Mixing of doses is safe, gives better immunity: CMC Vellore study

The results of a study to test the feasibility of mixing Covishield and Covaxin vaccines show that heterologous immunisation is safe and provides better results than homologous vaccination, a top source with the Health Ministry told BusinessLine. The findings of the study, conducted by the Christian Medical College (CMC), Vellore, is currently being reviewed by the National Technical Advisory Group on Immunisation (NTAGI), based on which a decision on mixing doses will be taken, the source further added.  

“The study was done to satiate scientific curiosity. It found that heterologous boosting is safe and more immunogenic than the homologous combination. NTAGI is currently reviewing it, based on which the Centre will decide on the mix and match boosting,” said a government source.  

According to sources, the study was done on more than 400 participants, divided into four groups with each group having more than 100 individuals. While one group had Covishield doses, another set had all three shots of Covaxin, and the remaining two groups had a mix of both vaccines -- two shots of Covishield with Covaxin as third dose and vice-versa. 

The Drug Controller General of India had given its nod to CMC Vellore for the trial last year in July. At present, India is giving the same dose as a booster dose to frontline and healthcare workers and those aged above 60 years.  These beneficiaries, who are being administered booster doses, are fully vaccinated and have completed nine months after they took the second dose. According to sources, the government is exploring heterologous vaccination for boosting, and it is also planning to extend booster doses to those below 60 years with comorbidities.

“Although boosting with the same vaccine is safe, heterologous immunisation is more immunogenic in response. This is not to solve any problem, but it is to be noted that it does improve the health response. However, I am okay with both the combinations,” said T Jacob John, a renowned virologist, when asked if mixing of doses is good for boosting.

Recently, in a Health Ministry conference, VK Paul, Member-Health, Niti Aayog, had said that a decision on booster doses is yet to be taken and that it is being discussed by the NTAGI.

It is to be noted that the Indian Council of Medical Research (ICMR) had also done a study on mixing of vaccine doses - adenovirus vector platform-based vaccine, followed by an inactivated whole virus vaccine. It found that the heterologous immunisation was not only safe, but also gave better immunogenicity than two doses of the same vaccine. The study was conducted on 98 participants having completed a period of two weeks or more after the second dose was included. The study duration was from May to June 2021. Participants were shortlisted in three cohorts -- 18 individuals from Uttar Pradesh, who had inadvertently got the first dose of Covishield and a second dose of Covaxin; 40, who had received both doses of Covishield; and the remaining were from the Covaxin group.

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