Explained: To roll out Covid-19 booster vaccines, or not to

What is a ‘booster’ dose?

Most Covid vaccines are administered in two doses, with a few given as a single dose. A booster is an additional shot given after the protection provided by the original shot(s) has begun to decrease over time, so that people can maintain their level of immunity for longer.

“How long immunity developed by an infection or a vaccination will last will vary depending upon various factors,” said leading immunologist Dr Vineeta Bal. For example, antibodies decay over time, and even memory T-cells will die after a few years or months.

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.

What is the published evidence of waning immunity following Covid vaccination?

In September, a study at ICMR–Regional Medical Research Centre, Bhubaneswar showed a significant drop in antibodies against Covid-19 within four months after complete vaccination.

Another study, on the immune response to mRNA vaccines, was published in Science earlier this year. At six months, it found declining antibody levels, but durable memory B cell and T cell responses. Most of the B cells were able to cross-bind with the Alpha, Beta and Delta variants.

A study in the US showed that antibodies reduce by more than 80% six months after receiving the second dose of the Pfizer-BioNTech mRNA vaccine.

A health worker shows empty vials after administering Covid-19 vaccines to beneficiaries at a government hospital in Bengaluru, Friday, Dec. 10, 2021. (PTI Photo/Shailendra Bhojak)

Have studies been done on the effect of a booster dose?

BioNTech and Pfizer have said a three-shot course was able to neutralise the Omicron variant in a laboratory test; the third dose increased neutralising antibodies by a factor of 25.

A recent study by Indian Council of Medical Research (ICMR) scientists suggested use of booster shots of Covishield to fight emerging variants. The study, posted on bioRxiv and yet to be peer-reviewed, evaluated the neutralising potential of blood samples from people who received two doses of Covishield.

Under what other circumstances can administering a booster be considered?

Breakthrough infections with Delta are associated with high viral loads. If community transmission is high, boosters would help in controlling viral spread, said Dr Sanjay Pujari, member of the ICMR national task force on Covid-19. He noted that daily case counts in Israel have remained low after booster coverage increased.

“In India, where there are no surplus stocks of different vaccines, the policymakers need to take a call whether to aggressively enhance the vaccine coverage and whether booster shots can be given, at least to those who were vaccinated more than six months ago. Particularly those with comorbidities and frontline health workers,” said scientist Dr V S Chauhan.

Experts such as Dr Bal said boosters are not an answer to surges, especially in an outbreak area. In any community, mass vaccination for specific diseases provides a robust way of preventing morbidity. The goal should be vaccination of the entire eligible population with the recommended number of doses, they stressed.

Indian Medical Association president Dr J A Jayalal stressed the need for the Centre to roll out a booster for healthcare and frontline workers and immunocompromised persons. “Immunity is sufficient to face normal infection. Now, with another variant, Omicron, there are reports indicating high transmissibility, and in such an event, it will be healthcare workers who will be most at risk due to increased frequency of exposure. The viral load average immunity will not be sufficient… Firstly, of course, all people in the country should be vaccinated,” he said.

What is the reasoning of some countries in administering boosters?

The US Centers for Disease Control and Prevention (CDC) has recommended booster shots, at least six months after their second dose, for all adults who received a Pfizer-BioNTech or Moderna vaccine. The European CDC has urged citizens to get fully vaccinated and adhere to recommendations on booster vaccination.

In many European countries where vaccinations started almost a year ago, there has been a surge in the pandemic. Armed with vaccine stocks, these countries and the US have rolled out booster shots for the last several months.

Experts pointed out, however, that the situation in the US and Europe is different compared to India. The number of Delta variant-caused cases is increasing in many countries.

Besides, there is some preliminary evidence that immunity mediated by mRNA vaccines (such as those from Pfizer-BioNTech and Moderna) starts waning earlier than that mediated by adenovirus-vector-based vaccines (such as Covishield), especially for T cell response. This was discussed in a letter published In the New England Journal of Medicine last month. “Hence mRNA vaccine recipients might be more in need for boosters earlier than Indians (since 80+% vaccine recipients in India have received Covishield),” Dr Bal said.

Why is the WHO not throwing its weight behind boosters?

According to a WHO statement in October, the degree of waning of immunity and need for booster doses may differ between vaccine products, target populations, circulating SARS-CoV2 virus, in particular variants of concern, and intensity of exposure. Introducing booster doses should be evidence-driven and targeted to groups in greatest need; improving coverage of primary vaccination series should be prioritised over booster vaccination, it said.

What is India’s policy on booster doses?

On Friday, the Health Ministry said two experts bodies are still examining this. About the WHO, Dr V K Paul, head of India’s Covid-19 task force, noted: “They have absolute clarity on the administration of a booster. It is under consideration. It has clearly and emphatically highlighted the need to complete primary vaccination, as the most important priority. Our thinking and the overall picture are also aligned with the same approach to complete the task of vaccinating adults with two doses,” Paul said.

ICMR chief Dr Balram Bhargava also said India is still examining the issue of booster doses. “The data (on antibody response) has shown that vaccines are still effective after nine months to a year. This depends on the type of vaccines,” Bhargava said.

Newsletter | Click to get the day’s best explainers in your inbox

How should we view booster shots in the context of Omicron?

Information on Omicron variant is still emerging. ICMR chief Dr Balram Bhargava said India will soon begin testing of Covaxin and Covishield against Omicron.

Experts said Omricon appears to spread very easily in the unvaccinated population, based on reports from Africa. Whether this will be the case in fully vaccinated people is unclear. Expert accounts from Africa suggest the disease itself is mild, and hospitalisation is mostly not needed. But once again, a lot more data from other places is needed before this can be confirmed as a general trend.

“Israel is an example of how mass booster vaccination have kept their case numbers low. Although some triple vaccinated individuals have tested positive for Omicron, they have had mild symptoms and there has been no rapid spread. However, further long-term data is needed,” leading immunologist Dr Vineeta Bal said.