British people should expect to receive repeated vaccinations against COVID-19 in future to keep pace with mutations of the virus, Prime Minister Boris Johnson said on Wednesday.

As vaccines are being rushed out across the world, researchers are considering tweaks and booster shots to make them more effective against new variants, some of which appear to spread more quickly.

Among those most concerning for scientists and public health experts are the so-called British, South African and Brazilian variants.

“I think we will have to get used to the idea of vaccinating and revaccinating in the autumn as we come to face these new variants,” Johnson told parliament.

Earlier this week Health Secretary Matt Hancock said Britain had agreed to buy 50 million new vaccine doses specifically for new COVID-19 variants.

The head of the Oxford Vaccine Group said on Tuesday it is not yet clear whether the world needs a new set of vaccines to fight different variants of the novel coronavirus but scientists are working on new ones so there is no reason for alarm.

The Oxford vaccine developed with British drugmaker AstraZeneca appears to offer only limited protection against mild disease caused by the South African variant of COVID-19, based on early data from a trial.

However, AstraZeneca said it thought its vaccine could still protect people against severe illness caused by the South African variant.

Britain has already injected over 12.6 million first doses of COVID-19 vaccines and is on track to meet a target to vaccinate everyone in the top most vulnerable groups by mid-February.

The COVID-19 pandemic has killed 2.34 million people worldwide since it emerged in China in late 2019, according to a Reuters tally, with Britain among the very worst-hit.