Bharat Biotech's Covaxin induces wider immune response, say experts

A recent, yet to be published study on heterologous vaccine boosting by Christian Medical College Vellore showed that Covishield-boosted and Covaxin-primed individuals display high levels of antibodies against Sars-CoV-2, the virus that causes Covid-19.

However, experts say that Covaxin induces a wider immune response and thus antibody production against the spike protein may be less intense.

Samiran Panda, head of epidemiology and infectious diseases at the Indian Council of Medical Research (ICMR) that has worked with Bharat Biotech to co-develop Covaxin, said there was no need to panic because of studies. “Every vaccine tries to induce a humoral (antibody) response as well as cellular response. The human body is a complex mechanism and both these immune responses work together, not in exclusivity,” he explained.

Covishield fares significantly better as a booster or third shot after primary vaccination with either Covishield or Covaxin, according to the CMC Vellore study on heterologous vaccine boosting.

Covaxin is an inactivated virus vaccine, while Covishield is a vector-based vaccine.

K Srinath Reddy, president, Public Health Foundation of India, explained the differences lucidly: “Covishield carries the code for production of the spike protein in the recipient’s body, to trigger strong antibodies against that antigen. Other parts of the SARS-CoV-2 virus are not introduced to the body. Covaxin, on the other hand, is an inactivated virus that offers the body’s immune system a larger platter of antigens to react against.”

He added, “As the body’s immune response is spread wider, antibody production against the spike protein antigen may be less intense. It is the water jet effect versus the shower effect.”

In an interview with Karan Thapar, microbiologist and professor at CMC Vellore Gagandeep Kang said that heterologous boosting with Covishield as a third shot after two doses of Covishield boosted antibody-led immune response by 58 times. Bharat Biotech refused to comment on the study as the data was not yet public.

Kang confirmed that the CMC Vellore study found out a third Covaxin after two Covaxin increases antibodies six times but from a very low base; a third Covishield after two Covishield increases antibodies 6.8 times but from a very much higher base; whereas a third Covaxin after two Covishield only increases antibodies 2.5 times. She clarified that the study was only testing antibody responses and not T-cell responses.

Panda said both the widely used vaccines in India — Covishield and Covaxin — were working just fine. Covid-19 vaccines are disease modifying vaccines that are preventing us from experiencing severe bouts, he said, adding that we are used to seeing infection prevention vaccines in the National Immunisation Mission, such as the polio vaccine.

In a recent study by ICMR scientists — published in the preprint server, bioRxiv — on protective efficacy of Covaxin against Delta and Omicron in Syrian hamsters, the researchers have mentioned that a decline of neutralising antibody response was observed after six months of a two-dose Covaxin immunisation.

However, persistent Sars-CoV-2 specific CD4+ and CD8+ T cell memory response (phenotype) was observed. T cells are immune cells that remember an antigen and produce specific antibodies when the body encounters it again.

The bioRxiv publication further noted that when a third booster was given it led to pronounced increase in the neutralising response against homologous and heterologous SARS-CoV-2 variants in humans.

“The booster dose of Covaxin was found to improve the neutralising antibody response against the VoCs (variants of concern) including Delta and the Omicron,” it said.