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Explained: What is the difference between a booster shot and an additional Covid-19 vaccine?

While the additional Covid dose would be a “full” dose of the vaccine, booster shots being offered right now have a lesser volume, since the third dose is only supposed to increase the efficacy range.

Written by Rounak Bagchi , Edited by Explained Desk | Kolkata |
Updated: December 7, 2021 8:21:19 am
A health worker administers a dose of a Covid-19 vaccine in Chennai. (PTI/File Photo)

With the Omicron variant of Covid-19 spreading across the country, the need for a booster shot or an additional jab may feel more urgent than ever.

The National Technical Advisory Group on Immunisation (NTAGI) will be meeting on Monday to deliberate upon issues of child immunisation and administering additional doses of Covid-19 vaccine to the immunocompromised. The expert panel is also expected to come up with a comprehensive policy for both the additional Covid-19 vaccine dose and child immunisation, as informed by the chairman of India’s Covid-19 task force, NK Arora.

“The policy will deal with who will require the vaccine, when and how. This needs to be seen in the context that a new variant is coming and with time, we will get more information about it. Therefore, the relevance and effectiveness of the current vaccines will also become apparent with time only,” Arora had said.

What is an additional dose of a Covid-19 vaccine?

An additional dose, originally called a third dose, is given to people with moderately or severely compromised immune systems to improve their response to the initial vaccine series. The term “third dose” was used to refer to additional doses for the two mRNA vaccines, but now the term is “additional dose” because those who received a Johnson & Johnson “one dose” vaccine may also be eligible for a dose based on their immune systems.

People with weakened immune systems might not develop enough immunity after vaccination with two doses of a vaccine. An additional dose, thus, might improve their protection against the novel coronavirus.

What is a booster shot?

A booster shot is nothing but a means of strengthening one’s immune system against a particular pathogen. It may be exactly the same original vaccine, in which case its goal is to increase the magnitude of protection by producing more antibodies.

The booster shot is an additional dose after the protection provided by the original shot(s) has started to decrease over time. Typically, you would get a booster after the immunity from the initial dose(s) naturally starts to wane. The booster is designed to help people maintain their level of immunity for longer. What a booster shot does is that it gives the memory cells the crucial signal to re-engage when the virus attacks.

So, what is the difference between the two?

A Covid-19 booster is given when a person has completed their vaccine series, and protection against the virus has decreased over time. Depending on the original series you had, some details will vary.

An additional dose, however, is administered to people with moderately to severely compromised immune systems. This additional dose is intended to improve immunocompromised people’s response to their initial vaccine series. Experts say that offering such beneficiaries a third dose could help them match up an immune response similar to generalized, healthy populations.

Third Covid-19 doses are thus offered to those with immune vulnerabilities, which could possibly include cancer patients (those in remission or undergoing chemotherapy), or those who have received an organ transplant. These can only be offered on a case-by-case basis, and may not exactly be subject to public rollout.

Are there differences in their dosage?

While the additional Covid dose would be a “full” dose of the vaccine, booster shots being offered right now have a lesser volume, since the third dose is only supposed to increase the efficacy range.

Experts have also suggested that there could be some differences in the side-effects one could expect. With booster shots, we have come to know about a higher intensity or symptoms similar to what one may go through with the second dose. However, it’s yet unknown how severe, or safe a third dose could be.

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