Covishield Photograph:( AFP )
The decline in effectiveness begins to first appear around three months, when the risk of hospitalisation and death is double that of two weeks after the second dose
A study published by the Lancet has revealed that the immunity offered by the AstraZeneca-Oxford’s COVID-19 vaccine wanes three months after receiving two doses, underlining the importance of booster shots.
The findings were based on the data of two million people in Scotland and 42 million people in Brazil who were vaccinated with the AstraZeneca vaccine, which is known as Covishield in India.
While analysing the data from Scotland, the researchers found that there was approximately a five-fold increase in the chance of being hospitalised or dying from COVID-19 nearly five months after being double vaccinated.
The decline in effectiveness begins to first appear around three months, when the risk of hospitalisation and death is double that of two weeks after the second dose, they said.
The researchers from Scotland and Brazil found that the risk of hospitalisation and death increases three-fold just short of four months after the second vaccine dose. Similar numbers were seen for Brazil, they said.
“Vaccines have been a key tool in fighting the pandemic, but waning in their effectiveness has been a concern for a while,” said Professor Aziz Sheikh, from the University of Edinburgh, UK.
“By identifying when waning first starts to occur in the Oxford-AstraZeneca vaccine, it should be possible for governments to design booster programmes that can ensure maximum protection is maintained,” Sheikh said.
The team was able to compare data between Scotland and Brazil as they had a similar interval between doses —12 weeks —and initial prioritisation of who was vaccinated — people at highest risk of severe disease and healthcare workers.
The dominant variant was different in each country during the study period —Delta in Scotland and Gamma in Brazil.
This means the decline in effectiveness is likely because of vaccine waning and the impact of variants, the researchers said.
However, the experts warned that these figures should be treated with caution because it is becoming harder to compare non-vaccinated people to those vaccinated with similar characteristics, particularly among older age groups where so many people are now vaccinated.
(With inputs from agencies)