New Zealand has reported 1.85 million confirmed Covid-19 cases - but the actual number of infections is likely much higher. Image / NIAID
By the books, nearly 1.9 million Kiwis have caught the coronavirus: close to 50,000 of us more than once.
Going off those figures, which the Government formally reports to the World Health Organisation, we'd assume that just a third of the population has ever been infected – and that a mere one per cent of us have had more than one bout of Covid-19.
But, it's always the hidden picture that matters more, and our infection iceberg is most certainly very large.
In fact, the odds are that you've already had Covid-19 – even if you don't think you have – and your chances of catching it again this summer are only growing.
Trying to fill in these blanks is a notoriously difficult exercise for our epidemiologists and modellers, who use what's called the case ascertainment rate, or CAR, to estimate the ratio of confirmed infections to all of those missed ones.
Right now, that's thought to be just 30 per cent - with a higher rate in adults but a lower one in children – meaning about two-thirds of cases are probably going undetected.
One tricky part of this is that the estimated CAR is always moving in line with our behaviour – fewer Kiwis are reporting positive test results, or indeed testing – and another is phantom infections.
Remarkably, international studies indicate asymptomatic transmission may account for four in 10 cases, yet, without a national sero-prevalence survey built on regular blood samples in the community, it's tough to say how many people have Covid-19 at any one time.
Similarly, it's hard to calculate how many Kiwis have had Covid-19 at all.
With two waves of the ultra-infectious Omicron having washed over New Zealand – and a third one infecting thousands more people each week – that proportion is likely to be at least two-thirds of the population.
Across the Tasman, there's good data to show the rate is upwards of 80 per cent; globally, scientists have just reported that two-thirds of people on the planet had probably been exposed – by vaccine or virus – as early as September 2021.
On top of what's already a tangled mess for our disease modellers is reinfection.
Consider that, if someone's initial infection wasn't recorded as a confirmed case, then their second wouldn't be classified as a reinfection.
Today's a new high for NZ Covid reinfections. Both the number of reinfections is the highest ever, and the percentage of new cases which are reinfections is the highest ever - 19% of today's announced cases are reinfections. https://t.co/LfA3tq7Wbt pic.twitter.com/QSJfvCJlRr
— Farah (@KiwiFarah) November 8, 2022
That means we could be under-estimating reinfections, or even over-counting the number of individuals who've had the virus.
But there's some reliable assumptions we can make.
"If someone's had two infections, there's more chance that they will have missed one of those than caught both," explained Dr Dion O'Neale, of Covid-19 Modelling Aotearoa.
Although reinfections have been slowly turning up more in official numbers – currently accounting for about one in 10 daily confirmed cases – the true proportion is, again, likely greater.
"I'd be unsurprised if the reinfection rate is in the mid to high 20s," O'Neale said.
"Even though most people have had a previous infection, many of those infections are semi-recent, so that slightly reduces the reinfection rate compared with what it would otherwise be.
"This is why we desperately need seroprevalence data to calibrate what our profile of past infection looks like."
Reinfections aren't at all uncommon in viruses, particularly in coronaviruses like the common cold and the much more threatening scourge that causes Covid-19.
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.
Yet there are two problems.
One is that these neutralising antibody levels fall over time - studies testing Pfizer vaccines against Omicron have shown notable drops over the space of months – leaving the door to reinfection open ever-wider over time.
Another is that Omicron subvariants, with their constantly evolving array of genetic machinery, are naturally clever at evading our immunity – and particularly if we're relying upon vaccines alone.
Early UK data showed the first-generation booster offered just 50 per cent protection against infection with the original Omicron type, before falling to 30 per cent over four to six months.
Studies have since indicated BA.5, which powered our winter wave, to be four times more resistant to antibodies from vaccines than its predecessor BA.2 was.
Now, a complex soup of circulating, nascent lineages of those two – BA.2.75 and the fast-rising BQ.1.1 among them - are proving yet more adept at infecting us.
"These do seem to have a growth advantage over BA.5, so we'd expect these to be driving more infections and reinfections," Otago University virologist Dr Gemma Geoghegan said.
"But having said that, BA.5 likely still has a way to go at finding new susceptible hosts."
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.
One recent Ministry of Health analysis suggested two or more doses of vaccine accounted for a 62 per cent reduction in risk of death from Covid-19, with boosters likely to lower the danger yet more.
But for people whose immune systems were last primed back toward the start of the year – and that's likely a good chunk of us – rebooting this hardware took more time.
"So, while your immune system is waking up, kicking into gear to produce more antibodies and generate an army of T cells that kill infected cells, the virus can be doing some damage," University of Auckland immunologist Dr Anna Brooks explained.
"Essentially, the speed of our immune system being able to ready itself once it detects the virus depends on how recent your last immune response was, at least with respect to how well our circuiting antibodies will be able to protect.
"Antibodies provide that first layer of protection while our specialised immune cells are waking up.
"That's why, if we're heading into a wave and you've only had one boost over six months ago, getting another one gives you at least a temporary amount of more immediate protection."
For more vulnerable people, immune responses tended to be poorer, and most of New Zealand's more than 2100 Covid-attributed deaths had occurred among those older than 80.
This year, Brooks was among 150 health experts who urged the Government to adopt a strategy they dubbed "Vaccines Plus" and recommended a range of virus-fighting measures.
In doing so, they singled out the ever-present danger of Long Covid: a constellation of persisting symptoms thought to accompany at least 10 per cent of infections.
Despite implications for nearly every organ system in our bodies, there remains no established treatment or cure, but merely a recently-introduced "clinical pathway" to guide doctors.
Amid the troubling unknowns still surrounding Long Covid, Brooks was concerned some people considered themselves safe as they'd already had the virus.
Firstly, she said any infection – whether our first or third – posed a Long Covid risk.
That's been highlighted by US research that found that, compared to those with a first infection, reinfected people had at least one condition linked with Long Covid that still lingered on even six months afterwards.
Secondly, reinfection could add to the woes of people already battling long-term symptoms – something reported among nearly one in five Americans who've had the virus.
In one recent global study, researchers surveyed hundreds of people who'd developed Long Covid, most of them after their first infection.
Among those still suffering lingering problems at the time they were reinfected, about 80 per cent saw their symptoms worsen, with a similar proportion reporting new or resurgent ones.
Of about 60 per cent who were recovering from the condition, reinfection was enough to cause a recurrence.
"So, if you're reinfected during your window of recovery, that might trigger Long Covid – if not the first time, then the next, or the time after that," Brooks said.
"We need real education on how reinfection is a long-term health risk – and this just isn't being spoken about at all right now."
Another paper published in the journal Nature Medicine on Friday, also indicated reinfected patients' risk of death more than doubled – and their risk of hospitalisation tripled – with elevated risks for everything from lungs, heart, blood and kidneys to mental health and neurological conditions.
Troublingly, vaccination also couldn't guarantee reinfected people they'd be spared these consequences.
"Even if one had prior infection and was vaccinated - meaning they had double immunity from prior infection plus vaccines - they are still susceptible to adverse outcomes upon reinfection," said the study's leader, Dr Ziyad Al-Aly of Washington University School of Medicine in St. Louis.
Here is the latest from our team in @NatureMedicine
— Ziyad Al-Aly, MD (@zalaly) November 10, 2022
Should you protect yourself from Covid-19 reinfection
Yes!
Reinfection is not benign; it is best to avoid it
by @Bcbowe @Biostayan @zalaly
A thread 🧵https://t.co/RVENVKRucU
As such, Brooks urged anyone to be vigilant about any symptoms - regardless of how minor – and seek a test.
Aside from more people being relaxed about Covid-19 – and the behavioural effects of September's widespread restriction roll-backs – Otago University epidemiologist Professor Michael Baker suspected waning immunity to also be contributing to the rise in cases we've seen.
"And the longer that we co-exist with this virus, the higher the proportion of cases that will be reinfections," he said, adding these could account for the bulk of new reported cases within a year or so.
"The consequences of being reinfected is still one of the biggest questions we have.
"So, I'd say we should all be defaulting to the position of minimising the frequency of getting infected and reinfected with this virus."
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