According to a study published in the journal The Lancet, the protection provided by the Oxford-AstraZeneca Covid-19 vaccine, known in India as Covishield, declines after three months of receiving two doses.
According to a team of researchers led by the University of Edinburgh, the findings suggest that booster programmes are needed to help maintain protection against severe disease.
Researchers from Scotland and Brazil looked at data from two million Scots and 42 million Brazilians who had received the Oxford-AstraZeneca vaccine, which uses an adenovirus, specifically a common cold virus from chimps, to train the immune system to fight the virus.
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In Scotland, there was a five-fold increase in the chance of being hospitalised or dying from Covid-19 nearly five months after being double vaccinated when compared to two weeks after receiving a second dose.
According to the experts, the decline in effectiveness begins around three months, when the risk of hospitalisation and death is double that of two weeks after the second dose, and the risk increases three-fold just short of four months after the second dose. Brazil had numbers that were similar to the United States.
“Vaccines have been a critical tool in the fight against the pandemic, but their effectiveness has been waning for some time. Governments should be able to design booster programmes to ensure maximum protection is maintained by identifying when waning first occurs in the Oxford-AstraZeneca vaccine, according to Professor Aziz Sheikh, Director of the University of Edinburgh.
“If you are eligible for a booster and have not yet had one, I would strongly advise you to schedule one as soon as possible,” he added.
The study also compared the outcomes of people who had been vaccinated with those who had not been vaccinated at similar fortnightly intervals to estimate vaccine effectiveness.
Experts cautioned, however, that the figures should be interpreted with caution because it is becoming more difficult to compare unvaccinated people to vaccinated people with similar characteristics, particularly among older age groups where vaccination rates are high.
Meanwhile, a recent study led by Pune’s BJ Government Medical College (BJMC) and Sassoon Hospital found that even three to seven months after taking two doses of Covishield, over 500 healthcare workers had high levels of protection (seroprevalence) against Covid-19.
The study also found that antibody prevalence was above 90% and immunity levels were high months after two doses were completed, indicating that booster doses were unnecessary.