ICMR study on mixing Covishield and Covaxin could open possibilities

In the initial months of the pandemic, when the quest for vaccines had just begun, a section of epidemiologists suggested the possibility of using mixed immunisation as one strategy against the unpredictable pathogen. Using a pair of two different vaccines was also advocated as a panacea to supply shortages. Now an ICMR study on 18 people, who were administered Covaxin as their second shot after having received Covishield as the first jab, has yielded encouraging results. The inadvertent mixing of shots triggered a superior immunogenicity profile — including against the virus’s more infectious variants — as compared to that produced by two doses of the same vaccine. However, the researchers have rightly cautioned that more detailed analysis is required before the mix-and-match approach becomes an accepted part of the anti-Covid strategy.

For at least two decades, researchers have been trying to find potent immune response combinations against several viral diseases, including HIV. In recent years, the endeavour seems to have met with some success with respect to the Ebola vaccine. The Moscow-based Gamaleya Research Institute that has been at the forefront of this research used the approach to develop its anti-Covid vaccine, Sputnik V. The vaccine uses two different vectors for its two shots — the first dose has the same adenovirus as in the Ebola virus, Ad5, while the second shot uses Ad 26. In the past three months, a slew of studies has suggested broadening the frontiers of the combination approach — pairing shots that use fundamentally different technologies. Preliminary studies show that adenovirus-based vaccines induce strong T-cell response — cells critical to early recognition and management of viral infections — whereas messenger RNA vaccines generate a high number of antibodies. Training the immune cells by a viral vector followed by an antibody boost could, therefore, lead to a potentially stronger defence against a SARS Cov-2 attack. A similar quest is reportedly behind a joint initiative by Gamaleya and AstraZeneca that explores the possibility of combining vaccines developed in the two laboratories. The ICMR study indicates the possibility of Covaxin, which relies on chemically inactivated viruses, becoming part of another potent pair.

Mix-and-match trials have not reported significant side-effects so far. But experts say these trials need to have several times more participants to pick up rare events. Caution is also imperative because the endeavour combines shots that have different adverse event profiles. Studies are already underway to ascertain the most potent gap between doses in a combination approach. ICMR needs to upscale its endeavour in these directions.

This editorial first appeared in the print edition on August 10, 2021 under the title ‘Mix and match’.