The protection provided by two doses of the Pfizer/BioNTech and the AstraZeneca coronavirus vaccines starts to wane within six months, new research suggests.
A reasonable worst-case scenario could see protection fall to below 50% for the elderly and healthcare workers by winter, an expert has said.
The Pfizer jab was 88% effective at preventing Covid-19 infection a month after the second dose.
But after five to six months the protection decreased to 74%, suggesting protection fell 14 percentage points in four months, latest analysis from the Zoe Covid study indicates.
With the AstraZeneca vaccine, there was a protection against infection of 77% one month after the second dose.
After four to five months protection decreased to 67%, suggesting protection fell by 10 percentage points over three months.
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.
The Zoe Covid Study launched an app feature on 11 December 2020 to enable logging of Covid-19 vaccines and monitor real-world side-effects and effectiveness in its cohort of over a million active users.
Zoe used data from vaccines that were logged from 8 December 2020 to 3 July 2021 and from infections that occurred between 26 May this year when the Delta variant became dominant, and 31 July.
The results have been adjusted to give an average risk of infection reduction across the population.
While protection appears to decrease steadily, individual risk may vary due to individual variation in antibody duration, researchers say.
Across the UK, vaccines were rolled out among the older and the most vulnerable in society along with health workers before rolling out vaccines to younger age groups across the UK.
This means the majority of people who had their second dose five to six months ago will be older or considered vulnerable due to other 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 illustrate 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: "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."
J&J says second dose of Covid vaccine boosts protection
Johnson & Johnson has said that a second dose of its Covid-19 vaccine boosted immunity beyond the first dose.
In two clinical studies that have not yet been peer-reviewed, the additional shot evoked a nine-fold increase in antibodies to the coronavirus, the company said in a news release.
US President Joe Biden's administration has announced plans for all Americans vaccinated with mRNA vaccines - Pfizer and Moderna - to receive a third dose eight months after the second in the series.
The Centers for Disease Control and Prevention (CDC) has said it expects a second dose of the J&J vaccine will be required, but that it needs more data before making a firm recommendation. Today's announcement will likely help inform that strategy.
The company's news release did not specify when the second dose was taken, but information about the trials on government websites shows the company was testing multiple intervals, beginning at six months.
The news comes after the CDC announced the effectiveness of Covid-19 vaccines overall have fallen after the hyper-contagious Delta variant became dominant.
In an ongoing study of US health workers who were overwhelmingly vaccinated with the Pfizer and Moderna shots, effectiveness against all forms of infection fell from 91% prior to Delta to 66%.
But protection against severe Covid-19 and deaths has remained more stable, exceeding 90% according to most studies.
Additional reporting AFP