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.
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 Serum Institute of India - originally developed by AstraZeneca and Oxford University in the UK.
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%, as opposed to its 53% 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 for a longer (12-week) gap.
Davinder Gill, a vaccine expert based in Massachusetts, told Mint, “My view is that this 53% efficacy for two full doses given less than six weeks apart is just slightly above the threshold of 50%, 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% with a gap of less than six weeks and 51.08% with a gap of six to eight weeks.
However, the efficacy reached 60.55% with a dose interval of 9-11 weeks, and 78.79% with 12 weeks or more, said the company insert.
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.”Dr Shahid Jameel
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 analyzed 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% 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% efficacy is a result of combining data from two studies that used different dose regimens and arrived at 62% and 90% 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 metaanalysis but even there we look carefully at the studies involved before drawing inferences.”Anant Bhan
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 UK and Brazil trials to arrive at 70% efficacy. The dosing was different.”
(With inputs from Mint)
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