Thedecision to increase the gap between two Covishield doses from 4-6 weeks to12-16 weeks is based on scientific reason regarding behaviour of adenovectorvaccines, said Chairman of India's Covid-19 Working Group of the NationalTechnical Advisory Group on Immunisation (NTAGI), N.K. Arora.
Citingthe 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 vaccineefficacy varied between 65 per cent to 88 per cent when interval is 12 weeks.
"Thiswas the basis on which they overcame their epidemic outbreak due to the Alphavariant. The UK was able to come out of it because the interval they kept was12 weeks," the doctor said as questions were raised when the governmentannounced to increase gap between Covishield doses.
"Wealso thought that this is a good idea, since there are fundamental scientificreasons to show that when interval is increased, adenovector vaccines givebetter response. Hence the decision was taken on May 13, to increase theinterval to 12-16 weeks."
Thisalso gives flexibility to the community, since everyone cannot come atprecisely 12 weeks or so, said Arora, adding "we have a very open andtransparent system where decisions are taken on scientific basis".
TheCovid Working Group took that decision, with no dissenting voice, he clarified.
"Thisissue was then discussed threadbare at an NTAGI meeting, again with nodissenting notes. The recommendation was that the vaccine interval has to be12-16 weeks."
Arorasaid that the earlier decision of four weeks was based upon the bridging trialdata available then and that the increase in gap between two doses was based onstudies that showed higher efficacy with increase in gap.
"Initialstudies on Covishield were very heterogeneous. Some countries like the UK wentfor a dose interval of 12 weeks when they introduced the vaccine, in December2020.
"Whilewe were privy to this data, when we had to decide our interval, we went forfour weeks interval based on our bridging trial data which showed good immuneresponse. Later we came across additional scientific and laboratory data, basedon which after six weeks or so, we felt we should increase the interval fromfour weeks to eight weeks, since studies showed that vaccine efficacy is about57 per cent when it is four weeks and about 60 per cent when it is eightweeks."
Speakingabout 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 otherbiggest user of AstraZeneca vaccine)."
Healso said that there were other examples like Canada, Sri Lanka and few othercountries which are using 12-16 weeks interval for AstraZeneca vaccine which isthe same as Covishield vaccine.
Aroraexplained how emerging evidence and reports regarding efficacy of partialversus full immunization were being considered by NTAGI.
Healso referred to a study by PGI Chandigarh which compared effectiveness ofpartial versus full immnunization.
"APGI Chandigarh study very clearly shows that vaccine effectiveness was 75 percent for both partially immunized and fully immunized. So at least in the shortrun, effectiveness was similar whether you are partially or fully vaccinated.This was in relation to the Alpha variant which had swept Punjab, NorthernIndia and came to Delhi. This also meant that even if you received only onedose, still you are protected."