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Covaxin, Covishield effectiveness highest when gap between doses is 6-8 weeks, ICMR study finds

Covishield is administered in India at an interval of 12-16 weeks, and Covaxin at 4-6 weeks. Study, yet to be peer-reviewed, also says vaccines are effective against Delta and other variants.

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New Delhi: The effectiveness of the two Covid vaccines — Covishield and Covaxin — is the highest when the interval between two doses is between six to eight weeks, a new study from the Indian Council of Medical Research (ICMR) has said.

The study, yet to be peer-reviewed and posted on a pre-print portal on 11 November, also said two doses of the vaccines are effective against Delta and other variants.

While Phase 3 clinical trials have evaluated the efficacy of Covid vaccines, it remains important to evaluate its effectiveness in the real world, particularly from a healthcare perspective.

Researchers from ICMR-National Institute of Epidemiology (ICMR-NIE) reviewed the evidence of the effectiveness of the two vaccines being used in India until 31 October 2021.

The team reviewed data from the National Library of Medicine article database and MedRxiv, the pre-print publication portal, using the terms effectiveness, BBV152, Covaxin, AZD1222, Covishield, India and Covid-19. BBV152 and AZD1222 are the official names of the two vaccines.

The researchers found that the overall effectiveness of two doses of Covishield was 80 per cent while that for Covaxin was 69 per cent against severe Covid-19.

“Our findings indicate a substantial reduction in the risk of severe Covid-19 and particularly against the Delta strain,” the researchers wrote.

According to the team, the results from the study suggest that the strategy for vaccination in the country is effective, particularly since most parts of India have high seroprevalence.

“Continued expansion of the two-dose vaccination coverage and stringent application of control measures remain warranted to prevent a surge in cases in the near future in the country,” they added.

Talking about the objective of the study, the researchers said they found very few studies in India describing the effectiveness of Covaxin or Covishield vaccines against Covid have been conducted across various geographic areas, using different sampling and recruitment strategies, as well as study design.

Most studies are limited to healthcare workers and policemen in smaller subnational areas, with a small sample size, they said, adding that this is because these frontline workers were among the first to be vaccinated in India.

The team noted that, according to these studies, the vaccine effectiveness ranges from 77 per cent to 92 per cent with wide confidence intervals. Moreover, no study provided a vaccine effectiveness estimate specifically against the Delta variant.

“This multicentric hospital-based study across 11 cities represents vaccine effectiveness at the national level, covering India’s large expanse and during the period of dominance of the Delta strain of SARS-CoV-2 in the country,” the team wrote in the study.

Also read: ‘Die in 2 yrs, become a monkey’: Why this Nagaland district has India’s lowest vaccine coverage

Overall effectiveness

The team found that vaccine effectiveness was highest for an interval of 6-8 weeks between two doses of either vaccine. This was significantly higher when the gap was less than six weeks.

When calculated separately, the vaccine effectiveness was highest — at 92 per cent — for an interval of 6-8 weeks for Covishield. When the gap between the two doses was less than six weeks, effectiveness dropped to 76 per cent. The vaccine is currently being administered in India at an interval of 12-16 weeks.

For Covaxin, vaccine effectiveness was 91 per cent when the two doses were administered at an interval of 6-8 weeks. If the dose gap was under six weeks, the efficacy dropped to 65 per cent. The current interval in India is 4-6 weeks.

There was not enough data to evaluate the efficacy of two doses taken after a gap of over 12 weeks, the researchers said.

(Edited by Neha Mahajan)

Also read: ‘Cut Covishield gap, skies won’t fall’, says Covid task force member as vaccines pile up


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