Explained: How Covid-19 vaccine gaps affect immunity

The rise of the Delta variant could push India back to a shorter gap to improve protection

One of the enduring debates that have run through the course of Covid-19 waves and vaccinations is the question of gaps between jabs. The contentious issue has had numerous studies conducted on it and governments flip-flopping on what the right policy should be.

The government on Friday asserted that there was no need to panic over an immediate need to change the dosage interval of the Covishield vaccine, which it had earlier widened to 12-16 weeks, stressing that reducing the time gap required proper scientific study in the Indian scenario.

Real world data from the United Kingdom had previously supported a gap of 12 weeks between doses as more effective, but with the rise of the new Delta variant, first identified in India, those inferences have lost some of their sheen. The UK itself has reversed its policy and shortened the gap between doses and newer studies have shown that a single dose may be less effective against the new variant than previously thought.

Read | Extending vaccine intervals may leave you vulnerable to variants: Dr Fauci

Why gaps are important to vaccines

Many Covid-19 vaccines are administered in two doses, with the first spurring an immune response, and the second “booster” shot redoubles that immune response to increase protection.

For some vaccines, a longer delay before a booster dose generates a stronger immune response, though the studies that test the gap need real-world data, which means identifying a sure shot and highly effective interval may take some time.

Why is it difficult to set policy regarding gaps?

Policymakers have struggled to balance the most effective gap between doses, vaccine supply, variants and the need to get the maximum number vaccinated as quickly as possible. While real-world data informs scientists recommendations, it may not always be possible to implement their advice.

A question most countries face with the rise of newer, more virulent variants is whether it is better to cover more people with a single dose or prioritise the second dose as vaccine stocks are limited.

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What do the experts say?

The most recent findings from Public Health England and a study published in the Lancet Journal have found that a single dose of the the AstraZeneca vaccine—Covishield in India—and the Pfizer vaccine both register a decline in efficacy to around 33 per cent, raising calls from some experts for India to reduce the vaccine gap.

The new findings go against real-world studies conducted in the UK that had not accounted for the Delta variant. The studies had shown that a longer gap of about 12 weeks actually improved the immune response, which had informed the Indian government’s decision to lengthen the gap between Covishield jabs.