockland County became paralyzed from the disease. According to the New York Times, this was the first reported case of polio in the United States since 2013.

Since then, wastewater surveillance has detected several other samples of poliovirus in Rockland and Orange Counties, as well as New York City. In addition, polio was detected in wastewater in Rockland County 25 days before the first patient developed symptoms, suggesting that others had been previously infected.

"The fact that we see it in the sewage 25 days before means that he's probably not even the second case," said Joseph Eisenberg, an infectious disease epidemiologist at the University of Michigan.

"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. "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."

In addition, José Romero, director of the CDC's National Center for Immunization and Respiratory Diseases, 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."

So far, no additional cases of polio have been reported in the United States, but a new report from CDC suggests that community transmission of the virus in New York is ongoing, and those who are unvaccinated face a risk of paralysis.

According to the report, changes in the virus's genome suggest that it may been circulating around the world for up to a year, and the earliest positive sample detected in New York was from April. In total, 260 wastewater samples from Rockland and Orange Counties were collected, and 21 samples tested positive for poliovirus.

Additional tests of wastewater samples collected 41 days after the original patient developed symptoms continue to detect the virus. "This suggests that there is a lot of community spread under the radar," said John Dennehy, a virologist and wastewater surveillance expert at Queens College.

Polio vaccination rates in certain areas remain low

With polio likely continuing to spread undetected in the community, health officials have strongly recommended people get vaccinated if they haven't already.

"Unvaccinated persons in the United States remain at risk for paralytic poliomyelitis if they are exposed to either wild or vaccine-derived poliovirus; all persons in the United States should stay up to date on recommended poliovirus vaccination," CDC researchers wrote in their report.

Currently, polio vaccination rates in both Rockland and Orange Counties where poliovirus was detected are around 60%, much lower than the 80% average in the rest of New York. In addition, the CDC report found that coverage was "as low as 37.3%" in some zip-code specific areas of Rockland County.

Although the Rockland County Department of Health initiated a polio vaccination campaign after the first case was detected, "the number of doses administered at temporary and established clinics was not sufficient to meaningfully increase population [inactivated polio vaccine] coverage levels," CDC wrote.

"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. (Anthes, New York Times, 8/16; Hein, MedPage Today, 8/16)