Bharat Biotech’s Covaxin is 50 per cent effective as a vaccine against symptomatic Covid-19 infection, says a study conducted on AIIMS, New Delhi hospital workers during the second wave surge in India. The study, first of its kind of the effectiveness of a vaccine developed in India and published in The Lancet Infectious Disease journal. Over two thousand seven hundred symptomatic subjects at AIIMS, New Delhi who took RT-PCR testing for coronavirus between April 15 and May 15 this year were assessed for the study.
The study was carried out on a high-exposure population during the second wave when 80 per cent of the all confirmed cases was due to the Delta variant of SARS-CoV-2. Moreover, it was carried out among health workers who were administered both doses of Covaxin, presenting the best chance to evaluate real-world effectiveness of the vaccine.
Dr Manish Soneja, additional professor of medicine at AIIMS, New Delhi, said that the study offers a more detailed picture of why Covaxin’s performance should be considered in context to surge in cases in India due to immune evasive delta variant. And add up to the list of evidence that support the rapid rollout of vaccination to control the pandemic along with protective measures like masking and social distancing.
AIIMS exclusively offered Covaxin to 23, 000 health workers at the hospital out of which 2, 714 employees were used as subjects in the study and 1,617 out of them tested positive for Covaxin. Researchers then matched positive cases with negative tests reports using a 1:1 ratio based on gender and age. Moreover, odds of vaccination with BBV152 were compared between controls and cases and adjusted for exposure at work, previous infection and infection dates.
The findings say that the adjusted vaccine effectiveness against symptomatic Covid-19 over full vaccination was found 50 per cent, provided the 2nd dose was administered 14 days or more before undergoing RT-PCR testing. And was found to be 50 percent. The effectiveness remained the same for a seven-week follow-up period. Majority of the participants were tested during the 30-day study period when India peaked in its positive Covid-1 cases but request for testing declined towards the end of the period.
Another study conducted with both Covaxin and Covishield administered subjects in India found both the vaccines reduced risk of hospitalization or severe disease in aged 45 or above significantly. Covaxin proved 69 per cent effectiveness against severe Covid and for Covishield it is 80 per cent.
The effectiveness estimated in the multi-centre study including researchers of ICMR and pre-printed online is lower than the 77.8 per cent protection against symptomatic Covid reported in the phase three randomized control trial published in The Lancet earlier this month. Moreover, prevalence of circulating variants of concern might have contributed to the lower effectiveness, the authors said.
Factors responsible for lower vaccine effectiveness, the authors presumed, are the exposure of the subjects who were all employees of hospitals and working in high-risk conditions compared to the general population. Moreover, the data was collected during the peak of the second wave when test positivity was generally high.
The authors maintained that the study did not estimate effectiveness against severe disease, hospitalisation and death like other studies and for that further assessment is required. Other limitations of the study were, the vaccine effectiveness was not monitored at different time intervals post vaccination, or if it changed with time. Also, the results were not adjusted to data on co-morbidities and prior Covid infection that affects vaccine effectiveness and health-seeking behaviour.
Also, as the samples from subjects were nor genome sequenced for a variant wise, vaccine effectiveness against a specific variant could not be ascertained.