Researchers have found that those double vaccinated against Covid-19 could see immunity waning within six months of innoculation.
The Zoe Covid Study launched an app feature in December 2020 to log Covid-19 vaccines and monitor real-world side-effects and effectiveness in its more than a million active users.
It found that the protection provided by two doses of the Pfizer/BioNTech and the Oxford/AstraZeneca coronavirus vaccines starts to wane within six months.
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 by 14% in four months.
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%.
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.