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EXPLAINED: Gap Before Third Shot And What We Know About Waning Of Vaccine Immunity

PM Modi said on his Christmas day speech that the third dose for healthcare and frontline workers and the high-risk groups will start from January 10, 2022. (PTI/File)

PM Modi said on his Christmas day speech that the third dose for healthcare and frontline workers and the high-risk groups will start from January 10, 2022. (PTI/File)

Reports say that India will go for a gap of 9-12 months between the second and the third dose of a Covid-19 vaccine

India has joined a list of more than 120 countries that have already planned or started giving out a third shot of a Covid-19 vaccine to their eligible populations. Termed a “precaution shot" by PM Narendra Modi, these shots are being introduced initially for selected groups while clearance was also granted to vaccination for those aged between 15-18 years.

What Is A ‘Precautionary’ Shot?

PM Modi labelled the third vaccine shot to be rolled out a “precaution dose" and not a “booster", which is the term being used for it around the world. He said that the groups to be prioritised for these shots, to be administered from January 10, are healthcare and frontline workers and those citizens aged over 60 years who are diagnosed with comorbidities if they are so advised by their doctor.

“The idea is that the dose is being offered as a matter of abundant precaution as one view is that Indians already have hybrid immunity," a government source told News18, explaining the “precaution dose" branding.

The World Health Organisation (WHO) defines boosters as doses “administered to a vaccinated population that has completed a primary vaccination series… when, with time, the immunity and clinical protection has fallen below a rate deemed sufficient in that population". The goal of a booster shot is to “restore vaccine effectiveness".

The global health body also talks of “additional doses" of a vaccine, which it says “may be needed as part of an extended primary series for target populations where the immune response rate following the standard primary series is deemed insufficient". The “primary series" is the normally stipulated dose of any given vaccine, that is, two shots for the likes of Covishield and Covaxin used in India and of most other vaccines around the world, or single shot of a one-dose vaccine like that made by Johnson and Johnson.

WHO says that the objective of an additional dose “is to enhance the immune response to establish a sufficient level of effectiveness against disease". It is intended mainly for

immunocompromised individuals who often “fail to mount a protective immune response after a standard primary series". WHO adds that it can also be considered for older adults who “may respond poorly to a standard primary series with some vaccines".

The most commonly prioritised target populations for booster doses are older adults, health workers and immunocompromised persons, WHO says, noting, in fact, that “in immunocompromised individuals the booster dose is considered as an additional primary series vaccination dose" per its definition.

How Soon Does Vaccine Immunity Wane?

At least 126 countries around the world have issued recommendations on booster or additional does with more than 120 having already started handing them out. The majority of these countries, WHO notes, are classified as high-income, or upper middle-income.

Reports, citing official sources, said that the gap between the second and third doses of a Covid-19 vaccine in India is likely to be 9 to 12 months. More than 61 per cent of India’s adult population has received both doses of the vaccine with about 90 per cent having been given the first dose.

WHO says that new data keep pointing to a decline in vaccine effectiveness against the novel coronavirus with time since vaccination with a “more significant decline in older adults" having been observed by researchers.

However, noting that data are currently insufficient to assess the impact of Omicron — the newest Variant of Concern designated by it — on vaccine effectiveness and against severe disease, it has pointed out that the “vast majority of current infections and Covid-19 cases are observed in unvaccinated people".

“If breakthroughs occur in vaccinated persons, in most cases events are less severe than those in unvaccinated persons," it says, pointing out that the “primary global goal for the acute phase of the pandemic is to reduce deaths and severe disease due to Covid-19 and to protect the health system". It’s an objective that current vaccines are widely deemed able to match.

But WHO also acknowledges that vaccine effectiveness data coming in from “an increasing number of countries… demonstrate an improvement in protection against infection; milder disease; as well as severe disease and death".

Citing data related to four of the WHO-approved vaccines with the most data — Pfizer-BioNTech, Moderna, Covishield, Johnson and Johnson — it says that while “vaccine effectiveness against severe Covid-19 decreased by about 8 per cent over a period of 6 months in all age groups", for those above 50 years of age, the protection against symptomatic disease “decreased by 32 per cent". It said that with the Chinese-made vaccines CoronaVac and BIBP, it has already issued the recommendation for the administration of an additional dose to those aged 60 years or older as part of the primary series".

Should The Third Shot Be Of A Different Vaccine?

WHO notes limitation of data coming in on booster doses, including of follow-up time given that the earliest to introduce these doses, Israel, did so only in July this year. Which means that less than six months have passed since a third dose was first administered anywhere in the world.

But WHO says that “all studies to date" show a strong immune response triggered by such doses “achieving or improving upon the peak antibody levels following the primary immunisation series". It also said that “both homologous and heterologous booster regimens are immunologically effective". Homologous is a third dose of the same vaccine that was used for the first two doses while a heterologous booster implies one different from that used in the primary series.

India is yet to take a call on whether it should go for a mix-and-match approach on the “precaution dose" but various reports suggest that such an option may be on the table.

The US, which has authorised boosters for its 18 and above population, has said that people can get a third shot of any of the vaccines that are being used in the country and need not stick to the one they were initially jabbed with. The booster shots for all the vaccines cleared in the US are of the same formulation as the primary schedule except for the Moderna shot, where it is half the dose of the jab given for the primary series.

For two of the vaccines in the US — Pfizer-BioNTech, Moderna — the gap before the third shot is required to be at least six months following the primary series while for the Johnson and Johnson shot, the second dose has to follow two months after the single shot regimen.

Anurag Agrawal, the Director of CSIR Institute Of Genomics and Integrative Biology, and a member of the national genomic surveillance panel, Indian SARS-CoV-2 Genomics Consortium, had said that Covovax — a protein subunit vaccine — has greater efficacy as a booster dose than Covishield, an adenovirus-based viral vector shot.

Agrawal said in a series of tweets after Covovax became the ninth shot to be cleared for emergency use by the WHO that it is “better than a third dose AstraZeneca/Covishield as a booster (sic)" although he clarified, amid the rise of Omicron, that he does not think “the approved version, based on ancestral spike protein, will be directly very effective against Omicron".

Citing a trial held in the UK, he said that “the immune boost, after two doses of AZ/Covishield, was far better for this than inactivated virus vaccine… Also, rapid modification of protein is possible for variant-specific immune response".

Experts say that whatever the primary dose, the booster will involve being jabbed with only one dose even if the original recommendation for that vaccine was of two doses for full vaccination. Thus, if one is fully jabbed with two doses of Covishield, the booster, if of a separate two-dose vaccine, would still consist of just a single extra jab.

An expert told US-based non-profit Mayo Clinic that while “all of the boosters will dramatically boost your antibody response", the decision about a booster should be based on how one responded to the vaccine that they got originally. “For example, a younger man who initially got the Moderna or Pfizer vaccine might want a J&J booster, because the mRNA vaccines are linked with a slight risk of heart inflammation called myocarditis. And a woman under age 50 might prefer to get a Moderna or Pfizer booster, because the J&J vaccine is linked with a slightly higher risk of a rare blood clotting condition in younger women," said Dr Gregory Poland, head of Mayo Clinic’s Vaccine Research Group.

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first published:December 27, 2021, 16:09 IST