Covid-19 NZ: Are we speeding towards a ‘twindemic’?

On May 11, 2020, Prime Minister Jacinda Ardern announced that New Zealand would move to alert level 2. At that time there were only 90 active cases of Covid-19 in the country and the numbers were falling.

Aotearoa had essentially won that first skirmish against Covid-19. What Ardern and her Government did not realise was that, through lockdowns and border closures, they had also essentially eliminated another disease.

While Autumn 2020 was remarkable, so too was winter – albeit much more low-key. There was almost no influenza in New Zealand. This trend was repeated in winter 2021.

Modelling suggests hundreds of people die of flu every year in New Zealand. In a nutshell, this means in 2020 and 2021, there were certainly people – particularly the elderly and vulnerable – who had their life prolonged not only because New Zealand sidestepped Covid, but because it also vanquished the flu.

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This may well be evident in a recent British Medical Journal study that examined 37 upper-middle and high income countries. One particular measure, years of life lost, was higher than expected in all countries in 2020, except New Zealand, Taiwan, Norway, Iceland, Denmark, and South Korea.

And it wasn’t only the flu. New Zealand also banished RSV (respiratory syncytial virus) in 2020, a disease that caused major disruption when it reappeared upon the opening of the Australian travel bubble in 2021.

Our relationship with the flu is somewhat paradoxical. This is a dangerous disease, but we pretty much let it spread, and that spread appears to add to a collective immunity that builds up over time. It may be we need the flu to protect us from the flu.

By taking swift action against Covid-19, New Zealand almost certainly saved a lot of lives, but it also may have inadvertently created an ‘immunity gap’ of sorts for those seasonal illnesses to potentially push through.

While the country is focused on when Auckland reopens, the coronavirus is not going away. And it has not replaced the flu or other seasonal illnesses that stretch hospitals every winter. It’s another to add to the pile.

In short, winter 2022 is looming. Do we have any idea what that looks like?

Are we more vulnerable to the flu?

US media has reported on the possibility of a twindemic – the idea of Covid-19 and influenza rampaging through the country in winter, overwhelming fragile health services. This is very much the worst case scenario. But is it going to happen? Is the flu going to cause that much of a problem?

No one knows what happens when no one has had the flu for two years, Immunisation Advisory Centre (IMAC) director Nikki Turner tells me. The key question is will our history of immunity be enough?

Broadly speaking a sort of collective immunity – brought on by vaccination and earlier infections – protects people from the worst of the flu. Think of this collective immunity like a bathtub full of water. Over the last two years in this particular bathtub, the plug has been open, but the tap has been pretty much off.

Our immune response to getting the flu is “relatively broad and long-lived” according to a 2019 Nature study. Yet, over time influenza can escape this response. Flu mutates a lot – more than coronaviruses. This is primarily why we vaccinate against flu every year. It’s slippery, constantly drifting and changing.

Most flu infections are very mild or unnoticeable, and they top up that collective immunity. But we haven’t had infections over the last few years. We have fewer antibodies. Our protection against the flu has likely waned.

“But to what extent we don’t know,” virologist Dr Sue Huang says, echoing the uncertainty at the heart of this question.

This waning immunity doesn’t necessarily mean reinfections are going to be more severe. While the flu may be able to penetrate our first line of defence, that history of immunity (or memory) should kick in and stop most people getting very ill.

One concern is if influenza does take hold in people whose immunity has waned, it may be easier for the virus to take root and spread through the population.

There’s also the issue of the youngest children having no exposure to the flu. A recent report from the Academy of Medical Science in the UK said: “There will be a larger number of infants and young children who have never been exposed to influenza, so higher infection levels might be expected in these younger age groups.”

This is all a touch counterintuitive, isn’t it? Flu is bad. It kills people, but our protection against it is seemingly rooted in people catching it.

“Reducing exposure [to a virus] doesn’t necessarily reduce disease burden because the longer the gap between infections the more severe the illness and the more likely deaths may be,” Paul Hunter, a professor in medicine at the University of East Anglia in the UK, says.

The logic here is simple: if, for example, you get the flu – or the vaccine – regularly, you’re likely to be fine the next time it comes along, but once the gap gets longer your immune system isn’t as sharp.

Hunter, who recently wrote a piece for The Conversation on the challenge of dealing with both viruses, is less optimistic about flu.

“One downside of missing out on flu for a few years is that when it comes back it could be more common and more lethal than would have been the case otherwise,” he says.

There’s another problem, immunologist Professor Graham Le Gros suggests. The protection the flu jabs provides depends on how well the vaccine matches up with the dominant strains of flu. The choice of vaccine is based on what strains of the flu are doing the rounds in a given year. But when there’s very little flu circulating it’s hard to be clear which strains will dominate, and therefore it’s hard to be sure which vaccine to go for.

What’s worrying here is that the flu has had two years to change and evade our immune response. Some believe that this potential ‘drift’ poses a greater risk than waning immunity. That said, even if this happens a degree of protection should hold up.

Do we have any idea what all this looks like?

We kind of do. Earlier this year there was a significant outbreak of RSV, which seemingly arrived when New Zealand opened up the travel bubble with Australia.

There were thousands of cases and a lot of children hospitalised. RSV is quite common and mostly mild, but it can cause significant problems in babies and younger children.

There was almost no RSV in 2020, which meant there was a large pool of children who had never been exposed to the virus once it breached our shores.

There were hundreds of cases every week with a much higher than expected hospitalisation rate. For example, on July 19, Stuff reported there were 23 children with RSV and respiratory-type illnesses in Wellington Hospital and six were in ICU.

There were three children, including two babies in intensive care in South Auckland on the same day. All were thought to have RSV.

According to a US study cited by the CDC the RSV-associated hospitalisation rate in children under 5 is 2.9 per 1000 children, or 0.29 per cent.

For some context, the latest Te Pūnaha Matatini (TPM) modelling includes an assumption of a 0.01 per cent hospitalisation rate for children under 4 infected by Covid-19. Some 800 children aged 0-9 have caught Covid-19 in the current outbreak. None have ended up in ICU.

While Covid-19 is a much more dangerous disease across an entire population, RSV seemingly poses a greater risk to younger children.

“RSV is the dominant cause of severe acute respiratory infections in preschool aged children across the whole world,” Professor Cameron Grant, a general paediatrician at Starship Children’s Hospital explains.

The UK has also recently endured a surge of RSV infections – curiously not in winter – which put extra pressure on its hospitals. There’s no vaccine against this virus, by the way. Almost every New Zealand child is expected to have had it by their second birthday.

But it’s hoped next year RSV will be more manageable given most children have built up immunity, Huang says.

But the flu isn’t a big deal, is it?

It’s hard to say exactly how many people died because of the flu in any given year.

Some 670 people died of influenza or pneumonia in 2018, according to the Ministry of Health’s mortality tracker. And about 3000 died, however, from “all diseases of the respiratory system”

Deaths in New Zealand are coded in a particular way. So, for example, if someone dies of a heart attack triggered by a bad bout of flu, it may well be coded as a death from a circulatory illness.

A 2017 paper, co-authored by Professor Michael Baker, used a statistical model to estimate how many fatalities across five different causes of death were actually caused by influenza. The point of the paper, Baker tells me, was to essentially work out how many deaths would disappear if you removed the effects of the flu.

The report found, for instance, that 1.7 per cent of circulatory deaths were caused by influenza.

The overall results were: flu caused 1.8 per cent of all deaths in New Zealand. It found a mortality rate of 13.5 per 100,000 people.

The reason I’m writing so much about this study is that it’s why the media typically reports that there are 500 influenza-related deaths in New Zealand annually. A similar study found that 2400 people were hospitalised every year with the virus.

The expectation would be that flu hitting a population with less immunity would be much worse. The National Geographic recently reported on US modelling that suggested the upcoming flu season could be responsible for “600,000 hospitalisations, which is at least 100,000 more than an average season”.

Flu is significantly less infectious than the Delta variant of Covid-19. A person with the flu will, on average, infect 1.3 other people, according to The New York Times.

If there were no mitigations in place – lockdowns or vaccines – a person with the Delta variant would infect, on average, six people. However, flu moves very quickly, leaping from person to person in a few days, and there is a lot of asymptomatic or unseen spread.

Data from 2015 – a fairly normal year for the virus – showed about one in four people in Auckland caught the flu during the influenza season. The data also found about four in five children and adults didn’t show any symptoms.

Simply put, flu gets around, which is why no one really thought the Covid-19 measures New Zealand introduced in early 2020 would be enough to eliminate the virus. But they did.

How worried should we be?

Well, it’s early days in the Northern Hemisphere. Flu has been picked up in the US, but it’s still below the baseline levels.

There are four strains of influenza or flu: Type A, B, C and D. Types A and B are the ones that typically make millions of people sick annually. Type A flu is being picked up in the US and type B in China.

This is good news so far, Stephen Kissler, a Harvard research who studies the spread of infectious diseases, tells me. These are familiar strains, something our immune system should be able to cope with.

“Even in ‘normal’ years, it’s not uncommon for different strains to circulate in different parts of the world, so this doesn’t raise any alarm bells for me. It does mean, however, that it will be difficult to map one country’s experience onto what any other country will face from flu this year.”

Kissler believes the most likely scenario is that these few years of almost no flu will mean the virus comes screaming back and there’s a few bad years on the horizon. But there are no guarantees.

He outlines a fascinating (and hopeful) alternative: our reaction to Covid-19 may have essentially neutered the flu and made it very difficult for the virus to change.

Flu, he explains, makes trouble every year because it keeps on generating new variants that can sidestep our immune system. But to do this, there needs to be a lot of flu.

“Generating a new variant requires one virus to (a) have a mutation and (b) get lucky enough to spread, which is a rare event, requiring lots of shots on goal.

“By reducing the spread of flu to very low levels, it’s possible to ‘lock’ the virus into its current evolutionary state; basically, there’s not enough virus going around for it to create new variants, so the old variants keep spreading, but only at low levels, since our immune systems already recognise them.”

This would mean we kind of reach an equilibrium of sorts with the flu. Our immune system knows the old variants, meaning there’s less opportunity for new variants to emerge.

Flu would be a little like a person who wants to put on muscle with no access to weights or protein heavy food.

“And all of a sudden, influenza looks entirely different from it once did, now no longer with major annual outbreaks of new variants, but instead with persistent low-level spread of familiar variants … I think this second scenario is less likely, but is still plausible,” Kissler says.

Hunter’s take is much more pessimistic: “In the UK, I am more worried about a flu epidemic this winter than I am about Covid now, especially with the roll-out of the Covid booster vaccine.”

He tells me New Zealand should, of course, be more concerned about Covid-19 given we have almost no immunity in the population – other than via vaccinations.

“Your current surge happening as you go into summer may provide some added protection as you get to winter (much like we are seeing in the UK). The trouble is that Covid vaccination now probably will have waned quite a lot by your winter so your more vulnerable at least will need a booster before winter.”

There’s another problem: what happens if you get the flu and Covid at the same time? A UK study looked at just that, exploring what happened to patients who had both viruses between January and April 2020.

There were some interesting findings, including for example that people with influenza may have been at a lower risk of Covid infection. It also found patients with both viruses at the same time were around twice as likely to die than those who just had Covid-19.

What can we do to be ready?

The problem of waning immunity in the Covid vaccine – primarily against infection – is ahead. A recent study published in the Lancet found effectiveness against infections declined from 88 per cent during the first month after full vaccination to 47 per cent after five months. Crucially, protection against severe illness and death should last for much longer in the vast majority of people.

Last Monday, Medsafe approved booster shots of the Pfizer vaccine for over-18s, at least six months after their second dose.

While the science on just who needs the boosters is not settled, Le Gros suggests we should be offering the booster to everyone going into winter 2022, which he believes will offer a much more “durable immune” response.

“I think it’ll be absolutely necessary before we enter winter.”

Baker agrees. The flu vaccine, he says, may be more important than ever next year. Why not offer people a flu jab and a Covid booster at the same time?

We’re not badly placed to do that. An awful lot of New Zealanders are at their most immune from Covid-19 now, given the accelerated vaccine roll-out following Delta’s incursion.

Around the time of the flu vaccine roll-out, the efficacy of the Covid vaccines in stopping spread will have waned, to some extent, in many. It may well be optimal to offer both at the same time.

And as well as boosters, a number of antiviral drugs to treat Covid-19 are emerging.

The country is also unlikely to be ‘back to normal’ come winter. The residual behaviours we picked up dealing with Covid-19 – mask wearing and social distancing, for example – may well be enough to keep flu under control. Flu’s Reproductive or R number – with no restrictions – is about 1.3. It may be that the way we act in 2021 may push that below 1 and keep the flu muffled.

On this, Turner is not so sure. She makes the point that RSV still managed to take root in the winter of 2021. Why should it be different from the flu? Huang also says the R number for flu would be higher in schools where kids were behaving normally. Flu and RSV infections are primarily driven by children, unlike Covid-19 where they’re not the main drivers of spread in the population, as Otago University associate professor Tony Walls explains.

So is there an inevitability to what lies ahead, a year of ongoing illness? Kissler offers a slightly more optimistic take.

“I think it’s important that we don’t forget the lessons we’ve learned during the pandemic: that some basic behaviours can make a huge dent in disease spread while still largely allowing us to go about our lives. I’m cautiously optimistic that all of these changes will help keep people healthy for years to come.”