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Explainer: Why is the interval between AstraZeneca doses being reduced again?

Basically, it’s about efficacy against the Delta variant.

Vials of the AstraZeneca jab.
Vials of the AstraZeneca jab.
Image: Nathan Denette

THE GOVERNMENT HAS agreed to proceed with advice from the National Immunisation Advisory Committee (NIAC) to reduce the interval between doses of the Oxford/AstraZeneca vaccine from eight weeks to four. 

In Ireland the two mRNA vaccines, Pfizer and Moderna, are administered in two doses four weeks apart but there is a gap of eight weeks for the AstraZeneca jab. 

This reduction to a four-week gap comes following a previous reduction from 12 to eight weeks, but why is this being reduced further?

In short, the decision is a reaction to the forecasted increase in prevalence of the Delta variant of Covid-19 and how this will impact transmissions and potential hospitalisations

In his letter to the Minister for Health published today, Chief Medical Officer Dr. Tony Holohan noted that the Delta variant is likely to “grow sharply over the coming weeks”.

Holohan also says that there is evidence that vaccines offer “somewhat less protection” against the Delta variant.

Research has shown that these concerns are particularly acute following just one dose rather than two, with AstraZeneca showing a lower efficacy particularly after one dose.

Double shot

A study by Public Health England (PHE) found that the Pfizer vaccine is 88% effective against the Delta variant after two doses, compared with 93% efficacy against the Alpha variant.

Meanwhile, the AstraZeneca jab was 60% effective, compared with 66% against the Alpha variant over the same period.

Both vaccines were 33% effective against symptomatic disease from the Delta variant three weeks after the first dose, compared with about 50% against the Alpha variant.

A separate PHE study of hospital admissions found that the Pfizer vaccine is 94% effective against hospital admission after just one dose, rising to 96% after two doses.

The AstraZeneca vaccine was found to be 71% effective against hospital admission after just one dose, rising to 92% after two doses.

Unvaccinated people have twice the risk of hospital admission with the Delta variant as the Alpha variant.

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As early as possible

In NPHET’s letter to government published today, it notes NIAC’s advice was that people who had received a first dose of the AstraZeneca jab “should receive their second dose of the AstraZeneca vaccine to ensure the earliest protection” against severe Covid-19.

“On the same basis, where practicable, the dose interval for the AztraZeneca vaccine should be reduced to four weeks,” it adds. 

The government has said that while this advice from NIAC has been accepted it will be reviewed “from a planning, operational and clinical perspective in the coming days”.

It has also been confirmed that the AstraZeneca and Janssen vaccines are to be made available for all age groups

About the author:

Rónán Duffy

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