Influenza was virtually wiped out by New Zealand's Covid-19 lockdown - likely sparing hundreds of flu-related deaths and easing the annual strain on our hospitals.
The dramatic side-effect of our pandemic interventions on common winter viruses has been revealed in a just-published analysis showing post-lockdown flu rates were down 99.9 per cent on the four years before.
The results have been described as a once-in-a-century phenomenon and left even public health experts astonished.
"None of us could have predicted the lockdown could have interrupted flu to this degree," said Dr Nikki Turner, a GP and director of the Immunisation Advisory Centre.
"We've never really had a natural experiment like this before."
More than 200,000 New Zealanders catch the flu each year, and an estimated at least 500 people die from it - more than the annual road toll, and accounting for 2 per cent of all deaths.
New Zealand labs involved in national surveillance detected only 500 cases up to September 27 - and 474 of them were recorded before lockdown.
"Our Covid-19 interventions effectively blocked the chains of transmission between those influenza-infected people and uninfected people," study lead author and ESR virologist Dr Sue Huang said.
She added the warmest winter ever observed in New Zealand might have also contributed, as did high immunisation rates.
By the end of June, nearly all 1.768 million doses of influenza vaccine were distributed. An estimated 911,299 New Zealanders received shots - higher than any other season.
The new data further showed how the combined effect of heightened hygiene, social distancing, six weeks of lockdown and shut borders - flu is seeded by many of the 3.8 million tourists who annually visit - knocked back several other infectious diseases.
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.
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.
Huang, who has worked on influenza for over the past 20 years, said she was "blown away" by the data.
"Every year winter season, I expect to see flu, just like every morning I expect to see a sunrise," she said.
"It is an unprecedented observation, probably a once-in-100-year event."
While it wasn't clear whether the big drop freed up hospitals to deal with more elective surgeries, Turner said the disrupted flu season would have eased what was a big burden for emergency departments and GP clinics.
Up to July 31, for instance, there were just 291 hospitalisations from flu - and the vast bulk of them came before lockdown and the busy flu season.
"What we normally see through winter is hospital wards have a really hard time through winter, and then doctors breathe a sigh of relief in summer," Turner said.
Otago University epidemiologist Professor Michael Baker said although flu would return, the interventions could mean a rethink about how we combat winter ailments.
"We've accepted it as a fact of life - or a fact of death, you might say - these viral respiratory infections that plague us every year, and which are clearly responsible for much of our excess winter mortality," he said.
"If you take them out of the equation, as we have this year for the first time ever, we're suddenly seeing the effect of what has been a massive public health intervention.
"So it does seem that an extra 1500 people who die in winter could survive for another year - maybe several years - if they weren't exposed to these viruses."
While the once-off effects of lockdowns and closed borders couldn't be repeated when life returned to normal, Baker said there were several lessons from the pandemic that could be adopted for best practice.
"Everyone wearing masks on buses in winter may be a good thing because we know how transmissable these viruses are," he said, adding the value of practising good hygiene, getting vaccinated, and staying home or away from others had also been proven.
"We might also think about taking extra precautions around nursing homes, or around older people, over the winter months."