Study shows AstraZeneca’s COVID vaccine is more effective the longer you wait for the second dose

“The most important point, and this is more relevant to other countries compared to us – there does not appear to be any negative effect of increasing this interval.”

The remedy administration approved Two-dose vaccine from AstraZeneca last week. Australia has ordered 53.8 million doses.

The TGA allows nurses and doctors to give the second dose between four and 12 weeks after the first, but the federal government’s Australian Technical Advisory Group on Immunization recommends that it be given 12 weeks apart.

A single dose of the vaccine was 76 percent effective in preventing symptomatic COVID-19 from 22 days after administration, suggesting people enjoy strong protection while awaiting their second dose. It is not yet clear how long this protection will last.

The data, showing that longer delays boost the vaccine’s effectiveness, is supported by antibody data showing that people who waited longer for their second vaccination had more than twice as many antibodies.

It’s not yet clear why the effectiveness increases the longer you wait for the second dose.

AstraZeneca’s vaccine uses a modified chimpanzee virus to deliver the key component of the vaccine to human cells. It is possible that the human immune system itself develops resistance to the chimpanzee virus – which subsides when the second dose is delayed.

“It’s a theory,” Professor Triccas said. “It could just be a trait of the immune system.”

The study explains one of the key mysteries of AstraZeneca’s vaccine.


In an early study, a small group of people accidentally given lower doses of the vaccine appeared to have much better protection.

That lancet Study suggests it’s not the dose that matters, it’s the timing. Most people in the low-dose group received their second dose 12 weeks or more after the first.

The study was not designed to measure what effect the vaccine might have in preventing the virus from spreading. But there were some tantalizing clues.

Every volunteer in the study was swabbed for COVID-19 every week, regardless of whether they had symptoms. While the vaccine does not appear to reduce the number of people with asymptomatic virus, it did reduce the number of people testing positive by 67 percent overall.

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