NEW DELHI: A study conducted by AIIMS has shown that two doses of Covaxin , the indigenous vaccine developed by Indian Council of Medical Research in collaboration with Bharat Biotech, is 50% effective in preventing symptomatic infection due to Covid-19 . This is lower than the 77.8% efficacy claimed by Bharat Biotech after phase 3 trials of the vaccine.
However, there’s a caveat for the differing efficacy figures. The phase 3 trials looked at vaccine effectiveness in the general population, while the AIIMS study involved doctors, nurses and healthcare workers who were working in the hospital setting and, therefore, had a greater exposure to the coronavirus. Also, the study was conducted between April 15 and May 15, when the second Covid wave was at its peak in India and nearly 80% of the infection was caused by the Delta variant, which is known to have a higher rate of breakthrough infections compared with the other variants.
Dr Naveet Wig, professor and head of medicine, AIIMS, told TOI that it was actually a positive finding that Covaxin was effective in preventing symptomatic infection in 50% of the subjects in the middle of the tsunami of cases, that too in the hospital environs. “It means the vaccine works,” he said.
The study, results of which were published in top medical journal The Lancet Infectious Diseases, involved 2,714 hospital employees who were symptomatic and underwent RT-PCR testing for Covid between April 15 and May 15 despite full vaccination. Of them, 1,617 (60%) tested positive for SARS-CoV-2 infection, and 1,097 returned negative results, the study says.
When the positive cases were matched with the negative cases to remove confounding factors such as age and gender, the adjusted vaccine effectiveness against symptomatic Covid after two doses of Covaxin with the second dose administered 14 or more days before undergoing RT-PCR testing turned out to be 50%.
An AIIMS doctor explained that if one were to look at the effectiveness of the vaccine in preventive symptomatic infection now, when the infection rate is low, it would likely be very high. “Vaccine effectiveness increases as more and more people are protected. This is because the viral load in the environment reduces, lowering the risk of the disease spreading,” she said.
Dr Manish Soneja, additional professor of medicine, AIIMS, said the study offered a more complete picture of how Covaxin, which is also referred to as BBV152, performs in the field and the possible immune evasive potential of the delta variant. “Our findings add to the growing body of evidence that rapid vaccine rollout programmes remain the most promising path to pandemic control while public health policies must continue to include additional protective measures, such as mask-wearing and social distancing,” Soneja said.
The authors acknowledge several limitations in the study. Most importantly, this study does not estimate the vaccine effectiveness against hospitalisation, severe disease and death. Additionally, the study was not designed to estimate vaccine effectiveness for different time intervals after vaccination or to determine if vaccine effectiveness changed over time. Another limitation was the absence of data on comorbidities and prior Covid infection, all of which could affect health-seeking behaviour and vaccine effectiveness.
Covaxin, which is formulated from an inactivated SARS-CoV-2 antigen, was approved for emergency use in India in January for people aged 18 and above. WHO added the vaccine to its list of approved emergency-use Covid vaccines on November 3. A WHO statement said, “Technical Advisory Group, convened by WHO and made up of regulatory experts from around the world, has determined that the vaccine meets WHO standards for protection against Covid-19, that the benefit of the vaccine far outweighs risks and the vaccine can be used globally.” Covaxin is also under consideration for vaccination of kids in India, subject to the approval of the designated authorities.