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Explained: Why has the interval between Covishield vaccine doses been raised to 8 weeks?

Covishield is Serum Institute of India’s version of AZD1222, the vaccine developed by AstraZeneca in collaboration with the University of Oxford.

Written by Prabha Raghavan | New Delhi |
March 23, 2021 3:30:52 am
Covishield, Covishield doses, Covishield news, Covishield vaccines, Covishield efficacy data, Indian ExpressA dose of Covishield being prepared in Jodhpur. (Express photo by Nirmal Harindran)

The government has decided to increase the interval between the first and second doses of Covishield to up to eight weeks in its ongoing vaccination drive against Covid-19. Covishield is Serum Institute of India’s version of AZD1222, the vaccine developed by AstraZeneca in collaboration with the University of Oxford.

Some data from global trials of AZD122 suggests that extending the duration between doses to 12 weeks increased its efficacy much more. On the other hand, interim findings reported on Monday from trials in the US, Peru and Chile showed that the vaccine had an efficacy of around 79% even when the second dose was given four weeks after the first dose.

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

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The Ministry of Health and Family Welfare took the decision on the recommendation of two expert groups – the National Technical Advisory Group on Immunisation (NTAGI) and National Expert Group on Vaccine Administration for Covid-19 (NEGVAC). The groups, after looking into available scientific evidence from clinical trials of the vaccine, concluded that the protection that it provides against Covid-19 is “enhanced” if the second dose is administered between 6-8 weeks.

What do other studies say about this vaccine’s dosing interval?

According to data from trials of AZD1222 in other countries, the efficacy of the vaccine increased when the second dose was given more than six weeks after the first. Efficacy in this case is the vaccine’s ability to bring down cases of symptomatic Covid-19 in those inoculated, compared with those who are not.

AZD1222’s efficacy was around 54.9% when the second dose was given less than six weeks after the first dose, as per a February study analysing Covid-19 cases in phase 3 clinical trial participants across the UK, Brazil and South Africa. The efficacy increased to 59.9% when the second dose was given 6-8 weeks after the first dose, 63.7% when the second dose was at 9-11 weeks, and 82.4% when the dosing interval stretched to 12 weeks or more. This study, which was submitted to The Lancet in February, has not yet been peer-reviewed.

What about the latest findings?

According to Oxford University and AstraZeneca, interim results from phase 3 clinical trials conducted on 32,000 participants across the US, Chile and Peru show that the vaccine had an efficacy of 79% against symptomatic Covid-19 when the interval between doses was four weeks. More importantly, the efficacy in the cases of severe or critical symptomatic Covid-19 was 100%.

The efficacy found in these trials is much higher than its efficacy in trials conducted in countries like the UK and Brazil.

Why did India increase the interval to eight weeks, and not longer?

UNCONVINCING DATA: One reason is that the expert groups looking into the evidence concluded that the vaccine would not be able to provide enhanced protection if its dosage interval was increased beyond eight weeks, according to a letter written by Health Secretary Rajesh Bhushan to all states and Union Territories.

According to Dr N K Arora of NTAGI, one of the groups that looked at this issue, there was “no good scientific evidence” to support a recommendation to increase the interval beyond eight weeks, especially given that India does not have a shortage of the vaccine. “We have looked at every bit of data, whatever is available…we are not convinced,” he said.

“This recommendation (to increase the interval beyond eight weeks) is more for countries and societies where there is a deficiency of (the) vaccine. India is in a very unique position. We are vaccine sufficient,” he added.

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POTENTIAL RISKS: There are also potential risks of “breakthrough infections” without any “significant benefit” in terms of efficacy that the groups identified with extending the dosing interval, especially at a time when cases are rising in the country.

“Increasing the dosage interval (too much) is fraught with the potential of dreadful infection between the two doses. So, if I give the first dose and wait until 12 weeks, there is a possibility that some people might get Covid infection in between. (We) don’t want that,” said Dr Arora.

Even if extending the dosing interval would lead to an increase in antibodies, Dr Arora said that this does not necessarily mean that it would provide more protection. “There is no direct relationship between increased antibodies and better protection,” he said.

What does this mean for India’s ongoing vaccination campaign against Covid-19?

Delaying the second dose could potentially mean that more doses are freed up for a larger number of people to get their first dose of the vaccine sooner. However, the government also feels that allowing this increase in interval would make it easier for the priority group population, which mostly consists of the elderly, to be vaccinated.

“Now there is flexibility… you can get the vaccine anytime between 28 to 56 days,” said NTAGI’s Dr Arora.

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