COVID-19 booster vaccines are part of the plan, but who to give them to and when isn't yet clear
With just 43 per cent of Australia's adult population fully vaccinated against COVID-19, there are millions of people still waiting for their second dose — and many yet to even line up for their first.
But with reports of waning immunity and breakthrough infections in vaccinated people overseas, many Australians are starting to ask about booster shots.
Health authorities in Israel first began offering third doses of the Pfizer vaccine to elderly people and healthcare workers in early August, and have since made booster shots available to everyone who received their second COVID-19 jab more than five months ago.
In the US, third doses of the Pfizer and Moderna vaccines are only authorised for some people with weakened immune systems, but are soon expected to be rolled out most to Americans eight months after vaccination, pending FDA approval.
But with research showing COVID-19 vaccines remain more than 90 per cent effective at preventing severe disease and death, are third doses really necessary?
And what does the rollout of booster shots mean for the billions of people around the world yet to receive even a first COVID-19 vaccine?
Third doses and boosters not necessarily the same thing
It may sound like semantics, but there's an important distinction to be made between a third vaccine given as part of an initial round of vaccinations, and a booster shot administered when vaccine immunity wanes.
Most countries administering third COVID-19 doses are prioritising people with compromised immune systems, such as cancer patients and organ transplant recipients, who tend to generate weaker responses to only two doses of the vaccine.
These supplemental jabs should be thought of as the third dose in their primary COVID-19 vaccination schedule, rather than as a booster, says Nick Wood of the National Centre of Immunisation Research and Surveillance.
"Your primary course [of vaccines] is to get you over the high jump bar," said Dr Wood, a child health specialist at the University of Sydney.
"Some people might need three doses in their primary course to get them over that bar."
Research suggests a large proportion of breakthrough infections are occurring in people with compromised immune systems.
It's not unusual for vaccines to be administered in a series of three doses, with a gap of several months between the second and third shots, University of Melbourne professor Kim Mulholland said.
Generally, experts agree that a third COVID-19 shot is warranted for people who are severely immunocompromised, and may also be necessary for older adults, who are less likely to mount a robust immune response to vaccines.
Some experts have even suggested the primary course of COVID-19 vaccines could potentially be expanded from two doses to three for everyone, as researchers learn more about how to maximise vaccine effectiveness.
"There are quite a few people who believe we were a bit quick with the scheduling of these vaccines, and that they should have been three-dose vaccines," said Professor Mulholland, a member of the WHO's Strategic Advisory Group of Experts on Immunization.
But the European Centre for Disease Prevention and Control and the World Health Organization have both stressed there is currently no need to give third doses to people with no underlying health conditions.
"Third doses may be necessary for the most at-risk populations, where there is evidence of waning immunity against severe disease and death," WHO director-general Tedros Adhanom Ghebreyesus said last week.
"But for now, we do not want to see widespread use of boosters for healthy people who are fully vaccinated."
Is vaccine immunity waning, and would booster shots help?
Data consistently shows vaccine-induced protection against severe disease and death is holding strong months after vaccination, even against the Delta variant.
But health authorities in Israel have forged ahead with booster shots for the general population anyway, amid concerns the protection vaccines provide against getting infected and having mild disease is declining over time.
"At the sixth-month mark, we still have very good protection against hospitalisation and death, but perhaps a bit of waning immunity [against symptomatic infection] … down from about 90 per cent to 65 per cent," Dr Wood said.
In Israel, preliminary data suggests people vaccinated in April and May have better protection against infection than people vaccinated back in January.
It's difficult, however, to distinguish how much this waning effect owes to the vaccines, and how much is influenced by the rise of the Delta variant, the relaxing of restrictions, and the health status of people first vaccinated — many of whom are older and have underlying health conditions.
Pfizer recently reported that a third dose of its vaccine boosts antibody levels in the blood against SARS-COV-2, including against Delta.
But it's unclear exactly what this means for protection against COVID-19.
"This is where we haven't got really good evidence," Dr Wood said.
While an increase in antibody production is likely to benefit people with compromised immune systems (by helping them get over that "high-jump bar"), it's hard to say how much extra antibodies would help otherwise healthy individuals.
"For viruses, antibodies are important, but so is your cellular immunity," Dr Wood said.
Whether a third dose would meaningfully increase longer-term immunity — largely mediated by our memory B and T cells — is a question that still needs answering.
In Israel, preliminary data suggests a third dose of the vaccine dramatically reduces the risk of COVID-19 infection, two weeks after the shot is administered. This makes sense, as circulating antibodies help the immune system to prevent viruses taking hold.
But the real question is: how long do these high antibody levels produced by booster vaccines last? And are they likely to reduce overall virus transmission in a sustained way?
Some experts say waiting for a booster shot that specifically targets the Delta variant (or the dominant variant of the day) would make more sense, given the current vaccines already offer robust and lasting protection against severe disease and death.
"It could well be that in Australia, we try to get everyone fully vaccinated before Christmas time, and for people that are immunocompromised, that might mean three doses," Dr Wood said.
"And then next year we give everyone a booster [tailored to] the variant that is circulating."
While Australia's vaccination rollout has lagged, Dr Wood said the delay gives health authorities here the chance to observe how booster shots might be best used.
"Australia is in a fortunate position in some ways because we're able to see what's happening in the northern hemisphere, and see whether their booster program will actually translate to less breakthrough infections and better protection against disease."
'Giving out extra life jackets, while leaving others to drown'
It's not just scientific questions that need careful consideration, either.
As countries like Australia consider the potential benefits of COVID-19 booster vaccines, there are billions of people in low-income countries still waiting to receive their first jab.
"We're planning to hand out extra life jackets to people who already have life jackets, while we're leaving other people to drown without a single life jacket," Mike Ryan, the executive director of the WHO's Health Emergencies Program, said last month.
In August, the WHO called for a moratorium on administering COVID-19 booster shots, to reduce global vaccine inequality and prevent the emergence of new coronavirus variants.
Dr Wood said the "marginal benefit" booster doses might offer wealthy countries paled in comparison to the "much more significant benefit" that the primary COVID-19 vaccines doses would provide people in low-income countries.
"The other thing is that in wealthy countries, there are stronger healthcare systems — you're more likely to get access to oxygen and ventilatory support … than you would be if you lived in a low-income country," he said.
"And so the prevention [afforded by] the vaccine is even more important."
Professor Mulholland agreed that the administration of booster vaccines presented a serious "moral problem".
"There is such a desperate global shortage of vaccines, that for countries to be giving what seem to be not quite necessary boosters — certainly if you're talking about protection against severe disease and death — is a waste of vaccines."