AstraZeneca vaccine recipients warned not to hold off on second dose amid Delta variant

Immunisation expert says it is likely booster shots will be needed to extend protection

The State's main immunisation expert has warned people who got a first dose of the AstraZeneca vaccine against "holding off" for a second dose of a different vaccine.

Prof Karina Butler said this would be a concern as it would leave large numbers of people only partially protected against Covid-19 at a time when cases of the Delta variant are increasing.

It is likely booster shots will be needed to extend protection against Covid-19 “but hopefully not as soon as people think” due to immunity lasting longer than originally expected, she told The Irish Times.

And with vaccines now licensed for use in adolescents, she said recommendations in relation to the immunisation of young people in Ireland will be ready in advance of the reopening of schools in the autumn.

Hundreds of thousands of people in their 60s have had to wait longer to be fully vaccinated due to the 12-week interval between the two AstraZeneca doses administered to this age group. In contrast, younger people are being vaccinated with two doses of Pfizer over a four-week period.

Eight weeks

The interval between AstraZeneca doses is being reduced to eight weeks but this will take time to implement and is subject to supplies being available.

With data showing that one dose of AstraZeneca gives only 33 per cent protection against infection with the Delta variant, there have been calls for vaccines to be mixed so that those who got a first dose of the vaccine could be given a second dose of an mRNA vaccine (Pfizer or Moderna).

But Prof Butler, who chairs the National Immunisation Advisory Committee (Niac), said the best thing people could do is take the vaccine that is available soonest and to "stick with" the current two-dose regime for AstraZeneca.

She said AstraZeneca was a good vaccine but two shots were needed early to maximise its benefit. “It might be that it is not as fully protective against very mild symptoms but it is still very highly protective against someone ending up sick in hospital.”

In relation to mandatory vaccination, Prof Butler said her view was that this should only be a “situation of last resort” and the hope was that people can be adequately informed to help them make the right decision.

Niac has ruled out a “mix and match” approach to vaccines for now due to a lack of studies on its safety and effectiveness. But Prof Butler said the early data was promising and Niac will consider the issue after the results of detailed studies in the UK are published later this month.

“The water can look inviting but we have to be sure there aren’t rocks underneath,” she said. “The data is very promising and I would not exclude it as a future option. But for now the best option, given the proven effectiveness data of AstraZeneca in the UK, is to take the soonest vaccine available to you.”

Very rare

In relation to cases of very rare but serious clotting among people who received AstraZeneca, she said the latest British data was “slightly more reassuring”, with the incidence falling to 0.6 reports per million people in the 18-49 year old age bracket after two doses.

Prof Butler said the rise of the Delta variant was a concern but Ireland is in “a very different situation” from the UK.

The more transmissible delta variant now accounts for 70-90 per cent of infections in the UK, where it is dominant. In contrast, only 5 per cent of sequenced cases here are of the Delta variant.

Rise in anxiety

She said the rise in cases in England was contributing to a rise in anxiety and confusion here.

"We are concerned about it. We saw the number of cases that happened in India, while in the UK, which was leading the way, we're now seeing a change in the trajectory with the numbers going up again, and the risk of some ending up in hospital."

It was clear from early data that the variant passes more easily within households and to close contacts, and that it impacts on vaccine effectiveness after one dose.

However, Prof Butler said the latest data, which shows both AstraZeneca and Pfizer vaccines are more than 90 per cent effective in preventing hospitalisation, was “very reassuring”.

Paul Cullen

Paul Cullen

Paul Cullen is Health Editor of The Irish Times