New research has suggested that the protection provided by two doses of the Pfizer/BioNTech and the Oxford/AstraZeneca vaccines can start to deplete within six months, Press Association has reported.

A worst-case scenario would see protection fall to less than 50% for the elderly and healthcare workers by winter, as they were among the first to receive the vaccine.

The study revealed that the Pfizer vaccine was 88% effective at preventing infection a month after the second dose. However, after five to six months this decreased to 74% effectiveness.

This would suggest protection would decrease 14% in four months.

The ‘Zoe’ Covid-19 study also looked at the Astra Zeneca vaccine. It initially showed a protection of 77% one month after a second dose. But it too declined to 67% after four to five months. A fall of 10%.

However, decreased protection in vaccines over time is to be expected, say experts.

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

The mid-term efficacy trial by Pfizer observed an initial 96.2% risk reduction in infection (up to two months after the second dose).

There was an 83.7% reduction more than four months after the second dose, a 12.5 percentage point risk reduction.

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

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

When the Pfizer vaccine was approved at the end of last year, jabs were prioritised for those most vulnerable as well as health care workers on the front line.

This could mean the majority of people who had their second dose five to six months ago will be in the older or vulnerable category. This could suggest that these people are now likely to be at an increased risk of Covid-19 compared to those vaccinated more recently.

Researchers have said that more data is needed over a longer time to confidently say how vaccine effectiveness changes over time and across age groups.

Despite this, vaccines still provide high levels of protection, especially against the Delta variant, and can reduce the likelihood of serious illness.

Professor Tim Spector, lead scientist on the Zoe Covid Study app, said: “In my opinion, a reasonable worst-case scenario could see protection below 50% 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.

“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.”