What COVID-19 booster should you have?

Restrictions may be easing, but COVID-19 is still well and truly with us as much of Australia shivers through winter – our first with both COVID-19 and influenza widely circulating. Some Australians are now eligible to receive a “winter booster” vaccine against COVID-19, as recommended by the Australian Technical Advisory Group on Immunisation (ATAGI).

But what exactly is a winter booster, why is it only recommended for some of us, and does it matter which vaccine we receive?

What is a winter booster?

The winter booster is essentially a second booster dose of a COVID-19 vaccine, following the two primary doses (or three if you’re immunocompromised), plus a first booster. So, if you’re eligible for a winter booster, it will probably be your fourth (or maybe fifth) dose of a COVID-19 vaccine.

A winter booster works similarly to the first booster – by re-exposing your body to an antigen from the SARS-CoV-2 virus and stimulating your immune system against it, your immune response and protection against disease is strengthened.

Because immunity against COVID-19 wanes over time, the strengthened immune response triggered by a booster dose can increase protection against the disease. This is likely to be particularly important in winter, when respiratory illnesses generally peak.  

“While some vaccines we get as kids can protect us for a very long time, maybe even life, immunity to COVID wanes somewhat quickly,” says Daniel Layton, a research scientist in disease prevention and detection at CSIRO.

For those eligible, a winter booster is recommended from four months after the previous COVID-19 vaccine dose, or from three months after a confirmed COVID-19 infection.

Who is eligible for a winter booster, and why?

“Typically, we look at getting a second booster for very vulnerable populations,” Layton explains.

ATAGI currently recommends winter boosters for adults aged 65 years and over (or 50 years and older for Aboriginal and Torres Strait Islander adults) and for people aged 16 or older who are severely immunocompromised, have conditions that increase their risk of severe illness from COVID-19, or live in aged care or disability care facilities.

People in these groups are at greater risk of severe outcomes from COVID-19, and were some of the first to be eligible for previous vaccine doses, meaning their immune protection has likely waned. They stand to gain the most benefit from a winter booster dose.

“If you’re in one of those vulnerable groups, it’s ideal to have the highest possible immune protection you can, so getting that booster now is a great idea,” says Layton.

Does it matter which vaccine you get for your winter booster?

In terms of vaccine types, Australian guidelines for a second or winter booster against COVID-19 are similar to those for the first booster. mRNA vaccines are preferred, with Pfizer approved for people aged 16-17 and Pfizer or Moderna for those 18 years and over. AstraZeneca and Novavax can be used as boosters in certain situations: for example, if no other vaccines are considered suitable.

Aside from these guidelines, there are no particular recommendations for specific vaccine combinations at this stage.

“There’s been a lot of studies done where your primary immunisation courses is from one vaccine and then you followed up with a different vaccine,” says Layton. “Ultimately, you do get a high level of protection following the boost, no matter which vaccine type you’ve been given, and even if they’re not matched to your original course.”

So, if your first three doses were Pfizer, it doesn’t necessarily mean that you need to have Pfizer for your winter booster (unless you’re under 18).

There has been some evidence to suggest that mixing and matching different brands or types of vaccines between the primary course and first booster can stimulate stronger immunity, but it’s not yet clear how a second or winter booster fits in.

“The idea is really just to re-expose your immune system to the antigen, which is what the vaccines are doing in one way or another, and particularly to have your antibody titres at a higher level during the most risky times,” says Layton.

An ongoing large Australian study, known as the Platform Trial in COVID-19 Vaccine Boosting (PICOBOO), is currently investigating immune responses to COVID-19 booster vaccinations, and will hopefully shed further light on the best way to maximise protection using booster doses.