Extending the time gap between first and second shot could be a practical solution as vaccine is rolled out

According to original instructions, there should be a 21-day gap between doses of the Pfizer vaccine, 28 days in the case of Moderna vaccine and 12 weeks for the Oxford-AstraZeneca vaccine.

Ireland has been spacing the dose between the Pfizer and Moderna at 28 days. It now appears that will be unchanged.

It has left a three-month gap between the first and second shot of the AstraZeneca. For healthy under-60s that will now be moved out another month.

Professor Kingston Mills, Professor of Experimental Immunology at Trinity College in Dublin, believes it is very sensible to stretch out the length between doses, even up to 12 weeks.

He pointed to a recently published research paper in the United States showing some evidence that the first jab of Pfizer or Moderna vaccine had efficacy of around 80pc.

“That suggests that in the relatively short term one dose is very effective,” he said.

“The neutralising antibodies are the key to protection and if they are very high after a single dose they will persist for longer.”

The spacing out of dosages is a practical move and allows for more people to get a first dose and get some degree of protection against Covid-19, rather than waiting for longer with none at all.

The National Immunisation Advisory Committee has been examining all the real-world data around delaying the second dose.

Since the vaccines were rolled out in Europe and the United States from the end of last year, a significant amount of information has emerged about their impact across tens of millions of people, as opposed to the trials carried out by the drug companies making the vaccine.

However, it will be several more months before definitive answers can be given on the true impact of extending the gap between doses.

The UK has managed to give around half its population some level of protection by giving first doses of the AstraZeneca vaccine and the Pfizer vaccine 12 weeks apart.

This week it reopened pubs and restaurants for outdoor trade while also resuming business in shops, hairdressers and gyms.

British Prime Minister Boris Johnson said the lockdown, and not the vaccine, was the main driver in bringing down cases.

The coming weeks will tell a lot about whether people who still have had one dose there are vulnerable to infection.

Various studies have been carried out on the 12-week interval, but the data still needs to be firmed up.

One study from the University of East Anglia found a single dose of the Pfizer vaccine provided very high protection of 90pc against the virus after 21 days.

It is unclear whether the protection lasted beyond 21 days, although scientists say that it is unlikely there would be any major decline in immunity if the second dose was stretched out for another nine weeks.

Lead researcher Professor Paul Hunter said the finding supported the UK policy of extending the gap between doses by showing that a single dose could give a high level of protection.

“While we do not know how long this immunity will last beyond 21 days without a second booster, we are unlikely to see any major decline during the following nine weeks,” he said.

He pointed out that although there is no certainty that immunity will not decline between 21 days and 12 weeks, immunity levels “don’t decrease that quickly for anything”.

“We don’t know for certain that will last for a very long time, but with all the other information that we have about natural infection with Covid, the Oxford-AstraZeneca vaccine and vaccinations to pretty much every other viral disease that you could wish to name, we think that’s going to last at least six months with that sort of level and certainly it will last well enough to your second dose,” he said.

However, one of the scientists behind the jab said there should be no more than six weeks between doses.

Ugur Sahin, chief executive of BioNTech, said he would be comfortable pushing that gap only to six weeks.

“As a scientist, I wouldn’t mind if the second dose of the vaccine is given three weeks, four weeks, maybe five weeks, even up to six weeks might still be OK,” he said.

“But I wouldn’t delay that further. As a scientist I believe that it is not good to go longer than six weeks.”

Director of the US National Institute of Allergy and Infectious Diseases Dr Anthony Fauci is one of those who has taken a more conservative stand.

He contends that delaying a second dose of Covid-19 vaccine could put people in a zone of risk for catching a more infectious variant.

He believes people who have not had benefit of two doses of vaccines could be susceptible to infection with variants that could weaken its impact.

Dr Fauci has been emphatic that people in the United States need to get both doses of vaccines made by Pfizer and Moderna as per their licence recommendations.

One of the issues raised by having too long a space between administering the first and second doses is that some people may not come back for the booster – another consideration to be factored into the decision.