The protection provided by two doses of the Pfizer / BioNTech and Oxford / AstraZeneca coronavirus vaccines begins to wane within six months, new research shows.
The reasonable worst-case scenario could see protection drop below 50% for seniors and healthcare workers by winter, an expert said.
The Pfizer jab was 88% effective in preventing Covid-19 infection one month after the second dose.
But after five to six months, protection has dropped to 74%, suggesting protection has dropped 14 percentage points in four months, the latest analysis from the Zoe Covid study says.
With the AstraZeneca vaccine, there was 77% protection against infection one month after the second dose.
After four to five months, protection fell to 67%, suggesting that protection decreased by 10 percentage points over three months.
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The study drew on more than 1.2 million test and participant results.
Pfizer’s mid-term efficacy trial observed an initial reduction in risk of infection of 96.2% (up to two months after the second dose).
There was an 83.7% reduction over four months after the second dose, a risk reduction of 12.5 percentage points.
Real-world analysis would be expected to show less protection than clinical trials, and the vaccines have not been tested against the now-dominant Delta variant of the virus.
The Zoe Covid study launched an app feature on December 11, 2020 to enable Covid-19 vaccine logging and monitor real-world side effects and efficacy in its cohort of over 1 million active users .
Zoe used data from vaccines that were recorded from December 8 of last year to July 3, 2021 and infections that occurred between May 26 of this year, when the Delta variant became dominant, and July 31. .
The results were adjusted to give an average risk of reduction of infection in the population.
While protection appears to decrease steadily, individual risk may vary due to individual variation in the duration of antibodies, according to the researchers.
Across the UK, vaccines were rolled out among the elderly and most vulnerable in society as well as health workers before being released to younger age groups across the UK.
This means that the majority of people who received their second dose five to six months ago will be older or considered vulnerable for other health reasons.
This suggests that these people are now likely to be at an increased risk of Covid-19 compared to those vaccinated more recently.
The researchers say that to confidently illustrate how vaccine effectiveness changes over time in different age groups, more data is needed over a longer period of time.
Zoe Covid Study Senior Scientist Professor Tim Spector said: “In my opinion, a reasonable worst-case scenario could see less than 50% protection for the elderly and healthcare workers in here in winter.
“If high levels of infection in the UK, due to relaxed social restrictions and a highly transmissible variant, this scenario could lead to increased hospitalizations and deaths.
“We urgently need to plan vaccine booster shots and, based on vaccine resources, decide whether a childhood immunization strategy makes sense if our goal is to reduce deaths and hospitalizations.
“Decreasing protection is to be expected and is not a reason not to be 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 be fully immunized. “
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