Home / Science / Health /  The science behind increasing gap between Covishield doses

The government on Thursday extended the interval between two doses of Serum Institute of India’s Covishield vaccine to 12-16 weeks from four to eight weeks earlier, following a recommendation from a panel responsible for immunization. Mint examines the panel’s rationale behind the decision.

What has happened?

The National Technical Advisory Group on Immunization (NTAGI) on Thursday recommended increasing the interval between two doses of Covishield to 12-16 weeks from four to eight weeks earlier. The previous interval itself was a change from the four to six week gap that was originally prescribed for the vaccine when it got an emergency authorization from the Drugs Controller General of India.

However, two days later, on Saturday, the UK government, which was earlier using a 12-week interval, reduced the gap to eight weeks amid concern about the spread of the B.1.617 variant of the virus that originated in India.

Why was the interval changed?

NTAGI chairperson N.K. Arora said that the interval was changed following three sets of real-world data from the UK which showed that the vaccine was 65-88% effective if the interval was three months or more. The data he was referring to was published in The BMJ, a peer-reviewed medical journal run by the British Medical Association. The data from a large surveillance study conducted by the University of Oxford, along with the UK government, showed that infections fell by 65% after the first dose of either AstraZeneca or Pfizer vaccine, and researchers also found no evidence that the two vaccines differed in their ability to reduce infection rates. More importantly, the study was conducted between 1 December and 3 April, when the B.1.1.7 variant, first discovered in UK, was dominant.

While the community surveillance data was new, there had been indications as far as over three months back that a longer interval provided better protection. Other data sets, like AstraZeneca’s updated analysis of its UK trial in February showed that the vaccine is more effective when the interval between the two doses is longer. The factsheet for AstraZeneca’s vaccine, Vaxzevria, also shows a higher efficacy rate for an interval of 12 weeks or more at 80% as compared to 55% at less than six weeks and nearly 60% at six to eight weeks.

What are breakthrough infections, and is there a risk of such infections with longer intervals?

Breakthrough infections are illnesses, in this case covid-19, in which a vaccinated individual becomes sick from the same illness that the vaccine is meant to prevent. Arora said the surveillance study in the BMJ gave the panel confidence to increase the dosage interval because it showed infections fell sharply after the first dose itself. “Earlier, we didn’t have the confidence because of breakthrough infections. But this study gave us that confidence," he said. In this case, the panel wanted to understand the balance between putting a vaccinee at risk of breakthrough infection and the benefit of better protection.

Are allegations of increasing dosing interval due to shortage true?

There have been allegations that the dosing interval was increased because of the current shortage of vaccines, including Covishield. Arora denies it, saying that it is not going to cut off the need for a second dose either way. “What would be the point to it? Instead of giving it a month from first dose, the second dose will have to be given three months later, but it will still have to be given," Arora said.

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