Polio may have been circulating widely for a year, and was present in New York’s wastewater as early as April, according to a new report from the Centers for Disease Control and Prevention.
A wastewater sample collected in April in Orange County, NY, tested positive for the virus, pushing back the earliest known detection in the area. Officials had previously announced that the virus was found in wastewater samples dating back to May in neighboring Rockland County.
Changes in the genome of the virus suggest that this version has been circulating, somewhere in the world, for up to a year. Genetically similar versions of the virus were detected in Israel in March and in Britain in June.
The new study provides more details from a continuing investigation into a polio case detected in New York last month, when officials announced that a young adult in Rockland County had become paralyzed from polio. It was the first report of polio in the United States since 2013.
The findings are not altogether surprising, especially given that polio, which is highly contagious, often spreads without causing serious symptoms, said Joseph Eisenberg, an infectious disease epidemiologist at the University of Michigan. “It can be circulating quite extensively, being under the radar, before you actually start seeing paralysis cases,” he said.
Officials had previously warned that the Rockland County patient was most likely the “tip of the iceberg.”
Polio vaccination rates are at just 37 percent in some of the county’s ZIP codes, the new study revealed.
The patient, who had not been vaccinated against polio, was hospitalized in June after developing symptoms including a fever, neck stiffness and lower-limb weakness, according to the study. Poliovirus, which spreads mainly through feces, was subsequently detected in the patient’s stool.
Genomic sequencing revealed that the patient was infected with a version of the virus derived from the oral polio vaccine, which contains a weakened version of the virus. The oral vaccine has not been used in the United States since 2000. (American children are routinely immunized with an injected vaccine.)
The oral vaccine is safe and effective, but people who receive it can shed the weakened virus in their stool for weeks, potentially infecting others. In communities with many unvaccinated people, the virus can keep circulating and eventually acquire enough mutations to become dangerous again.
The discovery of the Rockland case prompted health experts to begin testing wastewater samples collected in the region, including those that had previously been collected for coronavirus surveillance.
Officials had previously announced that they had found the virus in 20 wastewater samples collected in Rockland and Orange counties and that all had been genetically linked to the patient sample.
The new study revealed that a 21st sample, collected in Orange County in April, also tested positive for the virus. However, there was not enough genomic information available to conclusively link it to the other samples.
Two hundred and sixty wastewater samples from Rockland and Orange Counties had been tested as of Aug. 10, and polio was detected in 8 percent of them, according to the new study.
“This suggests that there is a lot of community spread under the radar,” John Dennehy, a virologist and wastewater surveillance expert at Queens College, said in an email.
The virus has also been found in six wastewater samples from New York City.
The Rockland County patient was most likely exposed to polio one to three weeks before developing symptoms, the report noted. The patient did not travel abroad during this time, but did attend “a large gathering,” according to the study.
Polio was detected in wastewater in Rockland County 25 days before the patient developed symptoms, suggesting that others had been previously infected.
“The fact that we see it in the sewage 25 days before means that it’s probably not even the second case,” Dr. Eisenberg said.
People who have received three doses of the inactivated polio vaccine are well protected against the virus, but the virus poses a potential danger to unvaccinated people, including children who are too young to be vaccinated.
Nationally, polio vaccination rates are relatively high. But there are pockets of the country, including in New York, where vaccination rates are much lower, and the pandemic has set back childhood vaccination campaigns.
As of July 2020, just 67 percent of Rockland County children older than 24 months had received three doses of the polio vaccine, a figure that fell to 60 percent by this month, according to the study.
After the Rockland County case was detected, the local Health Department began a vaccination campaign, but the number of shots given “was not sufficient to meaningfully increase” vaccination rates, the researchers reported.