LONDON — People arriving in Britain must quarantine in hotels starting Monday as the government tries to prevent new variants of the coronavirus derailing its fast-moving vaccination drive.

On Sunday the government reached its goal of giving the first of two doses of vaccine to 15 million of the most vulnerable people, including health care workers and over-70s.

Health Secretary Matt Hancock said the vaccination drive is now being extended to people over 65 and those with underlying health conditions.

Health officials are concerned that vaccines may not work as well on some new strains of the virus, including one first identified in South Africa.

People arriving in England from 33 high-risk countries must stay in quarantine hotels for 10 days at their own expense. In Scotland the rule applies to arrivals from any country.

Critics say the move comes too late, with the South African variant already circulating in the country.