When do we get a third covid booster?

Some are impatient for their fifth dose, but it may be best to time these just ahead of the expected winter wave.

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ATAGI’s highly anticipated decision about whether and when to recommend a fifth dose of covid vaccine will factor in timing, risk, and logistics, say experts. 

With people in what Professor Julie Leask calls the “high demand” group already clamouring to be allowed to boost to their antibody levels with a fifth dose – or third booster – timing a fifth dose is going to be critical. 

“Sometimes it’s hard for that group to understand why you might hold back, but there are good epidemiological reasons for that, that are ultimately about protecting the population,” Professor Leask, public health and vaccination expert at the University of Sydney, told The Medical Republic.  

It’s all about getting people’s antibody levels boosted just ahead of the next wave which – despite covid’s frustrating lack of obvious seasonality – is likely to hit Australia in the southern hemisphere winter. 

“That’s why I suspect ATAGI has been holding off, because they face the dilemma of letting people have a fifth dose when they want it, versus timing it so that the bulk of fifth doses, for those who come and get it, happen in the period just before a winter wave is expected,” says Professor Leask.  

Another consideration might be what’s happening with newer variants, said Associate Professor Paul Griffin, an infectious diseases specialist and microbiologist at Mater Health Services in Brisbane and the University of Queensland. 

“It is clear that that some of the newer sub-variants of concern, like the emerging XBB.1.5, are more immune-evasive,” Professor Griffin told TMR. “That means we want to try and have people boosted as close to when we expect that to be an issue as possible, if that is the variant that becomes dominant.”  

This raises the question of which vaccine would be best to use for a fifth booster, given the recent approval of Pfizer’s bivalent vaccine, which targets the Omicron subvariants BA.4 and BA.5, and earlier approval of their Omicron BA.1-plus-original strain bivalent vaccine. 

Professor Griffin said the additional benefit of bivalent boosters, compared to the original booster, was still being debated. “Any booster is better than none – that’s very clear,” he said.  

One study published recently suggests that the BA.4/BA.5 bivalent booster is associated with a greater reduction in the risk of severe infection causing hospitalisation in a setting where the Omicron BQ.1 and BQ.1.1 subvariants were dominant. Another suggested the bivalent vaccine had similar effectiveness against the more recent XBB and XBB.1.5 Omicron subvariants as it did against the BQ.1 and BQ.1.1 subvariants. 

Another question ATAGI will be grappling with is whom to recommend for a fifth dose. It’s a fraught question, given how complex the recommendations have become for high-risk groups and how challenging the communication can be, Professor Leask said.  

“You’re trying to be as precise with your recommendations based on risk groups as possible, but when you make it too hard to communicate, and have too many qualifiers, the people who actually need it miss out because it’s too hard to understand.” Currently, only people who are immunocompromised are eligible for a fifth dose – their equivalent of a second booster after a three-dose initial vaccination schedule. 

One option could be to have a rolling recommendation that at-risk groups get a booster every six months, rather than frame it as a fourth or fifth or sixth booster, Professor Leask said.  

Professor Paul Griffin said there might also be merit in making boosters available for anyone who wants them, if it has been six months or more since their last booster. Singapore has taken this approach, recommending a minimum three-dose vaccination to start with, followed by an additional bivalent booster dose every five months or more after the last vaccination in people aged 12 and above. Similarly, Israel has approved a booster for anyone more than three months past their last vaccination or covid infection. 

“One of the biggest issues at the moment is there’s lots of people, myself included, who are very keen to get vaccinated as quickly as possible,” Professor Griffin said. “I think allowing people to get boosted regularly, like every six months, would be a really good strategy.”