Home / News / India /  Explainer: Impact of Covishield’s 4-8 week interval on India’s covid vaccination

The government-approved gap between the first and the second dose of Serum Institute of India’s Covishield vaccine has been extended to up to eight weeks from up to six weeks earlier. Mint explains the reasons for the move and its potential impact:

Why did the government decide to increase the dosing interval for Covishield?

The decision was based on a recommendation by the National Technical Advisory Group on Immunisation (NTAGI) and subsequently the National Expert Group on Vaccine Administration for COVID-19 (NEGVAC). The two groups made the recommendation after the NTAGI saw the available data from AstraZeneca and Oxford’s clinical trials for the vaccine in the UK and Brazil, as well as Serum Institute’s own bridging study in India. The data showed that protection provided by Covishield is enhanced if the second dose is administered between 6-8 weeks, but not so if it was more than eight weeks.

The panels’ recommendation follows a fresh analysis of interim efficacy data by AstraZeneca in February, which showed that the vaccine’s efficacy increased from 54.9% to 59.9% if the interval between first and second dose was increased from less than 6 weeks to 6-8 weeks.

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What intervals are foreign regulators following?

The two most prominent recommendations for the AstraZeneca vaccine and Covishield are from the UK Medicines and Healthcare Products Regulatory Agency and the World Health Organization (WHO). India’s decision varies from the two. While the UK MHRA suggests a 4-12 week interval, the WHO has suggested an 8-12 week interval.

However, N.K. Arora, chair of covid subcommittee of NTAGI, said the WHO recommendation and MHRA decision have to be viewed in the context of scarcity of vaccines, something which India does not face, being home to the world’s largest vaccine manufacturer, Serum Institute.

What are the benefits of increasing the interval?

The biggest benefit will be that it would free up doses for people to get their first Covishield shot, which in turn will allow the government to include a much larger proportion of the population in its covid immunisation programme. Covishield already accounts for around 90% of the covid vaccines administered in India, with the rest comprising of Bharat Biotech International’s Covaxin. So far, nearly 45 million people have received Covishield while less than a tenth the number have received Covaxin, according to government data.

What are the risks?

Some experts are concerned that increasing the interval could leave vaccinated with only partial protection for longer, which in turn would increase the risk of getting an infection before they get their second dose. A few have also raised concern that exposing more people with only partial protection against the virus could, at least in theory, increase the risk of the virus mutating.

Arora said the panel is aware of both the risks, especially with “breakthrough infections" after the first dose. He said NTAGI and the government are monitoring the data on infections that have taken place before participants received the second dose.

Will the AstraZeneca US efficacy data and risks of increasing interval affect future recommendations?

AstraZeneca on Monday also announced interim results from its US trials, which showed that the vaccine had an efficacy of 79% at preventing symptomatic covid-19 when two standard doses are given four weeks apart.

The US data, along with others that are generated in India, will be examined by the government panel, and recommendations will be updated later to adapt to the latest available scientific data, Arora has said, adding that even now, people will have the option of taking the vaccine at four weeks.

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