Flu is back — what can we expect?

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 nonexistent 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.

“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, health experts were expecting 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 midwinter peak, if we have less immunity, seasonality might not be as much a factor,” Geoghegan said.

“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.

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.”

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.

Normal weekly flu tracking would start earlier than usual this year with borders reopening. 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 and offered free of charge to pregnant women, older people, Ma¯ ori and Pacific people over 55, and those with 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 — but hygiene mattered too.