3 min read.Updated: 14 Jan 2021, 09:49 PM ISTLeroy Leo
Vaccine experts say that 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
The government’s decision to administer the second dose of Covishield vaccine just four weeks after the first may lead to efficacy of just 53%, the vaccine’s package insert showed. This is slightly above the cut-off of 50%, though a longer gap of at least 12 weeks would have raised the efficacy to as much as 79%.
“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," said Davinder Gill, a vaccine expert based in Massachusetts, US.
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Drugs Controller General of India (DCGI) V.G. Somani last week gave an emergency use licence to the Covishield vaccine. The subject expert committee, based on whose recommendation DCGI authorized the use, had recommended that the second dose should be administered between 4 and 6 weeks after the first dose. It, however, acknowledged there is data available for administration of the second dose up to 12 weeks after the first dose from the overseas studies.
The overseas studies referred to are those conducted by AstraZeneca Plc and the University of Oxford, the developers of the original vaccine. According to the package insert, which broke down efficacy levels for two full doses based on the interval between shots, the efficacy was 53.28% if the second shot was given at an interval of less than six weeks and 51.08% if it was six to eight weeks. However, both the results were of poor statistical significance due to the small sample size.
The package insert also showed that efficacy result was 60.55% if the interval between dosages was 9-11 weeks, and, more importantly, if the gap between the two shots was 12 weeks or more, the efficacy was 78.79%.
While this makes a case for increasing the interval between dosages, Gill believes it would lead to complications in a pandemic setting.
“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 vaccines might not remember three months later that they have to take a second dose," Gill said.
The licensed dosing regimen of 4-6 weeks in India was driven largely by Serum Institute’s own trial using two full doses four weeks apart, even as AstraZeneca’s much-criticized trial used varying doses across different intervals due to mistakes the researchers made during the study.
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.
The 70% efficacy that Kulkarni referred to is the result AstraZeneca got after pooling across dosages and gaps between shots. These included data from the lower dosage that had 90% efficacy, as well as standard dosage, which had 62% efficacy, and the gap across the trial varied as widely as 4-12 weeks.
However, experts said it is prudent to use efficacy data that matches the dosing regimen being used by the government.
“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," said Anant Bhan, a bioethics expert, adding that the data around the dosing regimen is more significant from a public health perspective than the average