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March 2021

Longer gap between doses increases vaccine efficacy

A 3-month interval between doses of the Oxford COVID-19 vaccine results in higher vaccine efficacy than a 6-week interval, with the first dose offering 76% protection in the 3 months between doses. These results, from post-hoc exploratory analyses from a phase 3 randomised controlled trial published in The Lancet, suggest that the interval between doses can be safely extended to 3 months given the protection a single dose offers.

Study lead author Professor Andrew Pollard, University of Oxford, UK, said: ‘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. 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. 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.’

Other vaccines, such as for flu, Ebola and malaria, also give greater protection and stronger immune responses after a longer interval between doses.

Looking at the interval between two standard doses and its impact on efficacy, participants who were given their doses 12 or more weeks apart had greater protection (81%, based on 8/1,293 cases in the COVID-19 vaccine group vs 45/1,356 cases in the control group), than people given their two doses less than 6 weeks apart (55%, based on 35/3,890 cases in the COVID-19 vaccine group vs 76/3,856 cases in the control group).

In addition, counting from 22 days after participants had received their first dose, there were no hospitalisations in the COVID-19 vaccine group and there were 15 in the control group.

The authors note that it is not yet clear how long protection with a single dose of the vaccine might last, as the trial results are limited to the 3 months maximum. For this reason, a second dose of vaccine is still recommended.

The authors also estimate that a single dose of the vaccine may lead to a 64% reduction in transmission in the community.

Practice Nurse 2021;51(2):7