Study shows higher efficacy in delayed 2nd dose of AstraZeneca vaccine

Study shows higher efficacy in delayed 2nd dose of AstraZeneca vaccine
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A new study has found a delay in the second dose of the AstraZeneca vaccine leads to a higher efficacy rate.

According to University of Oxford researchers, the AstraZeneca vaccine is at 76% efficacy in preventing symptomatic infection for three months after a single dose and this number increases with a longer interval between the first and second doses.


This means that more people can receive their first doses sooner since it's more effective to delay the second dose, easing the current problems with supply.

Results of the study

The study, published on Tuesday as a preprint at The Lancet medical journal, stated: “Vaccine efficacy after a single standard dose of vaccine from day 22 to day 90 post vaccination was 76%... and modelled analysis indicated that protection did not wane during this initial 3 month period.”

The researchers found that if there's at least a 12-week interval before the second dose, the efficacy rate increased to 82.4% but when the second dose was given less than six weeks after the first one, the efficacy rate fell to 54.9%.


They said: “These analyses show that higher vaccine efficacy is obtained with a longer interval between the first and second dose, and that a single dose of vaccine is highly efficacious in the first 90 days, providing further support for current policy.”

Justified UK strategy

Currently, the UK government implements a vaccination strategy that prioritizes giving as many people as possible their first dose of the Covid-19 vaccine and delay the second jab by up to 12 weeks.

This is based on the notion that a first dose provides at least some partial protection from the virus, thus allowing more people to receive the vaccine while they are in limited supply.


While this decision has raised controversy and questions regarding efficacy, the UK Joint Committee on Vaccination and Immunisation, as well as health experts, supported the strategy.

Results of the study may prove critical to the government's vaccination initiatives. The researchers also found a 67% reduction in transmission after the first dose of the vaccine.

Andrew Pollard, chief investigator of the Oxford vaccine trial and co-author of the study, said: “these new data provide an important verification of the interim data that was used by more than 25 regulators including the MHRA and EMA to grant the vaccine emergency use authorization.”

“It also supports the policy recommendation made by the Joint Committee on Vaccination and Immunisation for a 12-week prime-boost interval, as they look for the optimal approach to roll out, and reassures us that people are protected from 22 days after a single dose of the vaccine,” Pollard added.

Mass vaccination in the UK began in early January and the National Health Service (NHS) will be the first in the world to distribute the vaccine. According to AstraZeneca, it aims “to supply millions of doses in the first quarter” as part of its deal with the U.K. government to administer up to 100 million doses in total.

Based on the agreement, AstraZeneca is expected to inoculate up to 50 million people in the UK, which has a population of around 66 million.

The UK government said in a statement that more than half a million doses are already available “with tens of millions more to be delivered in the coming weeks and months once batches have been quality checked by the MHRA.”