Could Govt’s 4-Week Gap for Second Covishield Dose Lower Efficacy?

“In a pandemic when you are protecting those most at risk, it’s not prudent to wait 12 weeks,” Dr Jameel tells FIT.

3 min read
A health worker shows the ‘Covishield’ after the arrival of the first batch of the vaccines from the Serum Institute of India at the Civil Hospital in Ahmedabad.

As India gears up to launch the world’s largest COVID vaccination drive on 16 January, concerns regarding the optimum dose interval to maximise Covishield’s efficacy have emerged, according to a report by Mint.

The government has planned to administer the second dose four weeks after the first dose for the vaccine produced by the Serum Institute of India, originally developed by AstraZeneca and Oxford University in the United Kingdom.

According to Covishield’s package insert, the vaccine’s efficacy when given in a gap of at least 12 weeks can be as high as 79 percent, as opposed to its 53 percent efficacy with a gap of less than six weeks.

Experts are worried about the government-stipulated interval of four weeks between the two doses, but they also acknowledge that a pandemic situation may not allow a longer (12-week) gap.

Davinder Gill, a vaccine expert based in Massachusetts, told Mint, “My view is that this 53 percent efficacy for two full doses given less than six weeks apart is just slightly above the threshold of 50 percent, but even that is not enough to meet the statistical criteria to generate enough confidence.”

Two vaccine candidates – SII’s ‘Covishield’ and Bharat Biotech’s ‘Covaxin’ – were earlier given emergency use approval by India’s top drug regulator. The subject expert committee recommended a 4-6 weeks gap between the two doses for Covishield, also noting that data for a gap of up to 12 weeks was available from the international trials conducted by AstraZeneca and Oxford University.

Based on trials conducted on a small sample size, it was found that the efficacy was 53.28 percent with a gap of less than six weeks and 51.08 percent with a gap of six to eight weeks.

However, the efficacy reached 60.55 percent with a dose interval of 9-11 weeks, and 78.79 percent with 12 weeks or more, said the company inserts.

But Is Extending the Dose Interval Practical in a Pandemic?

Speaking to FIT, Dr Shahid Jameel, Virologist and Director, Trivedi School of Biosciences at Ashoka University, says,

“In a pandemic when you are protecting those most at risk, it’s not prudent to wait 12 weeks. You need some protection as quickly as possible.”

Davinder Gill made a similar point in the Mint report when he said, “Increasing the gap between intervals to two-and-a-half or three months is not pragmatic. It is a pandemic, and there is chaos. Ideally, you would want to complete the vaccination course as quickly as possible and be done with it. Even the vaccine (beneficiaries) might not remember three months later that they have to take a second dose.

Serum Institute Executive Director Prasad Kulkarni, in a statement replying to Mint’s queries, said that “the vaccine’s efficacy should be analysed in its entirety because the data is statistically powered".

“Based on the cumulative results from the UK and Brazil trial, the vaccine has an efficacy rate of 70 percent in a two-dosage regimen, irrespective of the dosing interval. This is a statistically valid figure, and we should rely only on it," Kulkarni said.

It must be noted here that the 70 percent efficacy is a result of combining data from two studies that used different dose regimens and arrived at 62 percent and 90 percent efficacy levels, respectively.

Anant Bhan, a bioethics expert, told Mint,

“A lot of people have already disputed that average is not the right way to look at it because they (Astra Zeneca) have pooled in data from two different trials. We do pool data in meta-analysis but even there we look carefully at the studies involved before drawing inferences.”

He said that the data around the dosing regimen is more significant from a public health perspective than the average.

In conversation with FIT, Dr Jameel also added, “I don’t agree with the pooling of the UK and Brazil trials to arrive at 70 percent efficacy. The dosing was different.”

(With inputs from Mint.)

(This story was first published on FIT and has been republished here with permission.)

(The Quint is available on Telegram. For handpicked stories every day, subscribe to us on Telegram)

Stay Updated

Subscribe To Our Daily Newsletter And Get News Delivered Straight To Your Inbox.

Join over 120,000 subscribers!