Protection against Covid-19 provided by two doses of the Pfizer and AstraZeneca vaccines starts to wane within six months, new research suggests.
Experts have warned that in a reasonable worst-case scenario, protection could fall to below 50 per cent for the elderly and healthcare workers by winter.
Vaccine effectiveness drops within six months
The Zoe Covid study analysed more than 1.2 million coronavirus test results and participants using data that was logged on the app.
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The study found that the Pfizer vaccine was 88 per cent effective at preventing Covid-19 infection one month after the second dose, but this dropped to 74 per cent after five to six months. This is a fall in protection of 14 percentage points in four months.
By comparison, the AstraZeneca vaccine offers 77 per cent protection against infection one month after the second dose. This dropped to 67 per cent after four to five months, indicating that protection fell by 10 percentage points over three months.
The mid-term efficacy trial by Pfizer observed an initial 96.2 per cent risk reduction in infection up to two months after the second dose, while after more than four months, this fell to 83.7 per cent - a 12.5 percentage point change in risk reduction.
The study suggests that real world analysis would be expected to show less protection than clinical trials. The vaccines were also not trialled against the highly infectious, and now dominant, Delta Covid-19 variant.
Individual Covid risk will vary
Researchers have said that while protection provided by the vaccines appears to decrease steadily, individual risk may vary as antibody duration will be different from person to person.
However, as the vaccines were offered to the elderly and most vulnerable first, along with healthcare workers, the majority of people who received their second dose five to six months ago will now likely be at an increased risk of Covid-19 infection, compared to younger age groups who received their jab more recently.
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 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.
“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.”
The Zoe Covid Study launched an app feature on 11 December last year to enable real-world side effects and effectiveness of the coronavirus vaccines to be logged from more than one million users.
The study assessed data from vaccines that were logged on the app from 8 December 2020 to 3 July 2021, along with infections from 26 May this year and 31 July, when the Delta variant was dominant.
The results from the study have been adjusted to give an average risk of infection reduction across the population.
Researchers have said that more data is needed over a longer period of time to confidently illustrate how the effectiveness of vaccines changes over time and in different age groups.
This article originally appeared on our sister site, NationalWorld.