Covid-19 protection is waning in those first jabbed with the Pfizer or the Oxford-born AstraZenica vaccine, a study suggests.

The real-world study included data on positive PCR test results gathered between May and July 2021 from more than a million people who had received two doses of Pfizer or AstraZeneca vaccine.

Protection after two shots of AstraZeneca, which was developed by the pharmaceutical company and Oxford University, decreased from 77 per cent to 67 per cent at four to five months.

The mid-term efficacy trial by Pfizer found a 96.2 per cent risk reduction in infection up to two months after the second dose.

But there was an 83.7 per cent reduction after more than four months, a 12.5 percentage point drop.

The study drew on more than 1.2 million test results and participants.

Real world analysis is expected to show less protection than clinical trials, and the vaccines were not trialled against the now dominant Delta variant of the virus.

The Zoe Covid Study launched an app on December 11, 2020 for its over 1million users to log Covid-19 vaccines and to monitor real-world side-effects and effectiveness.

Zoe used data from vaccines which were logged from December 8 last year to July 3, 2021 and from infections which occurred between May 26 this year, when the Delta variant became dominant, and July 31.

The results were adjusted to give an average risk of infection reduction across the population.

While protection appears to decrease steadily, risk may vary due to variation in antibody duration in different individuals, researchers say.

Across the UK, vaccines were rolled out among the older and the most vulnerable groups along with health workers before younger age groups.

This means the majority of people who had their second dose five to six months ago will be older or considered vulnerable due to health reasons.

This suggests these people are now likely to be at increased risk of Covid-19 compared to those vaccinated more recently.

Researchers say that in order to confidently identify how vaccine effectiveness changes over time in different age groups, more data is needed over a longer period of time.

Professor Tim Spector, lead scientist on the Zoe Covid Study app, said: “Vaccines still provide high levels of protection for the majority of the population, especially against the Delta variant, so we still need as many people as possible to get fully vaccinated

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

“If 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."

He added: “We urgently need to make plans for vaccine boosters, and based on vaccine resources, decide if a strategy to vaccinate children is sensible if our aim is to reduce deaths and hospital admissions.

“Waning protection is to be expected and is not a reason to not get vaccinated.

“Vaccines still provide high levels of protection for the majority of the population, especially against the Delta variant, so we still need as many people as possible to get fully vaccinated.”