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Greater efficacy from longer dose interval, Oxford Covid-19 vaccine study finds

Extending time period from six weeks to three months will also help with supply, say authors

A three-month gap between doses of the Oxford Covid-19 vaccine results in higher efficacy than a six-week interval, with the first dose offering 76 per cent protection in the three months between doses, results from post-hoc exploratory analyses are indicating.

The results from a phase 3 randomised controlled trial suggest that the interval between doses could be safely extended to three months given the protection a single dose offers, which may allow countries to vaccinate a larger proportion of the population more rapidly.

Factors associated with optimal vaccine rollout include the effect of different intervals on protection after the second dose, and the risk of infection between doses either due to lower efficacy of a single dose or rapid waning of efficacy while waiting for the second dose.

During the study, data from randomised controlled trials in the United Kingdom (UK), Brazil, and South Africa, involving a total of 17,178 people, were combined.

Participants were aged 18 years and over and either received two standard doses of the Oxford Covid-19 vaccine (8,597 participants) or a control vaccine/saline placebo (8,581).

In the UK trial, a subset of participants (1,396 people) received a lower dose of the vaccine as their first dose.

Results showed that participants who were given their doses 12 or more weeks apart had greater protection than people given their two doses less than six weeks apart – 81 per cent for three-month interval versus 55 per cent for up to six weeks.

In addition, a single dose of vaccine was highly efficacious in the first three months (76% efficacy from 22 days after vaccination onwards).

Lead author Prof. Andrew Pollard, University of Oxford, said vaccine supply was likely to be limited in the short term, meaning governments must soon decide how best to deliver doses to achieve the greatest public health benefit.

“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 2 doses,” Prof. Pollard said.

“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.”

Study author, Dr Merryn Voysey, University of Oxford, added the analysis confirmed previous findings of the higher efficacy of a low- then standard-dose regimen.

“However, with additional data available, we have found that the enhanced efficacy and immunity may be partly driven by the longer interval between doses that was common in this trial group,” he said.

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