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3-month gap between doses of Oxford vaccine more effective against Covid, Lancet study finds

According to Oxford University researchers, the first dose of the vaccine offers 76% protection, making it safe for the interval between doses to be extended to 3 months.

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New Delhi: The Oxford-AstraZeneca Covid-19 vaccine is more effective when the second dose is administered three months, rather than six weeks, after the first dose, a study in The Lancet has revealed.

The Centers for Disease Control and Prevention (CDC) in the US, the country’s apex health body, recommends a six-week gap between the two doses of the Oxford vaccine.

Researchers from the University of Oxford analysed phase 3 randomised controlled trials of the vaccine and noted that the first dose of the vaccine offers about 76 per cent protection, making it safe for the interval between doses to be extended to three months.

According to them, this could allow countries to vaccinate a larger proportion of the population more rapidly when vaccine supplies are limited.

“Vaccine supply is likely to be limited, at least in the short term, and so policy-makers must decide how best to deliver doses to achieve the greatest public health benefit,” Andrew Pollard, lead author of the study, said in a statement.

“Where there is a limited supply, policies of initially vaccinating more people with a single dose may provide greater immediate population protection than vaccinating half the number of people with two doses,” Pollard said.

He added: “In the long term, a second dose should ensure long-lived immunity, and so we encourage everyone who has had their first vaccine to ensure they receive both doses.”

Following the regulatory approval of a vaccine, it is important to understand the best dose interval to ensure optimal roll out of the vaccine.

This is determined by the effect of different intervals on protection after the second dose, as well as the risk of infection in the intervening period between doses either due to lower efficacy of a single dose or rapid waning of efficacy before the second dose.

Also read: Delhi scales up vaccination centres as those above 50 are next in line for Covid shot

Vaccine efficacy remained same over 3 months

The team of researchers combined data of 17,178 people from randomised controlled trials in the UK, Brazil, and South Africa. While 8,597 participants received two standard doses of the Oxford vaccine, 8,581 received a control vaccine/saline placebo.

The researchers looked at how many participants developed symptomatic Covid-19 more than 14 days after the first dose.

The single-dose analysis included participants who chose not to receive a second dose. Since the trial was originally set up as a single-dose vaccine trial, some participants chose not to participate after the first dose. Some others got infected with Covid-19 before receiving the second dose.

Participants who were given their doses 12 or more weeks apart had 81 per cent protection, while those people given their two doses less than six weeks apart had 55 per cent protection.

Vaccine efficacy from 22 days to three months after vaccination was 76 per cent, and modelling indicated that the protection did not reduce in this time. Furthermore, antibody levels against the SARS-CoV-2 spike protein remained at similar levels for three months.

It is still not clear how long protection with a single dose of the vaccine may last, as the trial results were limited to three months. For this reason, a second dose of vaccine is still recommended.

Also read: Slow vaccination deliberate, will soon rise to 70 lakh/day from 5 lakh: ICMR task force member


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