More than 840,000 Kiwis caught the flu last year, but three quarters of them would never have known they had contracted the highly contagious virus.
Dramatic findings from a multimillion dollar study, revealed today, have pointed to a "hidden iceberg" of flu carriers in New Zealand who spread the virus among their family, co-workers and classmates without ever realising it.
The findings show that one-in-five people in New Zealand - and one-in-three Kiwis under 19 - were infected with influenza last year.
Yet more than 70 per cent of them didn't have symptoms, and of those who felt sick, 80 per cent didn't bother visiting a GP.
The data came from the Southern Hemisphere Influenza and Vaccine Effectiveness Research and Surveillance (SHIVERS) study, which analysed 2500 adults and children randomly selected from general practices in Auckland.
"The study showed high rates of influenza infections in the community - a large invisible iceberg that is not captured by hospital and general practice-based influenza surveillance," said principal investigator Dr Sue Huang, of the Institute of Environmental Science and Research (ESR).
"The finding is important for thinking about how to prevent the spread of flu using scenarios such as case isolation and social distancing."
Before and after the winter season of 2015, study participants answered questions and had a blood sample taken.
During the winter, researchers maintained regular weekly contact to see if participants had cold or flu symptoms.
For participants who had a cough and fever, a nose or throat swab was taken and tested for viruses that cause the flu or other respiratory infections.
The study indicated that of an estimated 840,000 people infected with flu, 220,000 became sick with flu symptoms and 28,000 made a GP visit.
Nearly 1700 people would have been hospitalised with flu, including 63 admissions to hospital intensive care units, and 22 deaths.
Huang said while it was known that in influenza and many other viruses people could be infected without feeling ill, the sheer proportion of those who were asymptomatic caught her and her colleagues by surprise.
"It showed us that if we are only looking at those who get hospitalised or go to the doctor, we are only picking up the tip of the iceberg: there's a big part of it that has never been seen before, and through this survey, we've shown it's a fairly large proportion."
Dr Tim Malloy, president of the Royal New Zealand College of General Practitioners, was also surprised at the large number of asymptomatic flu sufferers.
"I suspect this reflects the fact, like many infectious illnesses, this is a spectrum disorder and there will be features related to us as individuals that determine the extent of our symptoms, such as age, health status, lifestyle, work, stressors and immune status."
Huang believed the findings re-enforced the need for people to maintain good personal hygiene, particularly over the winter months, and protect themselves through vaccination.
This year, at least 1.2 million Kiwis opted to have flu shots.
Those not protected by immunisation, but who still considered themselves fit and healthy, were just as likely to be infected.
More concerningly, they could pass it on through touch and through the air to those likely to get sicker, such as infants, pregnant women, the elderly and people with certain medical conditions such as asthma or heart problems.
Dr John Bonning, chairman of the New Zealand faculty of the Australasian College for Emergency Medicine, said the number of people turning up at emergency departments around the country with flu this season had been "drastically down".
"So I'd suggest that immunisation has been helpful this year."
It wasn't clear what cost flu was having on the country's employers, who could lose infected workers for five to 10 days.
Huang and her colleagues were now using the data to calculate influenza's economic burden.
Overseas studies have estimated frightful tolls; in the US, the impact was calculated at between 0.24 per cent and 0.80 per cent of the country's GDP.
The serological survey data was linked to data from the electronic management systems of hospitals, local general practices and laboratories.
"This is one of the largest and most comprehensive studies of its type worldwide," Huang said.
"It provided an estimation of the full range of the impact of the influenza virus on the health and well-being of the community, in the same population, at the same time."
The influenza infections included asymptomatic infections to mild illness that do not require a GP visit, moderate illness requiring a GP consultation and severe disease resulting in hospitalisations.
"This type of data is valuable in a global context for understanding the impact of influenza and guiding strategies for seasonal influenza control and preparing for future pandemics."
The New Zealand flu experts involved in the study are now briefing their international colleagues from the US Centers for Disease Control and Prevention (CDC) and the US-based St Jude Children's Research Hospital over a two-day session in Wellington.
The team running the study is continuing to build on New Zealand's world-leading influenza surveillance systems, which track viruses in real time using data from hospitals and sentinel GPs working in the community.
"A hidden iceberg"
• Nearly one-in-five New Zealanders were infected with influenza viruses last year.
• Nearly one-in-three young people under 19 years of age were infected by influenza.
• Over 70 per cent of those infected with influenza did not have flu symptoms.
• Of those infected with symptoms, 80 per cent did not visit a GP.
• Of those infected people, about one-in-30 visited a GP; one-in-500 were hospitalised, one-in-13,000 were admitted to ICU and one-in-60,000 died.