The science on vaccines is continually changing, and a new study might change when someone gets their second Covid-19 vaccine dose.

The Oxford-AstraZeneca vaccine, manufactured in India by Serum Institute of India and sold as Covishield, has shown greater efficacy when the second dose is administered after a gap of 12 weeks, according to a study published in the medical journal The Lancet on March 6. Currently, Indians getting the Covishield shot need to get their second prime-booster dose at the four-week mark.

The study, based on results from over 17,000 trial participants, found that the efficacy of the AstraZeneca vaccine rose from an average of 55.1% when two standard doses are administered at a gap of fewer than six weeks to 81.3% when the interval is at least 12 weeks.

It also revealed that those who received two standard doses of the vaccine showed a 63.1% average resistance to symptomatic Covid-19. This number significantly rose to 80.7% in participants who received a half-dose at first, followed by a full second dose.

This could mean that the Covishield vaccine is at its least potential efficacy because of the dosing regimen in India.

Various dosing scenarios

The conversation around a difference in the size of the dosage first emerged as a trial anomaly during Oxford-AstraZeneca phase 3 trials. Since then, the Swedish pharmaceutical company has been studying the benefits of various dosing schedules.

The Lancet study also found strong evidence supporting “a longer-interval immunisation strategy”, which was first adopted by the United Kingdom government amid much criticism.

The UK had recommended a long interval for both the AstraZeneca and Pfizer Covid-19 vaccines in January. This had the medical community worried because Pfizer had not conducted any trial studies to determine whether a longer gap would make its vaccine more efficacious. Doctors had then sought an “emergency review” of this decision.

But later, in February, a World Health Organization panel supported the 12-week interval, based on the higher immunogenicity and efficacy benefits that came with a longer gap.

Two days after the WHO panel’s recommendation, India’s expert panel on immunisations gathered on Feb. 13 to debate the possible scenarios.

But India chose to stick to the original plan. “As of now, the current recommendation (to start vaccinating after 28 days) stands,” Vinod K Paul, chairman of the National Expert Group on Vaccine Administration for Covid-19, had told The Times of India newspaper.

Four-week gap

India has often been criticised for not keeping up with the fast-evolving science of the various treatment plans for Covid-19, and setting up protocols that do not change rapidly with new information.

Currently, for instance, beneficiaries who receive either Covishield or India’s homegrown Covaxin vaccine are automatically scheduled to receive their second dose after four weeks. The Indian government’s Co-WIN platform for the vaccine programme does not allow registered users to choose a longer interval.

A reason for India’s hesitance to adopt a 12-week interval plan for its vaccine drive could stem from the fact that Covishield’s bridging studies in India did not include dosage variables. As such, the emergency approval for Covishield is based on the data it has collected from its local trials in India.

Another possible factor could be the logistical aspect of vaccinating 30 crore people by August, a target that the Indian government has set for itself. As of March 9, nearly 2 crore people have received the first dose of the Covid-19 vaccine, and a little over 40 lakh have also taken the second dose.

This article first appeared on Quartz.