Megan Redshaw

Health officials in the U.K. this month identified the country’s first polio outbreak in 40 years, and believe the outbreak was caused by a strain of polio found in the oral polio vaccine.

Health officials in Britain warned parents on June 22 to ensure their children have been vaccinated against polio after multiple closely related versions of the virus that cause the disease were found in sewage water at the London Beckton Sewage Treatment Works — the largest water treatment plant in the U.K.

The U.K. Health Security Agency (UKHSA) said it believes the virus was “vaccine-derived,” meaning it came from someone who received the live polio vaccine.