Effectiveness of Covishield (TM) vaccine against SARS-CoV-2 infection among elderly population in south India, 2021: A case-control study

Respiratory Diseases
Vaccine preventable diseases

Knowledge of real-world effectiveness of COVID-19 vaccines is vital for policy decisions on pandemic control and understanding the future of the pandemic. Vaccination in India was prioritised for the elderly with CovishieldTM being the predominantly used vaccine. Our primary objective was to estimate the effectiveness of CovishieldTM to prevent SARS-CoV-2 infection among the elderly (>=60 years) population. Secondary objectives included vaccine effectiveness against symptomatic infection and hospitalisation with COVID-19.
We did a case-control study in a district of a southern state in India among a simple random sample of elderly individuals with COVID-19 test results reported in the government database between January-May 2021. Cases included those who tested positive by RT-PCR/Rapid Antigen Test. Controls were negative by RT-PCR. 14 days after two doses of CovishieldTM at-least 28 days apart was considered as fully vaccinated. Trained volunteers telephonically collected data on socio- demographic characteristics and risk behaviours two weeks prior to testing for SARS-CoV-2 infection. We identified relevant confounders using directed acyclic graph and used multiple logistic regression to calculate adjusted odds ratio with 95% confidence interval.
Overall, 315 (30%) of 1054 cases and 430 (45%) of 947 controls had received at-least one dose of CovishieldTM , while 33 (3.1%) cases and 48 (5.1%) controls were fully vaccinated. After adjusting for age, gender, education and religion, effectiveness of full vaccination was 46% (95% CI: 14%-67%) for preventing any infection; 55% (95% CI: 24%-73%) against symptomatic infection; and 87% (95% CI: 6%-98%) against hospitalisation with COVID-19.
Covishield was moderately effective in preventing SARS-CoV-2 infection and symptomatic COVID-19 and highly effective against hospitalisation due to COVID-19 among the elderly. Increased vaccine coverage can effectively lower morbidity and potential mortality among the vulnerable population. Additionally, non-pharmaceutical measures, including masking, hand sanitation and social distancing, to be continued to control the spread of the pandemic.

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