A community-based cohort study has demonstrated the effectiveness of two doses of Covishield against the delta variant in the general population of Chennai. The vaccine efficacy against SARS-CoV-2 in the age group 45 years and older was almost 61% at least two weeks after receiving the second dose of Covishield.
The study, a collaboration between the National Institute of Epidemiology-Indian Council of Medical Research and Greater Chennai Corporation, was conducted from May 22 to September 30, 2021 during the waning phase of the second wave of COVID-19 in the city.
Of the 20,913 households in the three divisions in Chennai, 19,211 were enrolled in the cohort. There were a total of 69,435 persons living in these registered households. One fifth of households were in slums and 95.5% were pucca houses. Of the 69,435 individuals, 21,793 were individuals aged 45 and older, according to the study published in “Vaccines.”
Over the three-month period, the researchers followed up these 21,793 individuals. While 7,735 individuals had not been vaccinated, 10,762 individuals received the Covishield vaccine – 4,052 individuals received a single dose and 6,710 had taken two doses. A total of 59 individuals were excluded from the 6,710 individuals who had taken two doses due to, among other things, previous infections. As many as 3,296 individuals vaccinated with Covaxin and other vaccines were excluded.
Sharan Murali, one of the lead authors, said they have designed a community-based cohort to study the vaccine’s true effectiveness. “We followed them for three months and checked whether they developed the disease,” he said.
In the 45+ cohort of 21,793 individuals, 303 tested positive for COVID-19 during the study period. Of the 14,305 individuals (6,651 received two doses and 7,654 unvaccinated) included in the primary vaccine effectiveness analysis, 143 tested positive for COVID-19. Of the 143 subjects, 36 had received two doses and 107 had not been vaccinated. The study found an overall vaccine effectiveness of 61.4% in the 45+ category.
“As a result, we recommend ensuring vaccination with two doses. Those who are not yet vaccinated should start taking the vaccines because they will protect in some way,” he said.