Booster protection wanes after four months – what does this mean for winter in Australia?

While much is known about protection against COVID-19 after two primary doses, the literature around booster shots is still in its infancy.
 
But a recent report, published by the US Centers for Disease Control and Prevention (CDC) on 11 February, is helping to shed some light on what the future may hold, with effectiveness found to wane after four months.
 
Researchers analysed 241,204 COVID-related emergency department (ED) and urgent care centre (UC) encounters, along with 93,408 hospitalisations from August 2021–January 2022 across 10 US states.
 
Among the ED/UC encounters and hospitalisations, 10% and 12% had received a booster, respectively.
 
Focusing on ED and UC visits from the Omicron surge specifically, researchers found vaccine effectiveness was 87% for people who had received a booster two months earlier – and dropped to 66% for those boosted four months earlier.
 
Similarly, vaccine effectiveness also fell for hospitalisations, down from 91% after two months to 78% after four.
 
So far, more than 10 million Australians over the age of 16 have received a third dose, and millions more are currently eligible. But with Chief Medical Officer Professor Paul Kelly predicting another Omicron wave during winter – which is just short of four months away – the research has raised questions about what the future may hold for Australia.
 
For now, experts are calling for calm, given that waning efficacy is to be expected, while highlighting that the findings still demonstrate a relatively high level of protection against severe illness.
 
Dr Cassandra Berry, a Professor of Viral Immunology at Murdoch University, is among them. When it comes to testing efficacy, she notes that most studies measure circulating antibody levels in the blood, which are known to naturally wane and decline over time.
 
‘A better method to gain insight into vaccine immunogenicity is to measure cellular immunity,’ Professor Berry told newsGP.
 
‘Memory B cells and memory T cells can last for decades.’
 
Research suggests this type of lasting immunity could potentially be better achieved through heterologous vaccination regimes, and Professor Berry says this could be a potential strategy for Australia, as in addition to Pfizer and Moderna, which are mRNA-based, AstraZeneca’s viral vector vaccine has also been approved as a booster shot and Novavax is soon expected to apply for approval.
 
‘The CDC study only looked at hospitalisations for people given mRNA second and third doses and not AstraZeneca or Novavax vaccinated individuals,’ she said.
 
‘Therefore, we could consider using non-mRNA vaccine types as boosters in Australia for the majority that received Pfizer or Moderna vaccines.
 
‘Perhaps our vulnerable people should at least receive AstraZeneca or Novavax as a booster a month before seasonal influenza hits the eastern states. Remember that boosters will also help break virus transmission.’
 
In Israel, which commenced its booster rollout ahead of Australia, a second booster dose has been offered to people with weakened immune systems since the start of the year, whereas fourth doses are currently only approved in Australia for those are who severely immunocompromised.
 
However, Professor Adrian Esterman, Chair of Biostatistics and Epidemiology at the University of South Australia, believes this is too narrow a definition considering emerging data on vaccine efficacy.
 
To ensure those who are most vulnerable are protected in winter, he believes the next step should be extending the eligibility of fourth doses to all people who have underlying health problems, as well as anyone over the age of 70.
 
‘The fact of the matter is that the [Federal] Government already allow a fourth dose for those who are severely immunocompromised. But that’s a very small, select group of people on specific drugs with specific conditions,’ Professor Esterman told newsGP.
 
‘So I think sooner rather than later – and I’m talking about the next month or so – they’re going to have to start approving a fourth dose for less severely immunocompromised and more generally an older age group because their immune systems simply aren’t up to scratch.
 
‘I can see it happening, and I’d rather it happen sooner rather than later, especially with winter coming up.’
 
Going forward with borders open and public health measures easing, Professor Esterman foresees the need to go back to a tiered system approach, similar to the beginning of the vaccine rollout when high risk groups were prioritised.
 
‘That approach is still useful, especially now getting down to approaching the endemic phase of the pandemic,’ he said.
 
‘You’ve got countries like Denmark, which have removed all public health measures and are basically letting it rip, but that policy only works if you protect your vulnerable people.

‘The rest of the general public can wait, especially with Omicron.’
 
Associate Professor Sanjaya Senanayake, a specialist in Infectious Diseases at The Australian National University, has proposed that if a fourth dose is required that ‘the preference would be a variant-specific one’.
 
But while Professor Esterman agrees an Omicron-specific vaccine, such as those being developed by Pfizer and Moderna, would be ideal, he says by the time they become available another variant could have emerged.
 
‘So it’s this game between different vaccines and different variants,’ he said.
 
‘What everyone’s hanging out for, and I think will happen eventually, would be a generic vaccine against all coronaviruses. We’ve already got a group in Australia and several groups around the world working on that, and it does look promising, but it’s probably still 12–24 months away.’
 
And with booster uptake already slow, if a fourth shot is to be offered, Professor Esterman says getting the communication strategy right will be vital.
 
‘Generally speaking, communication from the different governments has been miserable,’ he said.
 
‘They’ve rarely tried to explain what they’re doing and why they’re doing it. And you can understand people saying, “Why should I do this?” when it’s simply not being explained very clearly why it’s important and how safe it is.
 
‘So I think better messaging by governments is absolutely essential.’
 
Beyond risks of severe disease, concerns remain for anyone who contracts the disease developing long COVID and the health system’s ability to respond and cope with these numbers, with researchers predicting that tens of thousands of Australians could experience some form of long COVID.
 
Professor Berry says that is why all people need to understand the basics of the infectious period and spread of Omicron, particularly given how transmissible the variant has proven to be, even among those who are vaccinated. 
 
‘Saliva and small droplet particles sprayed out when speaking, singing [or] exhaling air can transmit the virus to bystanders – this happens even if the infected person is asymptomatic or is vaccinated,’ she said.
 
‘A person is most infectious 2–3 days before symptoms develop, if they do, and for about three days afterwards, according to the CDC.
 
‘Therefore, we all need to take responsibility for our actions and behaviour inside and outside our households to limit the spread of this contagion.
 
‘We cannot drop our guard or become too comfortable in collective complacency.’
 
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boosters COVID-19 hospitalisations mRNA vaccines vaccine effectiveness