Tuesday, February 23, 2021

AstraZeneca vaccine: Second dose more effective after three months off


The Lancet reports new findings on AstraZeneca’s COVID-19 vaccine. It is more effective if there is a three-month interval between the first and second doses, rather than six weeks earlier, according to studies.

Once a vaccine has been approved by regulatory authorities, it is important to establish the optimal interval between doses. This is both the influence of this interval on the effectiveness of protection after the second dose and the risk of infection between doses. As a reminder, the risk of infection may be increased by both the lower efficacy of a single dose and the potential decrease in immunity associated with waiting for the second dose.

Scientists from Oxford conducted studies on the effectiveness of the vaccine, which was created thanks to the cooperation of their facility with AstraZeneca. They combined data from randomized controlled trials in the UK, Brazil and South Africa, including 8,948, 6,753 and 1,477 people, respectively (17,178 people in total). Participants were at least 18 years of age.

As it turned out, greater vaccine efficacy is obtained with a longer interval between the first and second standard doses (81% effectiveness for the three-month interval compared to 55% for the maximum six-week interval). Moreover, according to the authors of the study, a single dose of the vaccine is highly effective in the first three months (76% effective from 22 days after vaccination). The effectiveness assessment took into account the number of symptomatic COVID-19 cases confirmed by a positive COVID-19 test result and fever, cough, dyspnoea, loss of smell or loss of taste.

The efficacy evaluations were supported by the results of the immune response in subjects aged 18-55 years: levels of virus-binding antibodies were more than twice as high in the group receiving the second dose of the vaccine with the greater delay.

The results suggest that the dosing interval can be safely extended to three months, given the protection provided by a single dose, which could enable countries using the vaccine to immunize a larger proportion of the population more quickly.

According to the authors, the results obtained are further support for the current policy of extended dosing intervals in the UK and are in line with the new WHO recommendations.

Other vaccines, such as influenza, Ebola and malaria, also provide better protection and a stronger immune response over a longer dosing interval.

‘Vaccine supply is likely to be limited, at least in the short term, so policymakers must choose how to administer doses to achieve the greatest public health benefit,’ said lead author of the study, Prof. Andrew Pollard from Oxford University. – In a limited supply, a policy of initially vaccinating more people with one dose may provide more immediate protection for the population than vaccinating half of the people with two doses. In the long term, the second dose should provide long-term immunity. Therefore, we encourage everyone who received their first vaccine to make sure they receive both doses.

It is not yet clear how long protection can last with a single dose of the vaccine as study results are limited to a maximum of three months. For this reason, a second dose of the vaccine is still recommended.

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