Polio may have circulated widely for a year and was present in New York City’s sewage as early as April, according to a new report from the Centers for Disease Control and Prevention.
A sewage sample taken in April in Orange County, NY, tested positive for the virus, pushing back the first known detection in the area. Authorities previously announced that the virus was found in sewage samples from May in neighboring Rockland County.
Changes in the virus genome 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 about an ongoing 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 aren’t entirely surprising, especially since 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 circulate quite widely, be under the radar, before you start seeing paralysis,” he said.
Officials had previously warned that the Rockland County patient was most likely the “tip of the iceberg.”
Polio vaccination rates are just 37% in some county zip codes, the new study found.
The patient, who had not been vaccinated against polio, was hospitalized in June after developing symptoms including fever, stiff neck and weakness in the lower limbs, according to the study. The poliovirus, which is spread mainly through feces, was later 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 pass the weakened virus in their stool for weeks, potentially infecting others. In communities with many unvaccinated people, the virus can continue to circulate and eventually acquire enough mutations to become dangerous again.
The discovery of Rockland’s case prompted health experts to begin testing sewage samples collected in the area, including those previously collected for coronavirus monitoring.
Officials previously announced that they found the virus in 20 sewage samples taken from Rockland and Orange counties and all were genetically linked to the patient sample.
The new study found that a 21st sample, taken from Orange County in April, also tested positive for the virus. However, there was insufficient genomic information available to establish a conclusive link to the other samples.
Two hundred and sixty sewage samples from Rockland and Orange counties had been tested as of August 10, and polio was detected in 8% of them, according to the new study.
“This suggests that there are a lot of communities spreading under the radar,” John Dennehy, a virologist and wastewater monitoring expert at Queens College, said in an email.
The virus was also found in six sewage samples from New York City.
The Rockland County patient was most likely exposed to polio one to three weeks before developing symptoms, the report notes. The patient did not travel abroad during this time, but attended “a large gathering”, according to the study.
Polio was detected in sewage in Rockland County 25 days before the patient developed symptoms, suggesting others had already been infected.
“The fact that we see it in the sewage 25 days before means 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 too young to be vaccinated.
Nationally, polio vaccination rates are relatively high. But there are pockets of the country, including New York, where vaccination rates are much lower, and the pandemic has set back childhood vaccination campaigns.
As of July 2020, only 67% of Rockland County children over the age of 24 months had received three doses of the polio vaccine, a figure that has dropped to 60% this month, according to the study.
After the Rockland County case was detected, the local health department launched a vaccination campaign, but the number of vaccines given “was not enough to significantly increase” vaccination rates, the researchers reported. .