When is a booster shot after COVID recommended?

Immunity can vary between individuals. But the sooner you can boost, the better. Picture: Shutterstock.

Immunity can vary between individuals. But the sooner you can boost, the better. Picture: Shutterstock.

As the Omicron wave crests, thousands of Australians are currently recovering from COVID and facing a rapidly approaching vaccine booster appointment.

It's fair to ask, does the immunity you acquire from contracting the disease protect you as effectively - and for as long - as a booster? In short, is there any point in getting your booster so soon after a COVID-19 infection?

The Australian government recommends delaying vaccination only until after you've gotten over your acute COVID-19 illness - meaning until after you're symptomatic. But that's in reference to initial two-shot vaccinations rather than boosters. Presently, there is no unified advice from Australian authorities on how soon you should get your booster after a COVID infection.

According to the US CDC, the safest thing is to still get your booster once symptoms subside. And many epidemiologists say the immunity you get from a breakthrough infection will be less effective than the immunity you get from a booster shot - but that's mostly in reference to the risk of being re-infected, rather than the risk of severe disease.

According to Ian Frazer, the immunologist who co-developed the ground-breaking HPV vaccine, the short answer is we don't yet know the best timeline for receiving a booster after a COVID infection. But Frazer says the immunity you acquire from contracting the disease generally lasts for around three months, give or take.

Unlike vaccine-acquired immunity, however, natural infection can vary by degrees and so the immunity it precipitates can be harder to predict.

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"Some people make quite a good immune response, some people will make an okay response, some will produce a very poor response," says Stephen Turner, head of microbiology at Monash University.

"So, you can get people that are asymptomatic, and as a consequence the immune system doesn't need to get going as much. Because the infection is not as much of a challenge, it's not as much of a threat, so in these cases people might not have an antibody response."

With that in mind, "if you've been exposed, it doesn't necessarily mean you're going to be protected. So, in terms of covering all your bases, it's worth getting a booster."

Vaccine-acquired immunity versus naturally acquired immunity?

There is evidence that naturally acquired immunity is less effective than vaccine-acquired immunity. A study from the US CDC found that COVID-19 mRNA vaccines are around five times more effective at preventing hospitalisation than a previous COVID infection - but the study only looked at cases where the infection occurred more than 90 days prior. And immunity is a complex beast that works in nuanced ways.

According to Frazer, "the human response to illness is both T-cell mediated and antibody mediated, and T-cell immunity is the one that is most important for stopping you getting really sick, whereas the antibody immunity stops you getting infected."

As it turns out, vaccines produce a stronger antibody-based immunity than infection-acquired immunity (and thus stronger protection against reinfection), but for T-cell immunity, which protects against severe disease, it's the other way around.

"So, logically, if you're talking about immunity, you probably don't have as many antibodies as a result of being infected with the virus as you would have if you get the vaccine, but that's just half the story.

"The other half of the story is the T-cell response, which is strongly induced by infection but not so strongly induced by the vaccine," Frazer says.

Turner says the mRNA vaccines - such as Pfizer and Moderna - are more effective at producing a T-cell mediated response than traditional vaccines like AstraZeneca. But the T-cell response is still more limited than that found in a naturally acquired infection.

"The limitation is that the targets that the T-cells see are only going to be contained within the spike protein that's encoded in the RNA vaccine," Turner says. That's because the vaccine harnesses the spike-protein part of the COVID-19 virus to induce an immune response, whereas when you're naturally exposed to the COVID-19 virus you're getting all of the virus' parts".

It's a complex picture, and Frazer says we've got a long road ahead. So where do we stand? Ultimately, as always, you should contact your health provider to find out the best course of action for you, but Government advice remains to get a booster shot four months after your last dose.

  • This article is published in partnership with Cosmos Magazine. Cosmos is produced by The Royal Institution of Australia.

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