NHS England has invited people aged 56 to 59 to book COVID-19 vaccinations in the coming week, with letters to 850,000 people in that age bracket landing on doorsteps from Saturday and another 850,000 due to land Monday.

"The latest invites have been sent after more than eight in 10 people aged 65 to 69 took up the offer of a jab", the National Health Service said in an emailed statement on Sunday.

"NHS staff have vaccinated more than 18 million people across England, meaning more than one third of the adult population have already received the life-saving jab."

Britain's medical regulator on Thursday said it would fast-track vaccines for coronavirus variants, adding that makers of already-authorised shots would not need new lengthy clinical trials to prove their adapted vaccines work.

There is concern that some variants, such as those first found in South Africa and Brazil, may reduce the efficacy of the first generation of COVID-19 vaccines, and manufacturers are looking to adapt their shots.

The accelerated process is based on that used for seasonal flu vaccines each year, the Medicines & Healthcare products Regulatory Agency (MHRA) said, and would be based on robust evidence that the shots create an immune response, rather than full clinical trials.

AstraZeneca PLC, Pfizer Inc and Moderna Inc - makers of the three vaccines the MHRA has approved for use so far - have all said they aim to modify their shots to cope with variants this year.

Britain has so far offered shots made by AstraZeneca and Pfizer.