Explainer: Pros and cons of extending gap between vaccine doses

Minister for Health is awaiting a recommendation on whether to extend interval

‘Early data shows that the first dose of the two-dose Pfizer and Moderna mRNA vaccines is highly effective.’

To extend or not to extend, that is the question.

At a time of scarce Covid-19 vaccine supplies, the debate rages as to whether it is a good idea to delay the intervals between first and second doses in order to get more first jabs into more arms and vaccinate more people, albeit with the reduced level of protection that comes from one dose.

Early data shows that the first dose of the two-dose Pfizer and Moderna mRNA vaccines is highly effective, bringing a high level of protection to vaccinated people before the second booster jab tops up the level of immunity after the recommended interval of 28 days.

Total doses distributed to Ireland Total doses administered in Ireland
15,011,914 10,799,130

Hospital Report

The US Centers for Disease Control and Prevention found in a recent study of almost 4,000 healthcare workers that a single dose of the two vaccines was 80 per cent effective in preventing infections, though it said two doses were better with protection jumping to 90 per cent two weeks after the second jab.


A similar study from Israel showed equally beneficial results.

The AstraZeneca vaccine, which is being administered to people in their 60s, has also been found to be very effective at preventing severe disease and death from Covid-19 after the first dose. The recommended interval for the two doses of this vaccine is 12 weeks.

The UK government decided at the outset to maintain a 12-week gap between the two doses of the AstraZeneca vaccine to protect a greater number of people, to reduce deaths and ease pressure on the country’s health system as the country battled a third wave of infections.

Analysis of data from clinical trials on more than 17,000 participants in the UK, Brazil and South Africa found that the 12-week gap was better than a six-week gap: the vaccine was 81 per cent effective over 12 weeks compared with 55 per cent over six weeks. Even a single dose of the vaccine was 76 per cent effective between three and 12 weeks after it was administered.


The decision here rests on the Pfizer and Moderna mRNA vaccines. Minister for Health Stephen Donnelly is awaiting a recommendation in the coming days on whether to extend the interval between the two doses from four weeks to a longer period of up to 12 weeks.

Experts see upsides and downsides to a change.

"As long as the supply of vaccines is secure, we should give all the vaccines out – don't hold any back," said Trinity College Dublin immunology professor Luke O'Neill, pushing for a longer interval that will reach the end-of-June first-dose target of 80 per cent of adults by the end of May.

“The idea is basically to get to as many people as quickly as possible because it will have a bigger impact, because you are covering more people in a given period of time.”

University College Dublin professor and infectious diseases consultant Paddy Mallon sees benefits in extending the interval window but said the overall strategy is to keep transmission low while fully vaccinating (with two doses) as many people as possible to prevent new variants coming in.

“I don’t know that there is a right answer because both options have a lot of benefit,” he said.

He points out that the State is “well on the way” to fully vaccinating people most at risk of severe disease from Covid-19 and that the benefits from changing the interval period would only amount to a gain of weeks given the projected supply of vaccines due into the country.

“If you were wanting to ensure that your population has the best protection against new variants, you would really be wanting to optimise the overall immune response, which would be best achieved by getting people fully vaccinated,” he said.

In addition to potentially confusing people by making vaccinations more complicated, Anthony Staines, professor of health systems at Dublin City University, said extending the interval could leave the population more susceptible to vaccine-resistant strains being imported.

"If we get vaccine-resistant strains circulating in Europe, a bunch of half-vaccinated people are an ideal place for it to spread," he said.

Vaccine specialist Anne Moore, a senior lecturer at University College Cork's school of biochemistry, said the Government should stick with the plan and the clinical trial evidence supporting the current dose interval, particularly when mRNA vaccines were going to be supplied in larger numbers.

“We may be trying to solve a problem that actually isn’t a problem at all,” she said.