Indian government last week extended the gap between two doses of the Covishield Covid-19 vaccine to 12-16 weeks, up from the previous maximum of eight weeks, on the recommendation of the National Technical Advisory Group on Immunisation. A day later, UK cut the gap from 12 weeks to eight weeks in view of the spread of the B.1.617 variant that originated in India. The National Technical Advisory Group on Immunisation (NTAGI) also suggested that pregnant women be given the option to choose their vaccine while lactating women be made eligible for the shot after delivery. But there is no evidence of an improved response of the second dose within the 12-16 week interval.
vaccine doses will be safe no matter when they are taken but will not provide any great “boosting” if given within less than a month or so of the previous dose. So vaccine doses can be taken any time after at least four weeks have passed from either the first dose or after an actual episode of Covid-19,” Rath, from New Delhi’s National Institute of Immunology.
If there are enough doses available, the second shot should be given at the optimal time as it will provide the best possible immunity in the present circumstances,” Bal, guest faculty at Pune’s Indian Institute of Science Education and Research, told PTI. A vaccine-triggered immune response, she admitted, will start declining with time. However, at the end of 12-16 weeks, it is unlikely to come to a complete baseline level.
According to World Health Organization (WHO)’s chief scientist, Dr Soumya Swaminathan, there is some data for some vaccines, including Covishield that’s been developed by AstraZeneca-Oxford University, where delaying the second dose up to 12 weeks actually gives a better immune boost. Earlier guidelines stated that the second dose of Covishield was to be taken 4-6 weeks after the first. This interval was subsequently extended to 4-8 weeks. Then, in April, the Centre advised that the second Covishield dose be taken 6-8 weeks after the first.
According to a peer-reviewed study published in The Lancet in February, the coronavirus vaccine developed by AstraZeneca and Oxford University is more effective when its second dose is given around three months after the first, instead of six weeks later. Vaccine supply is likely to be limited, at least in the short term, and so policy-makers must decide how best to deliver doses to achieve the greatest public health benefit, Prof Andrew Pollard of the University of Oxford said in the study.
Rath said the basis for this step seems to be based on the fact that there are good antibody levels in most people who have had Covid-19 and that they last for at least six months. So they are likely to be well protected for that period, and therefore there is no pressing need to vaccinate them before that,” the scientist said.
Most viral infections, Bal noted, generate enough immunity to provide protection for a few months post-recovery. Based on that knowledge, this recommendation is not unreasonable. NTAGI suggesting no changes in the protocol for Covaxin, the other vaccine being administered in India, Rath said this is likely because the recommendations are based on UK-based evidence with Covishield. There is no such evidence for Covaxin.