A New York state health official told the BBC there are hundreds or even thousands of undiagnosed polio cases.
This follows an announcement last month that an unvaccinated person in New York’s Rockland County was paralyzed by the virus.
His case is genetically linked to traces of poliovirus found in London and Jerusalem sewage.
Developed countries have been warned to increase vaccination rates.
Dr. Patricia Schnabel-Rupert, Rockland County Health Commissioner, said she is concerned that polio is going undetected in her state.
“If you see a case of paralysis, there is not a single case of polio. The incidence of paralytic polio is less than 1%,” he said.
“Most cases are asymptomatic or mildly symptomatic, and these symptoms are often overlooked.
“So there are hundreds, maybe even thousands, of cases that have occurred for us to see a case of stroke.”
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Dr Rupert confirmed that scientists were looking for “a link” between the New York polio case and traces of poliovirus found in London and Jerusalem sewage, after genome sequencing of samples from all three sites.
“This is a very serious problem for our global world, it’s not just New York. We all need to make sure that all of our populations are properly vaccinated,” he said.
The paralyzed American has a “vaccine-derived” form of polio, which occurs because some countries use a weakened form of the virus in their vaccines.
In rare cases, it can mutate and then spread through poor hygiene to other people who have not been vaccinated.
Global travel means that these cases can arise in countries that are not used to seeing polio, but where vaccination is low.
Although weaker than the original or “wild” form of the disease, vaccine-derived polio can still cause severe disease. The virus can attack nerves in the spinal cord and the base of the brain. It usually causes paralysis in the legs but can be fatal if the respiratory muscles are also affected.
The US and most developed countries use a newer form of vaccine, which does not contain any live virus.
Dr. Rupert said she never thought she would see a case of polio in America in her lifetime.
Some areas of Rockland County historically have vaccination rates as low as 60%. In 2018, there was an outbreak of measles.
Field teams are now being sent to these areas to encourage polio vaccination, especially among children.
In the UK, further testing is currently under way after sewage surveillance in Becton’s sewage plants found traces of poliovirus for several weeks.
Britain’s Health Security Agency is expected to release more details about the worst-hit areas of London soon.
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