A booster dose against COVID-19 raises the number of circulating antibodies and is shown to increase protection from symptomatic infection with Omicron, scientists have said, underlining that boosters can be the simplest step forward, especially for the immunosuppressed.

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Reacting to the statement of UK Health Security Agency (UKHSA) said that a booster dose of Covishield vaccine is effective against Omicron and a third booster dose of COVID-19 vaccine provides 70-75 per cent protection against symptomatic infection from the Omicron variant, the virologists and epidemiologists underlined that booster doses of any vaccine (except live attenuated like oral polio vaccine or OPV, measles) exponentially raises antibody level.

Eminent virologist Dr Shahid Jameel said a booster shot after two doses increases the amount of circulating antibodies and is shown to increase protection against symptomatic infection with Omicron. "We don't know how well two doses continue to protect against severe disease," he stated.

On what India should do where the majority of the population is vaccinated with Covishield, the former head of the advisory group to the Indian SARS-COV-2 Genomics Consortia (INASACOG) said it must be ensured that those who got only one dose of Covishield get the second dose in 8-12 weeks instead of 12-16 weeks.

"Carry out lab studies with Omicron to learn how well sera from Indian vaccines of Covaxin and Covishield neutralise the virus. Make a policy on boosters. What vaccines to use? Who should get it? And when? Make a policy and start vaccinating children starting with adolescents," he said.

Jameel said in India, four vaccines can be used as boosters: Covaxin in people who got Covishield and vice versa, DNA vaccine ZyCov-D, Covovax protein vaccine from SII and Corbev ax-E protein vaccine from Biological E.

The Health Ministry told the Lok Sabha last week that the National Expert Group on Vaccine Administration for COVID-19 (NEGVAC) and the National Technical Advisory Group on Immunisation (NTAGI) are considering scientific evidence related to justification for booster doses against the coronavirus.

Noted virologist Dr T Jacob John said booster doses of any vaccine (except live attenuated like OPV, measles) exponentially raises antibody level: "Pfizer vaccine some 40-fold high". "If we are interested in being cautious about the unknown risks of Omicron, boosters for as many as possible is the simplest step forward, especially for the immunosuppressed, seniors and those with co-morbidities. This is in the best interests of their welfare," he reported.

John, the former director of the ICMR's Centre of Advanced Research in Virology, stressed that children must be vaccinated too since an unvaccinated large segment of the population can act as reservoirs -- "plus Omicron seems to go after children". "Better to prevent than to wait for evidence. Protection delayed can also mean protection denied," he said.

Dr Giridhara R Babu, professor and the Head of life course epidemiology at the Public Health Foundation of India, said the evidence is gradually mounting in stressing the need for boosters eventually to everyone. "However, data on physical outcomes are important in prioritising the booster doses. Protection against hospitalisation or deaths is important in analysing the need for prioritising boosters compared to receiving only two primary doses," he said.

Babu said for developing countries, the priority still remains to reach the unreached (providing two primary doses to those not already vaccinated). Among the fully vaccinated, the available evidence points to the usefulness of providing booster doses to the elderly and those at high risk, including immunocompromised, he said.

Dr Chandrakant Lahariya, Physician epidemiologist and public policy specialist, however, said booster is not a priority for the country and at least for the Indian context, Omicron has not changed anything and India should do to collect more indigenous data and evidence for decision making on the booster.

"The vaccines being used in India continue to protect against severe disease, hospitalisation and deaths. Therefore, the focus continues to be on administering a first and second shot to as many adults as possible," he stated. Elaborating further, he said, "In the current stage of the pandemic, every study on vaccine effectiveness (in general and against Omicron) should be interpreted in the context of that setting".

"What all vaccine effectiveness studies are pointing out is that existing vaccines continue to hold up against the key objective of COVID-19 vaccination drives that is to prevent severe diseases, hospitalisation and deaths. This holds true for the Omicron variant," Lahariya added.

Lahariya said neutralising studies are also pointing out that hybrid immunity through natural infection and at least one shot of vaccine provides far greater protection than vaccines alone. India has the situation of hybrid immunity where high seroprevalence and one dose coverage give assurance that people are protected, he said.

"Considering the effectiveness of the COVID-19 vaccines being used in India against all symptomatic diseases is unknown or very limited data is available, therefore, we should not use studies and arguments of administering booster doses to improve protection from symptomatic diseases. As preventing all symptomatic diseases is not, at least at present, the purpose of ongoing COVID-19 vaccine drive in India," he said.