There has to be a gap of 12-16 weeks between two doses of the Covishield vaccine in India after the Centre revised its guidelines in the middle of May. Around the same time though the UK cut the gap for jabs for certain populations from 12 to 8 weeks after finding that the newer variants of the novel coronavirus could blunt the effect of vaccines. Here’s the latest on the optimum interval between two doses and why some have called for the gap to be reduced.
Why The Gap Between Doses?
There has been considerable variation witnessed in how the two doses of this vaccine have been spaced by different countries. After earlier going for a 12-week gap between two doses, UK last month said that people who receive a first dose of the Oxford-AstraZeneca vaccine can come back for their second dose within 8, and not 12, weeks. Around the same time, India upped its gap to 12 weeks.
In the early days, the decision by the UK — which was also using the Pfizer-BioNTech mRNA vaccine — to extend the gap between two doses had generated controversy before experts cited data to say that a longer gap of about 3 months was shown to be effective in drawing the best immune response. The extra gap was also seen as being helpful because countries could then vaccinate a larger number of people with at least one dose as they waited for supply chains to free up. However, it is the rise of variants that has forced a review of this gap.
Why Did The UK Reduce The Gap?
Amid the rise of the B.1.617.2, or Delta, variant in the UK, the country’s health authorities decided to prioritise the population aged over 50 years and those with comorbidities for vaccinations with a shortened interval. The decision was based on studies that found the Oxford-AstraZeneca and Pfizer vaccines were only 33% effective against the Delta variant after the first dose, but the effectiveness went up to over 88% after two doses of Pfizer with the Oxford-AstraZeneca vaccine coming in at 60%.
What Have Indian Researchers Found?
A study by researchers at the National Centre for Disease Control (NCDC) and the CSIR Institute of Genomics and Integrative Biology (IGIB) in New Delhi has found that the second wave surge in cases in the national capital was mainly caused by the Delta variant.
The study reportedly found that the Delta variant is 50% more transmissible than the Alpha variant, or B.1.1.7, or the UK variant that had caused an earlier surge in that country. It added that prior infection and partial vaccination were “insufficient impediments” for checking the spread of the Delta variant. Further, the researchers said that the Delta variant was “over-represented", that is, more common, in post-vaccination breakthrough cases.
A report by the Kolkata-based The Telegraph quoted experts who said that India should reduce the gap between doses in light of evidence on how the new variant behaves.
In the meantime, it was reported that the Centre is looking at studying the effect of mixing shots from two different vaccines and also a single-dose of Covishield jab amid a shortage in vaccine supplies.