India has started handing out a third shot of Covid-19 vaccines but the “precaution" dose, as this extra, booster jab has been termed by PM Narendra Modi, is available at the moment only to frontline and healthcare workers and people aged over 60 years who are diagnosed with comorbidities. The third shot is being administered at an interval of nine months from the second dose and will see eligible persons getting jabbed with the same vaccine that they had initially received.
Experts note that much remains unknown about the durability of the immunity from the primary two-dose regimen of the existing vaccines and the ability of new variants like Omicron to escape the protection they provide. But here’s what we know about the factors underpinning the decision to go for a third dose.
How Long Does The Immunity From Vaccines Last?
According to experts, “waning protection is the main reason to get a booster shot". Because the view is that antibodies produced by vaccination normally last between 6-8 months, after which their levels have been found to drop. As the World Health Organisation (WHO) says, “data on immunogenicity of some vaccines suggest that antibodies persist for at least 6 months [and] waning of neutralising antibodies has been reported".
Further, the global health body notes that research points to a “more significant decline in older adults" vis-a-vis immunity imparted by vaccines.
Data related to four of the WHO-approved vaccines — Pfizer-BioNTech, Moderna, Covishield, Johnson and Johnson — show 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".
Why Is A Booster Dose Needed?
WHO says a booster is the dose given to those that have completed a primary vaccination series — that is, getting jabbed with one or two doses of a Covid-19 vaccine, depending on the product — when it’s found that “the immunity and clinical protection has fallen below a rate deemed sufficient in that population". The goal behind a booster “is to restore vaccine effectiveness from that deemed no longer sufficient".
But it adds that even if “data consistently show a decline in vaccine effectiveness against infection and milder forms of Covid-19 over time", all the vaccines authorised by it for emergency use afford additional layers of protection by way of T and B cells. “With respect to duration of protection against disease requiring hospitalisation, current data show an overall continued high level of effectiveness," says WHO, explaining that “protection against severe disease is more durably retained due to anamnestic humoral and cell-mediated immunity".
Experts say that the “other reason for booster shots is to protect ourselves against other variants". Newer variants, like Omicron — the newest Variant of Concern (VoC) — are seen as being able to slip past the antibodies produced by the current vaccines as they were created to target the spike protein — which the novel coronovirus uses to attack and latch on to human cells — of earlier variants that subsequent mutations are found to have altered.
However, noting that data are currently insufficient to assess the impact of Omicron on vaccine effectiveness and disease severity, WHO 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.
What’s The View On Mixing Vaccines For The Booster Shot?
The third shot being administered in India is to be of the same vaccine as that which was used for the first two doses. Thus, those who were jabbed either with Covishield, Covaxin or Sputnik V will receive an identical third shot with the scope for mixing boosters having been ruled out by health authorities. Five more vaccines — Covovax, Corbevax, ZyCoV-D, Johnson & Johnson, Moderna — have been cleared for use in the country but none of these has been rolled out yet.
Ahead of the rollout of the third shot, experts weighing in on the best booster strategy for India had opined that recourse should be taken to a vaccine different from that used for the two initial shots so as to ensure a more robust immune response.
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 that Covovax 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".
Another expert, Dr Gagandeep Kang, noted “data from study that shows Covishield booster on top of Covishield (vaccination) gives immune response" but pointed out that “according to the global data (available till now), mRNA vaccines have proven to be the best boosters". She had averred, therefore, that “either the Indian government should figure out how to bring these mRNA boosters in India or we can wait for the launch of Gennova’s mRNA vaccine".
Why Has India Decided Against Mixing Boosters?
Dr VK Paul, Member (Health) in NITI Aayog, had said while announcing the country’s booster policy that even though there was no issue in principle in going for a separate vaccine for the third dose, there was not enough data available at the moment in India on the viability of such an approach.
The WHO, too, has noted the limitation of data on booster doses, including of follow-up time, given that the earliest country 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.
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.
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.
Another factor in mixing boosters in India could be linked to vaccine availability itself. The vast majority of people in India have been jabbed with the Covishield vaccine, of which more than 1.32 billion doses have been administered. Mixing of boosters would mean that these people would have to be given a shot of either Covaxin or Sputnik V for their third dose. But supply-side issues may crop up in sourcing these vaccines for the booster campaign — of which less than 200 million doses have been cumulatively administered.