The flu has been squashed down to historically low levels in New Zealand, thanks to the raft of radical measures we used to crush Covid-19.
New data has revealed how the combined effect of shut borders, heightened hygiene, record vaccine uptake and six weeks of lockdown also knocked back several other infectious diseases.
More than 200,000 Kiwis catch the flu each year, and an estimated 500 people die from it - more than the annual road toll.
Experts have been left astonished at the impact that the fight against Covid-19 has so far had on flu rates - one likening it to a fascinating "natural experiment".
But they warn the common virus will likely creep back up as Kiwis mingle under the looser world of level 1 - and are urging Kiwis to keep up good hygiene this winter.
It's also unclear whether rates will stay low enough to avoid the annual pressure heaped on hospitals, or help district health boards make headway into an 18-month backlog of elective surgeries from the pandemic.
The latest ESR data showed that, for the week ending June 5, the rate of consultations for flu-like illness was five per 100,000, compared with a historical rate of 16.5 per 100,000.
While consultation rates soared to more than 25 per 100,000 in March - in line with a surge in patients being checked for Covid-19 - the following month, figures dropped away to just 4.3 per 100,000.
Usually at this time of year, there'd be about 22 flu-related calls per 100,000 people to Heathline; rates were instead sitting at about 15 per 100,000.
Similarly, reports of any cough or fever among people being tracked by the FluTracking website are at historically low rates.
And in Auckland hospitals, rates of severe acute respiratory infection were hovering at one per 100,000 people, compared with a historical average of 6.1 per 100,000. Of 154 patients tested for influenza, 2 per cent turned out to be positive.
"At this early point In the annual influenza season, transmission is low," a Ministry of Health spokesperson told the Herald.
For Associate Professor Helen Petousis-Harris, a vaccinologist at the University of Auckland, the drop in activity was "profound".
"There is virtually none at a time when the seasons would normally be in full swing."
Along with the six weeks the country spent in lockdown, breaking the chains of transmission, she also expected better hygiene practice and a record flu vaccine uptake would've played a part.
Almost all of this year's expected 1.768 million doses of influenza vaccine have been distributed, with an estimated 911,299 Kiwis having received shots at the end of last month - higher than any other season.
Many more likely would've been vaccinated without being recorded.
Otago University epidemiologist Professor Michael Baker was also taken aback at what he called "a very dramatic drop", adding our closed borders would've made a difference, too.
While influenza was always circulating in New Zealand, it was also seeded by many of the 3.8 million tourists who annually visit.
"We can expect that low travel volumes because of the closed borders, along with the 14-day quarantine in place, will greatly reduce the chances of these viruses entering the country," he said.
"You could say it's a huge natural experiment, in the sense that measures introduced as an intervention against Covid-19 effectively does alter the ecology of these viruses."
The trend has also been seen across the Tasman. Australia recorded 20,032 flu cases in the first three months of 2020, but just 504 in April and May when Covid-19 lockdowns were introduced, the ABC reported.
Petousis-Harris further pointed to rates of another key infectious disease, whooping cough, or pertussis, which had also recorded a remarkable decrease, diving from 82 in April 2019 to seven in the same month this year.
Using the same comparison, meningococcal disease cases fell from six to zero, salmonellosis from 97 to 23 and bowel infection yersiniosis from 64 to 23.
ESR's technical lead epidemiologist Andrea McNeill said the trend was seen across most notifiable diseases - particularly enteric diseases, or those typically spread through food or water.
She expected the change wasn't just due to the lockdown, but people generally behaving differently because of public health messaging about Covid-19.
But while less exposure had meant fewer risk factors, McNeill added that fewer people sought healthcare - and therefore weren't tested for many diseases.
Some rates also climbed over April, including rheumatic fever, which was linked to crowding and recorded in 24 cases, compared with 15 the same month a year ago.
Now that Kiwis were back to mingling under the looser life of level 1, would virus rates begin to climb again?
ESR virologist Dr Sue Huang expected so - but by how much depended on us.
"The influenza incidence rate is a result of population immunity, fitness of virus and environment and behaviour condition," she said.
"Easing restriction will favour virus transmission so we will see more cases in incoming months. But it is hard to say how high the rate will climb up.
"Therefore, it is important to maintain good personal hygiene, vaccination and keep away from someone who is coughing and sneezing."