Australia’s booster doses explained: how to decide if you should get a fourth Covid vaccine and how long to wait

As the BA.4 and BA.5 Omicron Covid-19 subvariants trigger a surge in cases and hospitalisations throughout Australia, more people will be eligible to get a fourth dose of Covid-19 vaccine from Monday.

All over-65s, as well as all residents of aged and disability care facilities, and people with certain chronic diseases, comorbidities and other high risk factors have been eligible for their fourth dose since March. So what's changed?

All Australians aged over 30 are now eligible for a fourth shot.

However, the expert group that advises the government, the Australian Technical Advisory Group on Immunisation [Atagi], hasn't recommended that everyone in that group must get the fourth vaccine.

While Atagi recommends that people aged 50 and above "should" have their fourth dose,, those aged 30 to 49 years "may choose" to have a fourth shot.

The focus is on people aged 50 and over, and vulnerable people who haven't yet had a fourth vaccine, who are being encouraged to make an appointment as soon as possible.

Infectious diseases physician and Kirby Institute professor, Greg Dore, said the stronger language for older age groups is because three doses still offers good protection against severe disease and death in younger age groups.

"Even though the data shows third dose protection against the disease appears to be holding up pretty well, particularly in people under 60, I think Atagi are addressing a concern within the community from those who haven't had a booster dose for several months," Dore said. "Many of those people are keen to make an individual choice to do more in terms of their individual protection."

The current vaccines do not offer as strong protection against infection and symptoms from Omicron as they offered against previous Covid strains, but they are still effectively preventing severe disease and hospitalisation in younger age groups. In those age over 65 and the immunocompromised in particular, a fourth dose is needed to achieve better protection against hospitalisation.

"It's not as if a fourth booster will have zero effect on infection risk in the younger populations, there's certainly some reduction of around 30 to 40% for a short period of time," Dore said.

Dore said Atagi and the government may also be hoping that when those aged 30-49 get a fourth dose, this may compel their older or more vulnerable family members to also get boosted. "That's not an unreasonable strategy to take," he said.

Unless you meet existing criteria, most people under 30 are still not eligible for a fourth shot.

International data has shown a fourth dose has minimal effect on healthy young people. Atagi determined that it is still unclear whether the benefits of a fourth dose outweigh the risks in this population group.

In May, the World Health Organization (WHO) shared a similar sentiment, saying that based on available research "data to support an additional dose for healthy younger populations are limited; preliminary data suggest that in younger people, the benefit is minimal".

If you've already had three doses, and were recently infected with Omicron, you should wait three months from your Omicron infection, Atagi has advised. Otherwise, wait three months from your third dose before getting your fourth.

Fourth dose appointments will be available from Monday 11 July.

Two vaccines specifically targeting the Omicron variant were granted "provisional determination" by the Therapeutic Goods Administration (TGA) on Wednesday, the first step in a process for vaccines to be approved for use.

"It is absolutely reasonable to wait for those," Dore said.

"If you have had three doses already, combined with being infected with Omicron since January, it is completely understandable that you may choose not to come forward for a fourth dose.

"I hope that the messaging comes through that for healthy younger people, it is OK to take this individualised approach to fourth doses."

Being infected with Omicron provides additional protection against the virus, though it is unclear how long this lasts.

Now that the TGA has granted provisional determination, Pfizer can apply for "provisional registration" of the Omicron-specific vaccines. Pfizer has six months to do this, and will need to submit further data to the TGA from human trials about how the treatment works and its safety. The TGA then may approve it for use in Australia.

Atagi and the federal health minister, Mark Butler, reiterated that people who have already been eligible for the fourth dose for months, including those aged 65 and over, those who are immunocompromised, and those with severe chronic diseases, remain at higher risk of severe disease and death from Covid-19.

Too many people from these eligible groups have not received their third or fourth booster doses, and they are over-represented in Covid-19 deaths data. As of 7 July, just 60% of people aged 65 years and over had received their fourth vaccine dose.

"The major focus of the fourth dose campaign is to reduce that risk of severe disease, hospitalisation and death, which is why there's such a strong focus on people aged over 50, as well as younger people who have compromised immunity or other health conditions," Butler said.

It is also important for these high risk groups to speak to their doctor early about antiviral treatments that protect against severe disease if they are infected with Covid-19, as these treatment need to be given within the first days of infection.