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Covid-19: Understanding heterologous and homologous boosters



Covid-19: Understanding heterologous and homologous boosters

With India still registering numerous Covid cases, experts are unsure whether or not the country will face a fourth wave. The health ministry recorded 3,205 new cases on May 3, showing a 24.8 per cent spike in cases as compared to last month — making it extremely essential to get vaccinated, especially amid XE variant. As such, Dr Haryax Pathak, a member of iCart, sheds light on heterologous and homologous boosters, and how they can be useful in the fight against Covid.

Understanding heterologous and homologous boosters:


According to the expert, homologous boosters use the same type of vaccine as the first and second doses. Citing an example, he said, “A person who got the first two doses of Covishield, a non-replicating viral vector, gets jabbed with the same.”

Heterologous boosters, on the other hand, “use a different vaccine platform from the first couple of doses”. “A person who got two doses of Moderna (mRNA vaccine), would get jabbed with a different vaccine type such as Covaxin (Inactivated/Dead Virus) or Sputnik Light (Non-Replicating Viral Vector),” he explained.

“According to a study published by Lancet, homologous boosters increased neutralizing antibody titers by a factor of 4 to 20, whereas heterologous boosters increased titers by a factor of 6 to 73. For all clinical outcomes considered, heterologous boosters showed higher vaccine effectiveness than homologous boosters, providing an additional support for use of a mix-and-match approach,” he added.


According to Dr Pathak, “the current situation of heterologous boosters is still a work-in-progress in India. CMC Vellore, which was permitted to conduct homologous as well as heterologous booster dose trials by the national drugs regulator last year, is working on generating adequate data to be able to present before the subject experts of Central Drug Standard Control Organisation. The data, if approved, will then be presented before NTAGI that is already looking at scientific evidence in favour of recommending mix-and-match booster dose against Covid-19 to the health ministry.”

At the moment, India has Covaxin along with Covishield and Sputnik Light vaccines available.

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The expert said that Covishield utilises modified spike proteins from a chimpanzee adenovirus, ChAdOx1virus, to elicit immunity in the body. During its booster trials, Covishield showed promising results. When given as a booster shot, it increased the immune response against Alpha, Beta, Delta, Gamma & Omicron SARS-CoV-2 variants.


“Covaxin is developed with Whole-Virion Inactivated Vero Cell-derived technology which means that it contains inactivated viruses that can not infect but give protection to a vaccinated person.”


According to the results derived from a preliminary analysis of an ongoing safety and immunogenicity trial, 90 per cent of individuals, when boosted with Covaxin, had efficacy being maintained for a period of six months. The boosted subjects showed high neutralizing activity against the Omicron variant as well.


Dr Pathak further said Sputnik Light is based on recombinant human adenovirus (medium-sized, non enveloped) serotype number 26 (the first component of Sputnik V). Based on data collected by the Spallanzani Institute and results of previous studies, heterologous boosting with Sputnik Light tends to increase the efficacy of other vaccines and extend the booster protection period as optimal adenoviral platform configuration provides better protection against Omicron and other variants.


Explaining the importance of a booster dose, the expert said, “The government has announced free administration of a third booster dose due to the rapid increase in the number of Covid cases and emergence of new variants. It is a precautionary dose to safeguard the population against a possible fourth wave. While it is still not mandatory to get a booster dose, those eligible must get it. This ensures protection against infection and even higher protection against severe symptoms or hospitalisation.”

As per a study by JAMA, full vaccination, with booster/precaution dose is also known to reduce transmission of infection to some extent, which coupled with proper masking and distancing is imperative in preventing the further emergence of new variants.

“It is important for the government to look at the data for mix and match boosting as is being done in most other countries. Let’s make sure we get through this pandemic safely together,” the expert said.


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