Eligible Kiwis are being encouraged to book an immunity top-up with the updated “bivalent” booster shot for Covid-19, available from Saturday. Science reporter Jamie Morton explains.
It’s been a long while since my last boost. How protected against Covid-19 will I be right now?
That depends on a range of factors: not least your age, vulnerability, or how recently you’ve been boosted or had the virus.
When we’re infected or vaccinated, our immune system produces a response - namely through training viral-specific immune cells capable of fighting infection, and creating antibodies that can swiftly spot the virus when it’s encountered again.
But these “neutralising” antibody levels tend to drop away, leaving the door to reinfection open ever more widely over time.
“According to recent data, estimates of protection against infection following Covid-19 vaccination suggest immunity will be waning after six months,” University of Auckland immunologist Dr Anna Brooks explained.
The good news was that infection or vaccination did give us some long-lingering immune memory to work against the virus, at least in any form it’s taken so far.
This included T cells, the natural hunter-killers of our immune system that can instantly recognise infected cells, as well as B-cells, which begin pumping out fresh neutralising antibodies once activated.
While this backup immunity wasn’t enough to stop the virus invading our bodies, in many healthy adults, it did stop it making us sick enough to need hospital care, or die.
Those who’d acquired “hybrid immunity” from being vaccinated and previously infected – as most Kiwis now would have – would be likely to have the highest magnitude and durability of protection, Brooks said.
“Although this immunity will also wane, albeit likely at a slower rate than vaccine immunity.”
Brooks stressed that “any” infection carried the risk of ongoing health complications.
Individual circumstances mattered, too.
One New Zealand study that assessed the first Pfizer vaccine against earlier variants reassuringly found Māori and Pasifika were no less protected than other groups - but antibody levels declined with age.
Levels in people aged over 75 years, for instance, were found to be three times lower than in young people.
Will the updated bivalent booster do a better job than the last one?
There’s now a wealth of data to show vaccination is the single best way to slash the odds of dying from Covid-19.
The updated booster differs from the first slightly, as it’s designed to protect against the original strain of the coronavirus, but also against more recent Omicron forms.
Because the updated vaccine carries two messenger RNA (mRNA) components of the virus, half of it targets the “ancestral” strain, and the other half the BA.4 and BA.5 subvariants, which happened to fuel last year’s winter wave.
These specifically stimulated populations of antibody-producing B-cells that recognise each of the respective lineages.
Get boosted! Our new research in Nature Medicine shows both normal and bivalent boosters increase antibody levels, though variant boosters give on average 60% more antibodies. #COVID19Abs https://t.co/oORo4o68CB @IAP_Sydney @KirbyInstitute @ProfStephenKent
— Deborah Cromer (@DebCro) March 26, 2023
“Compared with the booster we have already, it gives you higher neutralising antibody levels against some of the more recent variants that’ve been circulating,” Otago University immunologist Associate Professor James Ushher said.
“But it’s an incremental gain: and what we got from the previous booster was still very good as well.”
One recent US study showed bivalent boosters to be some 37 per cent more effective than older ones at cutting the risk of a severe infection.
“The increased protection against hospitalisation or death was seen regardless of age or whether individuals had previously received a different booster,” Brooks said.
“This may be especially relevant for individuals who have not had an omicron infection, given the updated booster broadens immunity to the Omicron variants.”
What Omicron variants am I likely to come across this winter?
Given the speed at which Omicron continues to proliferate into new branches of sub-lineages, that’s hard to tell.
At any point in time, there’s typically a dozen main Omicron types doing the rounds in New Zealand.
ESR’s latest genomic surveillance report found CH.1.1 - a subvariant linked back to BA.2, which helped drive our first Omicron wave – accounted for about four in 10 sequenced cases in New Zealand.
But scientists have been closely tracking the rise of XBB: a grouping of “recombinant” lineages that already make up more than a third of cases, and which include the fast-growing XBB.1.5.
New #COVID19 insight: XBB variant rises to 36% of all cases, equalling CH.1.1. XBB.1.5 responsible for 22% growth while other lineages decline. #Wastewater matches patient findings w/ XBB leading at 43%, followed by CH.1.1 at 28% & BA.2.75 at 25%
— Rhys White 🧬 (@RhysTWhite) March 17, 2023
More info https://t.co/8ucqpICcLP pic.twitter.com/Tt9ABF8A4V
In the US, XBB.1.5 was now responsible for a whopping 90 per cent of sequenced cases – and modellers say it has the potential to drive a bump in cases here.
The World Health Organisation has warned XBB.1.5 may have the “highest immune escape to date” - even several doses of vaccine and prior infection with BA.5 don’t appear to offer high neutralisation against it.
Importantly, though, the variant doesn’t pack any mutation linked with potential change in severity – meaning our existing vaccines and immunity should still give reasonable protection against severe illness.
Should I be concerned about Long Covid?
Long Covid – a constellation of wide-ranging, long-lingering symptoms - is indeed something to be concerned about when it comes to infection.
One recent New Zealand study found as many as one in five people reported Long Covid symptoms after their initial infection, and that patients were still struggling to get help.
“Long Covid and other long-term health impacts will always be a risk following infection, given we don’t yet know who is at risk of health harms following repeat infections even when protected by vaccine and/or hybrid immunity,” Brooks said.
“However, recent evidence, garnered from reviewing a large number of studies involving more than 860,00 individuals, suggested vaccination was associated with an overall 43 per cent reduction of Long Covid.
“Therefore, keeping up to date with vaccinations or boosters, especially for vulnerable individuals, may reduce the risk of Long Covid too.”
How do I book my booster, and am I eligible?
From Saturday (April 1), anyone aged over 30 will be eligible to get the bivalent booster, provided it’s been six months since their last boost or infection.
Also eligible are people aged over 16 who are at increased risk of a severe infection, such as through being immunocompromised or living with medical conditions.
University of Auckland vaccinologist Associate Professor Helen Petousis-Harris said people should think of the booster like the flu vaccine, also available from April 1.
“It’s about getting some additional coverage across the winter,” she said.
“For anyone who has underlying conditions, they should definitely make sure they get it.”
As to why the new booster isn’t available to others under 30 and who’ve been triple-vaxxed, the Ministry of Health states a two-dose primary course should provide good protection from the risk of serious illness, hospitalisation or death from the virus.
“As such, they are not eligible to receive a booster.”
To date, Medsafe has only granted provisional approval for the use of the bivalent vaccine as a booster, meaning it is not available to children under 16.
People can book their booster online through bookmyvaccine.health.nz, or by calling Healthline on 0800 28 29 26.
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