COVID-19 vaccine effectiveness in adults aged 60 years and older: A case-control study in south India

Objective: To evaluate the effectiveness of COVID-19 vaccines in preventing symptomatic and severe diseases among the adults aged 60 years and older, identify key risk factors that influence infection and severity, and explore strategies to enhance the protection for this high-risk group. Methods: A test-negative case-control study was conducted among individuals aged 60 years and older, comprising 300 COVID-19-positive cases and 300 COVID-19-negative controls. Data on vaccination status and other variables were collected via telephone interviews using a pre-tested questionnaire. Vaccine effectiveness was calculated as 1 minus the crude odds ratio ( OR ), with 95% confidence interval ( CI ). Additionally, univariate logistic regression and multivariable logistic regression were employed to identify factors associated with SARS-CoV-2 infection and disease severity. Results: Among 600 participants, 13.7% (82/600) were identified as unvaccinated individuals. Multivariable analysis revealed that vaccinated status was the only factor determining both the infection of SARS-CoV-2 (adjusted OR 2.63; 95% CI: 1.57-4.41) and severity of COVID-19 (adjusted OR 1.85; 95% CI: 1.00-3.62). The estimated vaccine effectiveness was 68.0% (95% CI : 0.45-0.82) for fully vaccinated individuals and 59.0% (95% CI : 0.30-0.76) for partially vaccinated individuals. Of the cases, 22.0% (66/300) were identified as severe. The vaccine demonstrated an effectiveness of 73.0% (95% CI : 0.47-0.86) against severe COVID-19 among fully vaccinated individuals and 68.0% (95% CI : 0.44-0.83) for partially vaccinated individuals. Conclusions: The effectiveness of COVID-19 vaccines among the adults aged 60 years and older is lower than anticipated. In addition to the administration of booster doses, other interventions such as using higher doses, adjuvants, and intradermal routes should be evaluated to protect this highly vulnerable group that is at the greatest risk of adverse outcomes.