Explainer: Booster shots of the Pfizer vaccine

The Therapeutic Goods Administration has provisionally approved a third dose of the Pfizer vaccine for people aged over 18, administered six months after they receive the first two doses. This “booster dose” is designed to improve immunity to COVID-19.

The Australian Technical Advisory Group on Immunisation (ATAGI), which is the official government advisor on COVID vaccines, had already recommended a third dose of the vaccine for immunocompromised people, and they have now extended this recommendation to the rest of the Australian adult population.

Want to know more? Read on to find out…

Why do we need a third dose?

There’s growing evidence that the efficacy of the COVID vaccines diminishes over several months.

Today, for instance, a paper in the New England Journal of Medicine revealed findings that immunity against the Delta variant of COVID waned six months after full vaccination with the Pfizer vaccine.

The paper examined data from the Israeli national medical database during 11–31 July this year, when Delta cases were rising across the country. Across all age groups, people who’d been vaccinated in January were more likely to have caught COVID in July, compared to people who’d been vaccinated in March or later. Other vaccines have also been shown to provide reduced immunity months after people are fully dosed.

“It is very timely that the TGA has approved booster shots of the Pfizer COVID-19 vaccine,” says Professor Bruce Thompson, dean of the school of health sciences at Swinburne University of Technology.

“There is clear evidence that immunity wanes over time. This is no different to other vaccines for common viruses such as influenza.”

Most vaccines originally required two doses to provide the strongest immune response, and it’s now becoming apparent that a third will keep the response strong after several months have passed.

Is it worth getting a third shot even sooner, then? Not if you’ve got an otherwise healthy immune system.

“There’s no need to get a booster before six months, because the level of immunity is still quite robust,” says Professor Stephen Turner, head of the department of microbiology at Monash University.

“And even then, though immunity does seem to be dropping, it still does afford quite good protection.”

Will we need a fourth dose, or even more booster shots?

Will we need to keep getting vaccinated for COVID every six months from now on? We don’t yet know. Six months ago, before the Delta variant was even called Delta, it wasn’t obvious that we’d need a third dose to be protected from COVID-19.

“It’s hard to say,” says Turner.

“What is clear is that the booster certainly does, like the name suggests, boost immunity – particularly when there’s been a bit [of time] between people getting their second dose.

“How long that lasts? Well, you’d imagine it lasts for quite a while, because the whole idea of boosters is to augment and increase the level of immunity that’s induced.”

The current booster shot approved will be identical in contents to the first two Pfizer doses, but this might not be the case for subsequent doses, if they’re necessary.

“I think what’s more likely is that [a third dose] might actually do for the time being, and then if the virus keeps evolving, we might need to then start looking at updating the vaccines to better match strains that come into circulation,” says Turner.

Should we mix vaccines?

Whichever vaccine you received initially, the TGA is currently saying that the booster shot will be a dose of Pfizer.

In general, mixing vaccine shots appears to work and protect you from COVID-19, but the data is more limited than information on full courses of single vaccines.

However, mixing two different types of vaccines – for instance a viral vector vaccine, as AstraZeneca is, and an mRNA vaccine like the Pfizer vaccine – might even be better for COVID immunity in the long term.

“If the long game is to induce broad immunity, then using a different vaccine brand (non-mRNA type vaccine) as a mixed booster would be better,” says Professor Cassandra Berry, a researcher in viral immunology at Murdoch University.

“With plenty of vaccine supplies this shouldn’t be a problem, but if the idea is to boost immunity in vaccinated people, then is Pfizer the best choice?

“Let’s think wisely now before we travel down the road that is too narrow.”

Who else has approved booster shots?

Much of the world still hasn’t had access to one vaccine dose, much less three, so which countries are advocating for third doses?

The US Centers for Disease Control and Prevention have currently recommended a six-month booster for some adults – those with medical conditions, and people who work in high-risk settings, for instance. This covers a large proportion of the population, but not all – as Cosmos writer Jeff Glorfeld learnt while asking for a third Moderna shot.

The United Kingdom is rolling out booster shots to similar groups of people, on the same timeframe.

The European Union’s medical authority (the European Medical Association) has recommended booster mRNA vaccine doses for everyone over 18, six months after the first dose.

In Canada, mRNA booster shots are also being made available for certain populations, and availability changes on a province-by-province basis.

The World Health Organisation draws a distinction between booster doses (extra doses of a vaccine after someone has completed the recommended course) and additional doses (a recommended course containing more than two vaccine doses, because the recipient’s immune response is compromised).

Currently, its recommendation is that additional doses are necessary to protect those with vulnerable immune systems, and boosters might be useful to address specific variant outbreaks. However, given that so much of the world can’t access initial doses, and new variants are most likely to develop in unvaccinated populations, it recommends against widespread booster doses in favour of sending them to places that need them more.

The WHO is continually reviewing the evidence.

“A meeting organised by the WHO on Monday night allowed the global COVID vaccine research and development community the chance to closely review all the safety and efficacy data around a third booster dose of vaccine,” says Professor Damian Purcell, head of the molecular virology laboratory at the Peter Doherty Institute for Infection and Immunity at the University of Melbourne.

Purcell says the committee found that mRNA boosters were safe and effective, with even lower chances of side effects than the first two doses, and they did increase antibody levels.

“The increased personal protection also translated to reductions in transmissions in the community, providing a more general benefit.”