Experts say it is to be expected and boosters may be needed, at least for some people, ahead of winter.
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Researchers say they are seeing some waning of protection against Covid infections in double-jabbed people.
The real-world study includes data on positive Covid PCR test results between May and July 2021 among more than a million people who had received two doses of Pfizer or AstraZeneca vaccine.
Protection after two shots of Pfizer decreased from 88% at one month to 74% at five to six months.
For AstraZeneca, the fall was from 77% to 67% at four to five months.
Waning protection is to be expected, say experts.
Although some breakthrough infections may be happening, vaccines are still doing a very good job at protecting people against severe Covid illness and deaths.
Public Health England estimates that around 84,600 deaths and 23 million infections have been prevented as a result of the Covid-19 vaccination programme in England so far.
Prof Tim Spector, lead investigator on the Zoe Covid Study app behind the research, said the findings could explain recent breakthrough infections that some fully vaccinated people have been reporting.
Prof Spector said: “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.”
He estimates that protection against infection could drop to 50% by the winter and boosters will be needed, but other experts urge caution about making predictions for the months ahead.
The UK is expected to begin offering some people a third Covid booster jab next month, but is waiting for recommendations from an independent advisory body called the JCVI which is looking at evidence to support a decision.
Prof Spector said: “Many people may not need them. Many people may have had a natural booster because they’ve already had a natural Covid infection, so will effectively have had three vaccines.
“So I think the whole thing needs to be much more carefully managed than just giving it to everybody which would be a huge waste and ethically dubious given the resources we have. I think we need a more targeted approach than last time.”
Dr Simon Clarke, an expert in cellular microbiology at the University of Reading, said infection levels in the community would alter a person’s chance of encountering and catching Covid at any given time, making it hard to draw firm conclusions about waning immunity.
Dr Alexander Edwards, also from the University of Reading, said it was important to understand when booster doses might be needed and for whom.
“Vaccination does not, unsurprisingly, make people invulnerable, and does not prevent all infections. Variants have real and significant impact on public health, and a lot of people are still tragically dying in the UK from this nasty virus.
“The vaccines we have are remarkably safe and effective, and still remain far better than other vaccines that give massive benefits.”
He added: “We must pro-actively plan our public health strategy to account for imperfect protection, and for the possibility of falling protection over time.”
A similar study was published by the Office for National Statistics and the Oxford Vaccine Group last week.
Based on PCR test results from nearly 400,000 people who had been infected with the Delta variant in the UK, it found two doses of the Pfizer vaccine was initially more than 90% effective against symptomatic Covid infection, compared to around 70% for the AstraZeneca vaccine.
But over the course of three months, the protection from Pfizer fell significantly whereas immunity with the AstraZeneca jab remained more stable.
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