The decision to increase the gap between two Covishield doses from 4-6 weeks to 12-16 weeks is based on scientific reason regarding behaviour of adenovector vaccines, said Chairman of India's Covid-19 Working Group of the National Technical Advisory Group on Immunisation (NTAGI), N.K. Arora.
Citing the data released in the last week of April this year by Public Health England, UK's executive agency of the Department of Health, Arora explained that vaccine efficacy varied between 65 per cent to 88 per cent when interval is 12 weeks.
"This was the basis on which they overcame their epidemic outbreak due to the Alpha variant. The UK was able to come out of it because the interval they kept was 12 weeks," the doctor said as questions were raised when the government announced to increase gap between Covishield doses.
"We also thought that this is a good idea, since there are fundamental scientific reasons to show that when interval is increased, adenovector vaccines give better response. Hence the decision was taken on May 13, to increase the interval to 12-16 weeks."
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This also gives flexibility to the community, since everyone cannot come at precisely 12 weeks or so, said Arora, adding "we have a very open and transparent system where decisions are taken on scientific basis".
The Covid Working Group took that decision, with no dissenting voice, he clarified.
"This issue was then discussed threadbare at an NTAGI meeting, again with no dissenting notes. The recommendation was that the vaccine interval has to be 12-16 weeks."
Arora said that the earlier decision of four weeks was based upon the bridging trial data available then and that the increase in gap between two doses was based on studies that showed higher efficacy with increase in gap.
"Initial studies on Covishield were very heterogeneous. Some countries like the UK went for a dose interval of 12 weeks when they introduced the vaccine, in December 2020.
"While we were privy to this data, when we had to decide our interval, we went for four weeks interval based on our bridging trial data which showed good immune response. Later we came across additional scientific and laboratory data, based on which after six weeks or so, we felt we should increase the interval from four weeks to eight weeks, since studies showed that vaccine efficacy is about 57 per cent when it is four weeks and about 60 per cent when it is eight weeks."
Speaking about why the NTAGI did not increase the gap earlier to 12 weeks, he said, "We decided we should wait for ground-level data from the UK (the other biggest user of AstraZeneca vaccine)."
He also said that there were other examples like Canada, Sri Lanka and few other countries which are using 12-16 weeks interval for AstraZeneca vaccine which is the same as Covishield vaccine.
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Arora explained how emerging evidence and reports regarding efficacy of partial versus full immunization were being considered by NTAGI.
He also referred to a study by PGI Chandigarh which compared effectiveness of partial versus full immnunization.
"A PGI Chandigarh study very clearly shows that vaccine effectiveness was 75 per cent for both partially immunized and fully immunized. So at least in the short run, effectiveness was similar whether you are partially or fully vaccinated. This was in relation to the Alpha variant which had swept Punjab, Northern India and came to Delhi. This also meant that even if you received only one dose, still you are protected."