British Prime Minister Boris Johnson said he is looking at toughening border quarantine rules because of the risk of “vaccine-busting” new coronavirus variants, Reuters reports.

“We have to realise there is at least the theoretical risk of a new variant that is a vaccine-busting variant coming in — we’ve got to be able to keep that under control,” Johnson told reporters at a vaccination centre.

“We want to make sure that we protect our population, protect this country against reinfection from abroad,” Johnson said. “We need a solution.”

New variants of the virus that causes Covid-19 are opening up the prospect of a much longer battle against the pathogen than previously thought. Scientists fear the new variants may be more deadly, and that vaccines may be less effective against them.