NEW DELHI: The first-ever data on the effectiveness of the Covishield vaccine in India has shown that it works faster in people already infected with Covid-19, but is also highly effective in those without antibodies against the virus after 28 days of the first dose.
The study titled 'Effect Monitoring and Insights from Vaccination program of Healthcare Workforce from a tertiary level hospital in India against SARS-CoV-2' has been carried out at the Max hospital in Delhi with the support of the CSIR-Institute of Genomics and Integrative Biology.
As part of the study, the researchers measured antibodies to SARS-CoV-2 directed against the spike protein in a group of 135 healthcare workers administered Covishield.
The Oxford-Astra Zeneca Covid-19 vaccine AZD1222 or ChAdOx1 has been an important part of the global vaccine roll-out against SARS-CoV-2, and a locally manufactured version Covishield by the Serum Institute of India is the most commonly used vaccine in India so far.
The researchers found 44 subjects (32.5%) who had already developed antibodies to SARS-CoV-2 at day 0 or before immunization and it was observed that antibody response was significantly higher at each time point, with the maximum increase seen between days 0 and 7.
In contrast, the seronegative group, which had 91 individuals, started developing antibody response only after 14 days or later.
Significantly, three seronegative individuals did not develop any antibody response even at day 28 of vaccination.
The scientists noted that median antibody response at 28 days in seronegative subjects was similar to that of seropositive subjects at baseline and was on a rising trajectory.
"Our data suggests that ChAdOx1 (Covishield) is highly immunogenic, particularly so where previous SARS CoV2 antibody-response is established," said the scientists in their paper.
Given the high background seropositivity in India, this may be useful in determining optimal timing of the second dose during mass immunization within the constraints of vaccine supply and administration, they added.
Based on the results, the scientists have argued that the data could be utilized to design an effective vaccine strategy where vaccines could be prioritized based on sero-prevalence studies.
"Our data supports safely delaying the second dose in recipient groups with high sero-positivity," they said. "This could be adopted as a universal strategy, given that the first dose seems to give adequate protection lasting for about three months in other studies, or could be one part of a dual strategy where high-risk or vulnerable populations receive the second dose earlier, while normal-risk subjects have a delayed second dose."