Eighteen people who were accidentally given the Covishied vaccine as the first dose and Covaxin as the second demonstrated better immunogenicity than those who got two doses of the same Covid-19 shot, a study by the Indian Council of Medical Research has showed.

Currently, the government has not approved the mixing of two different Covid-19 vaccines.

The ICMR study has not yet been peer reviewed. It was published on medRxiv, an online journal for unpublished reports on medical sciences, last week.

In May, 20 villagers in Uttar Pradesh’s Siddharthnagar district were administered one dose of Covishield and a second dose of Covaxin. The district’s chief medical officer had said that it was an oversight and there were no instructions from the government “to administer a cocktail of vaccines”.

Researchers from ICMR compared the immunogenicity profile of 18 of those people to 80 others who had been administered two doses of the same vaccine. This included 40 recipients of Covishield and 40 of Covaxin.

“Immunogenicity profile against Alpha, Beta and Delta variants [of Covid-19] in heterologous group [those who received two different vaccines] was superior; IgG [Immunoglobulin G] antibody and neutralising antibody response of the participants was also significantly higher compared to that in the homologous groups [those who got two doses of the same vaccine],” the authors of the study said.

They added: “The findings suggest that immunisation with a combination of an adenovirus vector platform-based vaccine [like Covishield] followed by an inactivated whole virus vaccine [like Covaxin] was not only safe but also elicited better immunogenicity.”

Viral vector vaccines use a “modified, harmless virus to deliver instructions to our cells that trigger an immune response”, according to the Centers for Disease Control and Prevention in the United States.

Inactivated whole virus vaccines contain dead virus, “incapable of infecting people but still able to instruct the immune system to mount a defensive reaction against an infection”, according to Covaxin maker Bharat Biotech.

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The authors of the study emphasised that “multi-centre” randomised control trials will have to be conducted to prove if mixing vaccines could help generate a better immune response, The Indian Express reported.

They added that the sample size of 18 people was small and the participants were studied just 60 to 70 days after receiving their first dose.

Nevertheless, the researchers said, the findings were important. “This is the first report of heterologous immunisation with an adenovirus vector based and an inactivated whole virion vaccine in humans demonstrating safety and significantly improved immunogenicity,” the study said.

The researchers added that mixing jabs will help “overcome the challenges of shortfall of particular vaccines”, The Indian Express reported.

The researchers said that adverse events following immunisation or AEFI in all three groups were similar.

The most common reaction after first and second dose was pain at the site of the injection, PTI reported. No participant had a serious adverse reaction within 30 minutes of receiving the first or second dose.