Protection against Covid-19 offered by two doses of the Pfizer/BioNTech and the Oxford/AstraZeneca vaccines begins to fade within six months, underscoring the need for booster shots, according to researchers in Britain.

After five to six months, the effectiveness of the Pfizer jab at preventing Covid-19 infection in the month after the second dose fell from 88 percent to 74pc, an analysis of data collected in Britain's ZOE Covid study showed.

For the AstraZeneca vaccine, effectiveness fell from 77pc to 67pc after four to five months.

The study was based on data from more than a million app users, comparing self-reported infections in vaccinated participants with cases in an unvaccinated control group.