Health authorities in Britain have declared a national incident after finding evidence suggesting local spread of poliovirus in London.

No cases of polio have been identified so far, and the risk to the public is low. But health authorities urged anyone who is not fully immunized against poliovirus, particularly children, to immediately seek vaccines.

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.

Routine surveillance of sewage in the country picks up poliovirus once or twice a year, but between February and May, officials identified the virus in several samples collected in London, according to Dr. Shahin Huseynov, technical officer for the World Health Organization’s vaccine-preventable diseases and immunization program.

Genetic analysis suggests that the samples have a common origin, most likely an individual who traveled to the country around the new year, Huseynov said. The last four samples collected appear to have evolved from this introduction, likely in unvaccinated children.

British officials are now collecting additional samples and trying to identify the source of the virus. But the wastewater treatment plant that identified the samples covers about 4 million people, almost half of the city, making it challenging to pinpoint the source.