The team led by researchers at the University of Bristol, UK, analysed the National Health Service (NHS) health record data on over seven million adults, finding protection in people aged over 65 years, and in clinically vulnerable adults.
They also analysed how quickly vaccine effectiveness waned over time in adults without prior SARS-CoV-2 infection and who received two doses of Pfizer-BioNTech or AstraZeneca COVID-19 vaccine compared with unvaccinated individuals.
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The researchers used records on 1,951,866 (19.5 lakh) and 3,219,349 (32 lakh) adults who had received two doses of Pfizer and AstraZeneca vaccnes, respectively and 2,422,980 (24 lakh) unvaccinated adults.
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They found that rates of COVID-19 hospital admission and death were substantially lower among vaccinated than unvaccinated adults up to six months after their second dose.
Vaccine effectiveness against these events was found to be at least 80 per cent for Pfizer, and at least 75 per cent for AstraZeneca vaccine, the researchers said.
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However, waning vaccine effectiveness against infection with SARS-CoV-2 meant that rates in vaccinated individuals were similar to or higher than in unvaccinated individuals by six months after the second dose, they said.
"Until now there has been limited and conflicting evidence relating to the rate of waning following second dose of COVID-19 vaccines, whether it extends to severe COVID-19, and whether the rate differs according to age and clinical vulnerability," said Elsie Horne, from the University of Bristol, the study's lead author.
"Although we found that protection against severe COVID-19 provided by two doses of vaccine wanes over time, the very high initial protection means that, despite waning, protection remains high six months after the second dose," Horne said.
This finding, the researchers said, was consistent across all adults, including older adults and those who are at risk of severe COVID-19.
"We found that the rate at which vaccine effectiveness waned was consistent across subgroups defined by age and clinical vulnerability. Studying how long COVID-19 vaccines remain effective continues to be important to scheduling and targeting of booster vaccinations," said Jonathan Sterne, a professor of at Bristol Medical School.
The researchers now plan to lead a follow-up study looking at vaccine effectiveness to one year post-second dose and into the era of the Omicron variant.
They are also investigating the rate of waning in vulnerable clinical subgroups, such as those with chronic kidney disease and with cancer.
(With inputs from PTI)
