The DGCI has given permission to conduct a trial study of mixed doses of Covaxin and Covishield. The study will get conducted by Christian Medical College (CMC) Vellore and will include 300 trial participants. The two vaccine uses different platforms: Covishield is an adenovirus vector platform-based vaccine, while Covaxin is an inactivated whole virus vaccine.

Earlier this year, in May, 18 villagers of district Siddharth Nagar, Uttar Pradesh had got Covishield as the first vaccine dose and Covaxin as the second dose mistakenly. This was not done purposely. The study compares the vaccine response of these 18 villagers with 40 recipients of two doses of Covishield and 40 recipients of two doses of Covaxin. The study illustrates that a combination of (heterologous) different vaccines shows a better immune response than the two doses of the same (homologous) vaccine. 

According to the World Health Organization, there is currently limited data on the immunogenicity or efficacy of a “mix and match” regimen. WHO said, “mix-and-match regimens are likely to work. However, we need to analyze the evidence in each of these vaccine combinations before any other recommendations can be made.”

Before Covid-19, studies have shown better and broader immune responses with the mixing of vaccines of different types. Such heterologous strategies are being studied in HIV, malaria, flavivirus (e.g. dengue), HPV, Ebola, and influenza. 

The idea of heterologous boosting initially emerged in Europe, when the AstraZeneca vaccine was found to be associated with blood clotting in a handful of individuals. A second dose with a different platform vaccine was recommended.