Is it safe and effective to get a different booster dose after your primary course of a COVID-19 vaccine?

At the time of publication, only about 61 per cent of Australia, or nearly 13 million Australians, had received a third vaccine dose.

Associate Professor Paul Griffin is an infectious diseases physician and microbiologist from the University of Queensland.

He told ACM that the "gold standard" dose was if an AstraZeneca first course was followed by a Pfizer of Moderna booster.

That, he said, has so far proven to have the strongest potential to boost immunity against the virus.

Switching from one brand of vaccine to another is "called heterologous boosting and there are some benefits in doing that," Professor Griffin said.

"It does seem to give you at least a little increase in protection."

According to Pfizer's Phase 3 trial results, released publically on March 24 in the New England Journal of Medicine, the booster has an efficacy of 95.3 per cent relative to the first two doses.

The trial followed 5,000 double-dosed participants who then received a booster and compared the effects with another group of 5,000 double-dosed people who received a placebo.

Both groups received the third dose 10 months after their first course vaccination, and the groups had not had an COVID-19 infection prior to their participation in the trial. The tests were completed while the Delta strain was dominant.

Of those who recieved the Pfizer booster, six tested positive for COVID-19 during the trial compared to 123 who had received the placebo.

At the moment in Australia, the mRNA vaccines - Pfizer and Moderna - are the recommended booster doses.

Professor Chris Blyth is the director of the Wesfarmers Centre of vaccines and infectious disease. He said the mRNA vaccines have greater potency, more so than the viral vector vaccines - Astrazenca or Novavax.

"Either Pfizer or Moderna, is advantageous over other boosters at this stage and so that's why all people are recommended, over the age of 16, to use an mRNA booster, Pfizer or Moderna, regardless of their primary course," said Professor Blyth.

ATAGI has also recommended a forth dose for over 65s and other vulnerable people such as Indigenous Australians over 50, those in disability care and those who are immunocompromised over the age of 16.

However, it may be a while until we see similar recommendations for all Australians, said Professor Blyth.

"We don't know at this stage whether we will need more frequent boosters coming forward. It is clear that there are some populations that may benefit in the future from additional doses, particularly those whose immunity may be waning, and particularly those at greatest risk of disease."

It may be the case that the COVID-19 booster dose will end up being treated like the seasonal flu vaccine, which is encouraged yearly.

Whether this happens, Professor Blyth said, will depend on the new strains of the virus as they continue to emerge.

Professor of Viral Immunology at Murdoch University Cassandra Berry believes the booster dose will become more imperative as winter approaches.

"[...] We can expect a bad flu season. With hard border controls, we have prevented many flu strains coming into Australia until now. However, we are still in a COVID pandemic with more SARS-CoV2 virus circulating than ever before. So, it is vital for people to stay immune and get this year's updated flu vaccine for protection.," Professor Berry said,

"Without immunity, we simply lose the war against these respiratory viruses. Once flu enters our body it essentially cracks our epithelial cell lining and opens our airways for different invading pathogens, like Omicron BA.2. If this happens, we will likely observe much higher numbers of hospitalisations.

"So, getting a fourth dose as a booster for those with poor or waning immunity is critical and will reduce the number of deaths."