The level of antibodies in a person's blood could be the most effective way to determine if they need a fourth Covid-19 vaccine, and if so when they should get it, new research has indicated.

Director of the UCD Centre for Experimental Pathogen Host Research (CEPHR) Professor Paddy Mallon said the university is part of a European Commission- funded consortium called Vaccelerate.

The consortium has brought scientists and clinicians from around Europe to explore questions around the fourth dose of the vaccine.

Speaking on RTÉ's This Week, he said: "We're actually leading a study called Boostavac, which is a clinical trial in adults over the age of 18 to try and explore what's the ideal timing for a fourth dose vaccine and that trial is just started recruiting in Ireland."

He added: "Specifically for the Boostavac trial we're looking for people who've had three doses of the Pfizer vaccine, because the trial involves a fourth dose of the Pfizer vaccine."

He said it is hoped to have it in operation in more than 20 sites across Europe to study when is it appropriate to get a fourth dose and what is the right timing between the third and fourth dose of the vaccine.

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Prof Mallon said it is based on data which suggests that the "level of antibodies that you have in your blood, whether induced by vaccination or just by infection, there may be a specific level that offers you sufficient protection and maybe the best thing to do is when your antibody level drops below that threshold, that's the ideal time for a booster vaccine."

He said UCD has developed an antibody test.

"It's a blood test, and you actually get what's called a quantitative antibody, so you get a level that tells you how much antibodies (are) in your blood we can track that level over time to see how quickly it drops off and the ideal scenario would be that if you can identify this protective threshold, once your level drops below that threshold, that's the right time to get your booster."

The trial is open to all adults over the age of 18.

He said that Sars-CoV-2, which causes Covid-19, is now "one of the most infectious viruses" and has mutated to a form that is exceptionally transmissible.

"It's one of the most infectious viruses that we know of, thankfully, that's been linked to less severe cases, because it's affecting a different part of the body in most cases in the upper respiratory tract rather than the lungs," he added.

Asked if re-introducing a mask mandate would have an impact on the current wave of Covid-19 cases in Ireland, Prof Mallon said that given the extent of viral transmission and the sheer burden of cases in society, "it would be doubtful that introducing mandatory mask wearing in particular settings is going to have a significant impact on the overall wave."

He said this current wave is well established and "I think if you're going to argue for a particular restriction or particular intervention, you've got to argue in the context of what impact you expected to have."

Prof Mallon said wearing masks in shops or crowded spaces may have a small effect, but people are socialising in lots of different settings "and you could be wearing a mask in one setting and not wearing a mask in another."