Explained: Selecting the Covid-19 booster vaccine

On Monday, the government began administering an additional “precautionary” dose of the Covid-19 vaccine to vulnerable groups, including those over age 60 and with comorbidities, as well as healthcare and frontline workers. The two main vaccines in India’s immunization program, Covishield and Covaxin, are given in two doses, so the “precautionary” or “booster” dose is the third one for those eligible.

Over 63 crore Indians have received both doses of their vaccine until Sunday, out of nearly 150 crore doses administered overall.

For those eligible for the booster, the Centre has not allowed mixing of vaccines as of now: If someone has received two doses of Covishield, she will be given the same vaccine for the third shot, and so also for Covaxin. Once more data is available, a heterologous approach (using a different vaccine for the third jab) will be taken, later on, the Centre has said.

What purpose do boosters serve?

The immunity provided by a regular course of vaccines is known to wane over time. Various studies in India and elsewhere have reported a decline in antibodies of vaccinated people, recorded at various points of time after the last dose. This decline happens not only with antibodies but even memory T-cells.

A booster dose is given so that people can maintain their level of immunity for longer. In the past, boosters were recommended for smallpox prevention every three to five years. Tetanus toxoid boosters are also recommended today for adults and pregnant women after childhood vaccination.

Regulators in some countries, such as the United States Food and Drug Administration (FDA) and the United Kingdom’s Medicines and Healthcare products Regulatory Agency (MHDA), have looked at booster dose data and given approvals based on that.

In India, a recent study found that six months after the second Covaxin dose, a booster using the vaccine as a booster dose led to neutralizing T cell and B cell responses. This suggests good immune memory responses and long-term protection from severe disease, manufacturer Bharat Biotech said in a statement.

There are regulatory and clinical aspects. From a regulatory and legal perspective, according to Dr. Gagandeep Kang, one of India’s best-known vaccinologists, it makes sense to give the same vaccine as the previous two doses until there is sufficient data to make a policy decision on using a different vaccine as the third dose.

“These vaccines are under emergency use authorization (EUA) which means that they are not fully licensed. For instance, in the national program on immunization, we have off-label recommendations for the rotavirus vaccine. The program itself says it can give the first dose up to 12 months, whereas the manufacturer says can give up to eight months. For the inactivated polio vaccine… the license is for a full dose given intramuscularly. What this means is that off-label use for products that are licensed is one thing, but the same for ones that are under EUA is another issue,” Dr. Kang said.

“We also have to consider the issue of indemnity. What if something goes wrong when you use another vaccine: then who is responsible?” she said.