A plan for booster vaccines is “urgent” as the protection provided by two doses of the Pfizer and the AstraZeneca shots could fall to below 50 per cent for the elderly by winter, a leading scientist has warned.

Professor Tim Spector, lead scientist on the Zoe Covid study app, said the results showed that protection from two vaccines starts to wane “even within six months”.

He said: “In my opinion, a reasonable worst-case scenario could see protection below 50 per cent for the elderly and healthcare workers by winter.

“If there are high levels of infection in the UK, driven by loosened social restrictions and a highly transmissible variant, this scenario could mean increased hospitalisations and deaths.

“We urgently need to make plans for vaccine boosters, and decide if a strategy to vaccinate children is sensible.”

The latest analysis from Zoe, drawing on test results from 1.2 million app users, suggested that the Pfizer jab was 88 per cent effective at preventing Covid-19 infection a month after the second dose, but that this fell to 74 per cent after five to six months.

For the AstraZeneca vaccine, protection fell from 77 per cent a month after the second dose to 67 per cent four to five months later.