Explained: The need for COVID-19 vaccine boosters amid Omicron scare

Body’s response to primary doses will wane in a few months, say experts


Amid Omicron scare, there has been a growing demand for COVID-19 vaccine booster shots in India.

Even as the government is dilly-dallying over administering boosters, experts say it makes sense to provide an extra layer of protection against the disease rather than waiting for the Omicron shot.

A new study published in the Lancet shows that six COVID-19 vaccines--Oxford-AstraZeneca, Pfizer-BioNTech, Novavax, Janssen, Moderna, and Curevac--are safe and effective to be used as booster doses.

Veteran virologist Dr T. Jacob John is all for booster doses. “It is more important against Delta and Omicron. They are mutating viruses. Higher the fence, the better the protection. If the antibody level is good, the virus will not be able to infect,’’ says John, retired professor and head of departments of clinical virology and microbiology, Christian Medical College, Vellore.

Booster shots offer increased protection against the infection. They are administered six months to one and a half years after the primary doses. “The primary dose primes the immune system. There is some response. But long-lived memory cells are not induced. Only short-term memory cells get induced by the primary dose. The booster dose, on the other hand, stimulates long-term memory cells. It raises the immunity very high, even higher than infection-induced immunity,’’ explains John who was also the former director of ICMR’s Centre of Advanced Research in Virology.

Booster doses also induce antibody manufacturing cells. “Even if you do not get exposed to the virus, your body will keep on making antibodies, without any stimulus. So, the antibody level does not come down. Even if it comes down a bit later on, any exposure to the agent will immediately boost it, because you have memory,’’ says John. “What we need now is B Cell immunity or antibody immunity which is necessary for protection from infection. To get B Cell immunity, one needs primary as well as booster doses,’’ he adds.

The primary doses are for ‘firefighting’. They offer immediate protection. A study conducted at Sri Jayadeva Institute of Cardiovascular Sciences in Bengaluru found that antibody levels induced by COVID-19 vaccines sustain for a period of six months. The study conducted in a group of 250 healthcare workers showed that 99 per cent of them had high antibody levels six months after vaccination.

The response to the primary doses, however, will wane in a few months and hence it is important to administer booster doses.

“Continuous presence of antigens is the key for maintaining long-lasting immunity,’’ says Amit Awasthi, associate professor at Translational Health Science and Technology Institute (THSTI).

Dr. Awasthi's group has established animal models and cellular assay platforms to support preclinical and clinical evaluation of COVID-19 vaccines in India.

So, when can booster doses be given? “In my opinion, they may be given six months post second dose,’’ says Awasthi.

John maintains that the immunisation schedule is complete only with the booster dose. “Every vaccination includes primary and booster doses. COVID-19 is not an exception.’’

That is the case with infections like tetanus, too, says Dr Subramanian Swaminathan, director, infectious diseases, Gleneagles Global Health City, Chennai. “A Tetanus vaccine is only valid for five-10 years. After that you need a booster.”

Majority of Indians have taken either Serum Institute’s Covishield vaccine or Bharat Biotech’s Covaxin. The kinetics of humoral and cellular immune response differs for different vaccines, says Awasthi.  “Previous studies have shown that the antibody response in mRNA vaccinated individuals is high but rapidly declines after six months. In contrast, adenovirus-based vaccines provide durable antibody responses even after six months (Collier et al., 2021). The correlates of protection against severe disease of COVID-19 largely depends on the levels of neutralising antibodies and the T cell immune response. These responses vary among vaccines depending on several factors such as the type of vaccine, duration between the two doses, the adjuvants used, age, ethnicity etc, and require detailed investigation.’’

Since the durability of immunity protection differs among different vaccines, the timing of booster dose should also vary accordingly, adds Awasthi.

 Frontline workers and those with comorbidities such as long-standing diabetes, heart diseases, kidney diseases, cancer and lung diseases should have priority access to booster doses, say experts.   Dr C. N. Manjunath, Karnataka COVID-19 task force says: “Now we have vaccines in surplus. I think government of India should come out with a policy on vaccine boosters.’’