Mixing Covid vaccines - which some data suggests could generate better immunity or more antibodies - is a "definite possibility" but more information is needed before a decision can be taken, AIIMS chief Dr Randeep Guleria told NDTV on Saturday morning.
"This is something that has been looked at in the past - giving one vaccine as the priming shot and another as the booster. Some data suggests mixing vaccines leads to slightly higher side effects, but other data suggests it may lead to better immunity and antibody protection," Dr Guleria said.
"One needs more data... a large number of vaccines will be available in the future... you will have Pfizer, Moderna, Sputnik V and Zydus Cadila. Therefore which combination is better is something we don't know at this time... but yes, initial studies suggest it may be an option," he explained.
The government is working on this and results of trials will be available in a few months, it has said.
Last month the preliminary results of a British study were published in The Lancet.
Participants were given a first dose of the AstraZeneca shot (Covishield) and a second of Pfizer (not yet in India), and reported more short-lived (but mild) side effects. Data on efficacy is still pending.
A Spanish study reported by news agency Reuters found the combination to be safe and effective.
Dr Guleria also played down fears existing vaccines may be ineffective against the 'delta plus' variant, saying more data is needed to establish the mutated strain's potential immune escape abilities.
He stressed the importance of being vaccinated despite these fears. "... if you are vaccinated fully and come in contact with the virus you may still be infected, but the severity may be much lesser.
He also expressed concern a single dose may be insufficient against 'delta' variants. Published medical research suggests one dose offers 33 per cent protection and both doses nearly 90 per cent.
"It is a cause of concern that the primary dose of the vaccine may not be enough to tackle the 'delta' variant. We might need to give the booster dose much earlier to ensure better safety," he said, as scientists argue the government's decision to expand interval between two doses.
On the topic of the 'delta plus' variant of COVID-19 Dr Guleria said the government is "closely monitoring" the situation, but the 'delta' strain is of more immediate concern.
"We are very closely monitoring that (the 'delta plus' variant) (but) at the moment 'delta plus' is not the dominant variant in India... the 'delta' variant is. So we need to actively track it... do genome sequencing to see how this variant is behaving in our population and prepare accordingly," he said.
There are 48 cases of 'delta plus' COVID-19 in 11 states, the government said Friday.
Dr Guleria also said the third wave of infections in India is unlikely to be as severe as the second.
"There are a lot of debates on whether the third wave will be more severe than the second... my feeling is the subsequent wave will not be as bad," he said, while also cautioning against underestimating the virus; he said "we need to learn from the second... to deal with the third".
A study by the ICMR and the UK's Imperial College London indicated the same thing; it said the third wave required "extreme scenarios for abrogation of (existing) immunity" to be as bad as the second.
The second Covid wave hit India hard with lakhs infected and thousands dying every day. At its peak in early May the country was reporting over four lakh cases and 4,000 deaths per day.
Worryingly, the number of deaths in some states has since been revised amid accusations of undercounting; the new data suggests lakhs of deaths were ignored or not reported.
The second wave also triggered a massive healthcare crisis - hospitals struggled to deal with the influx of patients and critical resources like oxygen, medicines and ventilators were scarce.
Experts have flagged a potentially complete breakdown of the system in the event of a more virulent third wave of infections, and urged the government to improve health infrastructure.
Dr Guleria, like other experts, has warned against rushing to lift Covid restrictions, particularly if there is even a slight surge in cases.
"Wherever in the country we notice even a slight surge, it should be a red flag. We need to take immediate measures in those areas immediately rather than wait for it to spread," he said.
This week Maharashtra - which was set to lift lockdown after seeing off the worst of the second wave - revised its plans and said all districts had to enforce Level 3 restrictions in light of the 'delta plus' threat.