Although health authorities indicated that the use of the term “national incident” was used to outline the scope of the issue, no cases of polio have been identified so far, and the risk to the public is low.
“Most of the UK population will be protected from vaccination in childhood, but in some communities with low vaccine coverage, individuals may remain at risk,” said Dr. Vanessa Saliba, a consultant epidemiologist for the UK Health Security Agency.
The last case of polio in Britain was in 1984, and the country was declared polio-free in 2003. Before the introduction of the polio vaccine, epidemics were common in Britain, with up to 8,000 cases of paralysis reported every year.
Polio was eradicated in the United Kingdom due to historically high vaccine uptake, which provides nearly complete protection against paralysis, a symptom that affects 1% of polio cases.
However, the UKHSA believes the outbreak can be traced back to a person who received the oral polio vaccine (OPV), which contains a live version of the virus that does not cause disease, in Pakistan, Afghanistan, or Nigeria and then traveled to the UK in early 2022.
Since 2004, the United Kingdom has used an inactivated polio vaccine (IPV), which provides strong protection against the disease but less protection against virus transmission – something that would not normally be an issue in a country where polio has been eradicated.
The OPV was widely used in the United Kingdom until 2004, and it is effective at reducing polio in populations with high vaccine coverage because it creates a milder version of the virus in the gut that can spread and contribute to community-wide natural immunity.
However, in communities with low vaccine coverage, this milder version can lead to “vaccine-derived poliovirus,” or type 2 polio.
This outbreak of a vaccine-derived poliovirus, combined with lower vaccine coverage in London and the use of IPV rather than OPV, is now causing concern.
Wild poliovirus has been eliminated from every country in the world, except Afghanistan and Pakistan. But vaccine-derived polio continues to cause small outbreaks, particularly in communities with low vaccination coverage.
Doctors advised to be on the lookout
The UKHSA stated that there was no evidence of polio-related paralysis, but they are issuing an alert to doctors, particularly those in London, who may not be on the lookout for polio symptoms.
While experts stress the risk to the wider population is low, public health officials are now in a race to track down the outbreak out of a population of four million in north and east London, whose homes are covered by the Beckton sewage treatment works.
Genetic analysis suggests that the samples have a common origin, most likely an individual who traveled to the country around the New Year, Dr. Huseynov said.
The last four samples collected appear to have evolved from this initial introduction, likely in unvaccinated children.