The first polio case in America in more than 30 years was reported late last month.
The case in Rockland County, around 35 miles north of New York City, has been attributed to a strain related to the live oral polio vaccine, which is used in some parts of the world but has not been used in the US since 2000.
Over the past six weeks, traces of this form of the virus have also been found in sewage samples in Kolkata and London.
Meanwhile, in June, the World Health Organization (WHO) reported cases of vaccine-derived polio in Eritrea, Ghana, Togo, Ivory Coast, Israel, Yemen, Nigeria and the Democratic Republic of the Congo, prompting a mass vaccination campaign.
While the incidents appear unconnected, there is growing concern amongst experts as global vaccination levels against the largely preventable disease have dropped to their lowest levels in 15 years.
Calls are now being made for many countries, especially those that are densely populated, to redouble their efforts on vaccinations and surveillance to ensure polio doesn’t make an unwelcome comeback.
In 2018, Papua New Guinea suffered an outbreak of vaccine-derived polio, 18 years after the country was declared free of polio. While, so far, there is no evidence that these recent cases are the start of a mass spread of the disease, caution is still being urged.
The surge in vaccine-derived polio cases has reopened the debate among some experts over the benefits of the two main types of immunization commonly used against the disease.
The first, known as the IPV, uses an inactivated virus and is injected. The other, administered orally, is known as the OPV and carries a weakened but active virus that, after circulating in a population, can mutate and occasionally lead to outbreaks.
Despite their differences, both ensure that those who have received all their doses are mostly immune to the vaccine-derived virus, whether they encounter it in air or water.
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