The study, published in The Lancet Infectious Diseases journal, assessed 2,714 hospital workers at AIIMS, New Delhi, who were symptomatic and underwent RT-PCR testing for the coronavirus between April 15 and May 15 this year.
The study period coincided with the second wave of the pandemic in India, when the Delta variant of SARS-CoV-2 was dominant, accounting for approximately 80 per cent of all confirmed cases of Covid-19. It was carried out among a high-exposure population of healthcare workers who were primarily given Covaxin, thus presenting a unique opportunity to evaluate the real-world effectiveness of the vaccine.
Covaxin, developed by Hyderabad-based Bharat Biotech along with the Indian Council of Medical Research (ICMR) under the research name of BBV152, is a Vero cell-derived, inactivated whole-virion vaccine formulated with a novel adjuvant, and administered in a two-dose regimen 28 days apart.
The vaccine, which was approved for emergency use in adults in India in January, was granted emergency use listing (EUL) by the World Health Organisation (WHO) earlier this month.
“Our study offers a more complete picture of how Covaxin vaccine performs in the field and should be considered in the context of Covid-19 surge conditions in India, combined with 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,” Dr Manish Soneja, additional professor of medicine at AIIMS, New Delhi, said.
From January 16, when India started vaccinating healthcare and frontline workers, AIIMS exclusively offered Covaxin to its 23,000 employees. Of the 2,714 employees in the study population, 1,617 tested positive for SARS-CoV-2 infection. Positive cases were matched to negative RT-PCR tests (controls) using a 1:1 ratio based on age and gender. The odds of vaccination with BBV152 were compared between cases and controls, and adjusted for occupational exposure to Covid-19, previous SARS-CoV-2 infection, and infection dates.
The adjusted vaccine effectiveness against symptomatic Covid-19 after full vaccination with Covaxin, with the second dose administered 14 days or more before undergoing RT-PCR testing, was found to be 50 per cent. The effectiveness of two vaccine doses remained stable over the seven-week follow-up period.
The majority of eligible participants were tested during the first 20 days of the 30-day study when the test positivity rate for Covid-19 was at its peak in India. Requests for testing declined towards the end of the study period (from May 6 to May 15).
Another study conducted across 11 hospitals between May and July this year had found that both Covishield and Covaxin significantly reduced the risk of severe Covid-19 and against the Delta variant among those aged 45 years and older. The overall effectiveness against severe Covid was 69 per cent with both doses of Covaxin and 80 per cent with two doses of Covishield.
This multi-centre study, which included researchers from ICMR, was posted as a preprint online, and is not yet peer-reviewed.
The real-world performance of a vaccine often differs from trial conditions. The authors of the AIIMS study acknowledged that the effectiveness estimated in their study was lower than the 77.8 per cent protection against symptomatic Covid reported by a phase 3 randomised control trial study published in The Lancet earlier this month.
Among the factors possibly responsible for the lower vaccine effectiveness could be the facts that the study was conducted only among hospital employees who have a higher risk of exposure to infection than the general population, and that it was conducted at the peak of the second wave when test positivity rates were generally high in Delhi. Also, the prevalence of circulating variants of concern, especially delta, may have contributed to the lower effectiveness, the study said.
The authors acknowledged that the study did not estimate vaccine effectiveness against hospitalisation, severe disease, and death, which they said required further assessment.
Also, 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-19 infection, which may affect health-seeking behaviour as well as vaccine effectiveness.
While the surge was driven by delta, positive patients were not tested for SARS-CoV-2 variants. The study could not, therefore, definitively estimate the vaccine effectiveness against a specific variant.