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NEW DELHI : Two doses of Covaxin result in 50% effectiveness against symptomatic covid-19, shows the first real-world assessment of the vaccine developed in India, according to a study published in The Lancet Infectious Diseases journal. 

The study was conducted among healthcare workers, a high-exposure population, during India’s second covid-19 surge. These people were primarily offered Covaxin. 

The researchers assessed 2,714 hospital workers from the All India Institute of Medical Sciences (AIIMS)-Delhi from 15 April to 15 May. All these workers were symptomatic and underwent RT-PCR testing for covid-19. The delta variant was the dominant strain in India during the study period, accounting for approximately 80% of all confirmed covid-19 cases in that time. 

“Our study offers a more complete picture of how Covaxin performs in the field and should be considered in the context of a 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 wearing masks and maintaining social distancing," said Dr Manish Soneja, additional professor of medicine at AIIMS Delhi. 

Covaxin, developed by Bharat Biotech, is a vero cell-derived, inactivated whole-virion vaccine administered in a two-dose regimen, 28 days apart. In January 2021, Covaxin was approved for emergency use in India for people aged 18. The World Health Organization (WHO) added Covaxin to its list of approved emergency use covid-19 vaccines in November 2021. 

The study found that the effectiveness of two vaccine doses remained stable over a seven-week follow-up period. Requests for testing gradually declined toward the end of the study period (6 May to 15 May). The median interval between receipt of the last vaccine dose and the end of the study period for those who had received one dose was 37 days and it was 50 days for those who had received two doses.

The adjusted vaccine effectiveness of the first dose, estimated after 7 and 21 days, was low, which is consistent with the performance of other vaccines against the Delta variant and indicates the importance of a second dose to achieve vaccine effectiveness. 

The majority of eligible participants were tested for SARS-CoV-2 infection during the first 20 days of the 30-day study when the positivity rate for covid-19 was at its peak in India. “Findings from the study confirm previous research indicating that two doses of Covaxin are required to achieve maximum protection and that all vaccine roll-out plans must follow the recommended dosing schedule. More research is needed to better understand how these findings translate to Covaxin’s effectiveness against delta and other variants of concern, especially related to severe covid-19 infection, hospitalisation, and deaths," said Dr Parul Kodan, assistant professor of medicine at AIIMS, New Delhi. 

Several factors may be responsible for the low vaccine effectiveness in this study, the authors pointed out. First, this study population only included hospital employees who may have a higher risk of exposure to covid-19 infection than the general population. Second, the study was conducted during the peak of India’s second wave of covid-19 with high test positivity rates for both hospital employees and residents of Delhi. Third, the prevalence of circulating variants of concern, especially Delta, may have contributed to Covaxin’s lower effectiveness. 

The authors acknowledge several limitations in the study. Most importantly, this study does not estimate effectiveness against hospitalisation, severe disease, and death, which require further assessment. Additionally, the study was not designed to estimate effectiveness for different time intervals after vaccination or to determine if the effectiveness changed over time. Another limitation was the absence of data on comorbidities and prior covid-19 infection, which may affect effectiveness.

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