The yet-to-be peer-reviewed study, posted on the preprint server MedRxiv on Friday, also found that when compared to pre-vaccination baseline, both vaccines elicited statistically significant antibody levels in both seronegative individuals and seropositive or those who had recovered from Covid-19 infection.
Between June 2021 and January 2022, the researchers enrolled 691 participants in the 18-45 age group across four sites in urban and rural Bengaluru and Pune.
Participants received either two doses of Covaxin at 28 days apart or two doses of Covishield at three months apart.
The Omicron wave in early 2022 overlapped with the second dose of vaccine in two sites and with both doses in one site.
Participants were sampled at six timepoints for antibody analyses and at four timepoints for cellular analyses.
When compared to pre-vaccination baseline, both vaccines elicited statistically significant antibody levels in both seronegative and seropositive individuals, the researchers found.
Covishield elicited immune responses of higher magnitude and breadth than Covaxin in both seronegative individuals and seropositive individuals, across cohorts representing the pre-vaccination immune history of the majority of the vaccinated Indian population.
Immunologist Vineeta Bal noted that in young adult population there is a difference in the response to Covid vaccines, if individuals are already infected with SARS-CoV-2 and recovered (seropositives) versus not infected.
“In seronegatives, two doses of Covishield lead to higher magnitude of antibody levels in higher proportion of vaccine recipients as compared to Covaxin recipients,” Bal, from Indian Institute of Science Education and Research (IISER), Pune and one of the study authors, told PTI.
“In terms of waning of immune response with time also Covishield vaccinated individuals retain antibodies in their blood for longer period in larger proportion of individuals,” she said.
The study does not directly look for or document protection from COVID-19 in the recipients.
“Levels of neutralising antibodies as well as T cell responses, according to different investigators, are indirect indicators of protection,” Bal said.
“The interim results so far show that Covishield vaccination induces robust neutralising antibodies against the original SARS-CoV2; Covaxin also does it but somewhat less efficiently,” she added.
“Against later variants of the virus such as Delta and Omicron, especially Omicron, neutralising antibodies triggered by Covishield are not as effective, however, they appear marginally better than in Covaxin recipients,” Bal added.
Prior studies comparing Covishield and Covaxin were limited to addressing only antibody responses and in particular in the health care worker population who were immunised prior to the Delta wave.
There is, however, limited data on cellular immune responses elicited by these vaccines and no direct head to-head comparisons or stratification by pre-vaccination serostatus.
Vaccination following exposure to the Delta or Omicron variants of SARS-CoV-2 is likely to affect the quality, quantity and duration of immune responses.
For determining future COVID-19 vaccination policy when pan-coronavirus or sarbecovirus vaccines become available, it becomes important to take into account pre-vaccination immune history of the majority of the vaccinated Indian population.
“Some unpublished work from other investigators in India seems to suggest that heterologous boosting i.e. Covaxin in Covishield recipients and vice versa works better than homologous boosting,” Bal added.
(With inputs from health)