New Zealand may see little of the flu again this season, after the virus was all but wiped out here by last year's dramatic Covid-19 interventions.
But experts are nonetheless urging people to get their jabs as usual, with a transtasman bubble just one way the "notoriously unpredictable" nasty could soon spread about our communities again.
More than 200,000 New Zealanders catch influenza each year, and an estimated at least 500 people die from it.
That's more than the annual road toll and accounts for 2 per cent of all deaths.
Last year, however, flu rates were dramatically crushed by 99.9 per cent due to the circuit-breaking effects of closed borders, a nationwide lockdown, record flu vaccinations and other measures to stop Covid-19.
New Zealand labs involved in national surveillance detected just 500 cases up to September 27 - and 474 of them were recorded before lockdown.
As at this month, the country remained virtually flu-free.
The most recent data showed rates of influenza-like illness in New Zealand hovering at historically low levels, and the sole three detections of the virus this year had been among returnees in MIQ.
Of more than 28,000 people in the country participating in the FluTracking website, just 0.4 per cent reported having fever and coughs - and none of these tested positive for the flu or Covid-19.
"At the moment, we do not have any influenza virus circulation in our community," ESR virologist Dr Sue Huang said.
"The likely source of influenza virus importation would be returnees bringing influenza virus from those overseas countries where the virus is circulating."
Before the pandemic, flu was typically seeded by many of the 3.8 million tourists who visited the country each year - and Huang expected border restrictions staying in place would prove an extra barrier to it getting back into the country.
"However, influenza is notoriously unpredictable and there are other factors that may lead to the influenza virus reaching New Zealand and establishing community transmission," she said.
"Therefore, we need to maintain active surveillance to monitor the situation."
She said it was possible a transtasman bubble - expected to open soon - could provide a way for flu to spread again here.
"The virus can come back as long as the opportunity exists for influenza virus to pass from one person to another because influenza virus is still circulating globally."
Like the SARS-CoV-2 virus behind Covid-19, she said flu could also spread silently within a population.
"For example, an asymptomatic person can be infectious and pass the virus on to another healthy person," she said.
"This is one of the main reasons that it is difficult to predict its impact this year."
Just how quickly could it become re-established?
ESR and Otago University virologist Dr Jemma Geoghegan said that largely depended on sub-types of the virus, and how many mutations were involved.
"When we've had big flu years, that's because there's been a lot of change in the virus, and our natural immunity that recognises flu has been somewhat decreased."
"But until flu picks up again, we honestly have no way of telling what's going to happen."
In this year's influenza programme, running from April 14 through to the end of the year, three flu vaccines were being delivered to target four strains circulating in different parts of the world.
Huang said being vaccinated still offered the best protection - a point echoed by Immunisation Advisory Centre director Dr Nikki Turner.
"There's just no way that we can predict what's going to happen with flu, so therefore having a vaccination for it is still important," she said.
Flu wasn't the only infectious disease to have taken a major knock here over the past 12 months.
Rates of respiratory syncytial virus (RSV) - the most common cause of lower-respiratory-tract infection in infancy or childhood in New Zealand - was similarly down 98 per cent on the 2015-19 average last year.
Other big drops were recorded in human metapneumovirus (down 92.2 per cent), enterovirus (down 82.2 per cent), adenovirus (down 81.4 per cent), types one to three of parainfluenza virus (down 80.1 per cent).
The main cause of the common cold, rhinovirus, was also down 74.6 per cent - although rates were seen to quickly spike when New Zealand's restrictions were relaxed to level 1 in June.
But Huang said ESR had received reported detections of some of these viruses, so expected they'd still be circulating in our communities.
"As our behaviours regarding person-to-person interaction do not have much restriction within New Zealand this year, it may indicate a normal seasonal circulation pattern."
The effect of Covid-19 interventions on flu and other seasonal viruses has meanwhile been explored in a just-published paper, finding there was good evidence they'd been "severely curtailed" around the world.
The same trends reported here in Australia last year were now being observed in the UK and Europe, the study's international authors reported in the Journal of Medical Virology.
However, some areas were seeing a resurgence of these viruses late in the season - suggesting the lockdown may have simply put a temporary pause on their transmission.
The researchers said it was also possible that populations had achieved a level of herd immunity to some existing seasonal respiratory viruses - to which they'd now require a higher "exposure dose" of them before developing successful infection and disease.