Scientists have already detected early cases of influenza in New Zealand, after a two-year hiatus. What can we expect from the familiar nasty this season? Jamie Morton explains.
It might seem like a lifetime ago now, but 2019 was the last time New Zealand experienced a flu season.
When Covid-19 arrived early the next year, the unprecedented steps we took to block it also brought the hammer down on influenza.
With closed borders stopping the main way the virus is seeded here - through international visitors - and a national lockdown stamping out what was still circulating, flu rates swiftly plummeted by an incredible 99 per cent in 2020.
Since then, the virus has remained virtually non-existent here.
"The only flu cases we've had in the last two years have been in managed isolation," Immunisation Advisory Centre (Imac) director Dr Nikki Turner said.
With New Zealand poised to reopen to the world, health experts are concerned that two years of living in a flu-free environment would have left our levels of immunity vulnerable to the virus.
And for good reason: the flu traditionally infects around one in four Kiwis each year, while causing an estimated 500 deaths - making it hitherto our single infectious disease killer.
"Our immune naivety will be the big issue here, as most people will not have had flu for two years, if not longer," Otago University virologist Dr Jemma Geoghegan said.
She noted how a respiratory syncytial virus (RSV) outbreak that hit our children and hospitals hard last year – its true size was likely much greater than the 6000 cases reported – followed an Australian outbreak and the temporary resumption of transtasman travel.
While the flu continued to circulate globally, mostly still at low levels, health experts were expecting to cases to be arriving at our borders now – and a handful of cases of influenza A have already been detected here.
When might flu really take off?
"That's a good question, because, while the typical flu season runs from around May until October with a mid-winter peak, if we have less immunity, seasonality might not be as much a factor," Geoghegan said, adding that more people being inside would still exacerbate spread.
"We saw that in Australia with RSV, which is typically a winter-associated virus but it happened there in summer – so it's possible we could see an outbreak any time from when our borders open."
Just how many cases we could expect wasn't clear – but it didn't take many more cases than seasonal volumes to put hospitals already grappling with Covid-19 under pressure.
In 2018, for instance, Auckland DHB was prompted to issue an appeal to the public after emergency department staff were seeing between 250 and 270 adult flu cases a day, compared with the usual daily number of 200 over winter.
A reopened New Zealand also meant the return of a host of other common viruses.
ESR public health physician Dr Sarah Jefferies said Northern Hemisphere countries experienced unusually high levels of RSV infection at the end of last year.
"This unusual RSV activity is similar to that experience in June in New Zealand last year, which was likely the result of lower population RSV immunity due to Covid-19 control measures in 2020 and increasing population mobility, including fewer lockdowns and some quarantine free travel, in 2021," she said.
"There is not currently a vaccine to protect against RSV."
Similarly, health experts are worried about human metapneumovirus (HMPV), enterovirus, adenovirus and the highly-contagious measles – not to mention another wave of Omicron re-infections as our immunity to the variant waned.
Like the flu, these familiar nasties were also crushed by our elimination strategy.
HMPV levels plunged by more than 92 per cent, while even tracked rates of rhinovirus, which causes the common cold, fell by three quarters.
Despite widespread Omicron driving flu-like symptoms, experts still expected to be able to track a range of viruses in our communities this year.
Dr Andrea McNeill, ESR's epidemiology technical lead, said surveillance drew on a range of community and hospital-level data, including calls to Healthline and sampling of a small number of cases presenting to GP clinics.
While the pandemic had had an impact on how many flu cases were being picked up, the Covid-19 response has also spawned the development of a new and enhanced system to monitor sentinel respiratory viruses.
"This system will be implemented in a phased approach as soon as possible, and will include the establishment of a national respiratory virus testing repository, which will allow us to monitor national testing rates for respiratory viruses," McNeill said.
Normal weekly flu tracking would start earlier than usual this year with borders reopening, with ESR expected to begin reporting rates within weeks.
Otago University immunologist Dr Dianne Sika-Paotonu said monitoring for new coronavirus variants would remain important - particularly when vaccinated travellers from outside Australia began entering the country from the start of next month.
"Community spread will be rapid if any new Sars-CoV-2 variants are capable of evading protection given by currently-available Covid-19 vaccines."
While Covid-19 had been the big focus for two years, she said childhood vaccinations were ever as important – especially with the pandemic disrupting the immunisation schedule.
"This means, there is potential risk of a measles or whooping cough outbreak, for example."
Over the Northern Hemisphere, scientists tracked the spread of two specific flu strains: most commonly the A or H3N2 strain, followed by the B/Victoria strain.
The Southern Hemisphere vaccine - now available here and offered free of charge to pregnant women, older people, Māori and Pacific people over 55, and those with certain medical conditions – is targeted against four common strains, and has been tweaked to better protect against H3N2.
Jefferies said vaccination still offered the best protection against serious illness with flu, as it did for Covid-19 measures - but hygiene measures mattered too.
Turner pointed out that Northern Hemisphere countries had been spared severe flu outbreaks – something that may partly have been down to pandemic practices.
"We've actually learned a lot from Covid and people have really changed their behaviour," she said.
"More people now stay home when they're sick, they practice social distancing, and there's still mass masking – all of this makes a big difference and we can expect this to have an impact.
"So, while we can't predict what this flu season is going to look like, or say whether it's going to be severe or not severe, we need to be prepared for it."
They allege the Crown ignored Treaty obligations by not engaging with them.