The flu is killing about 500 New Zealanders each year - and men, Maori, Pacific Islanders and those living in poverty are at greatest risk of premature death from it.
That's according to new research from Otago University scientists who say the flu is likely New Zealand's single biggest infectious disease killer, accounting for about 1.8 per cent of deaths in the country.
A new study published this week has, for the first time in any country, estimated the distribution of flu deaths in relation to ethnicity and social deprivation, as well as gender and age group.
It found that, in the key 65-79 year age group, Maori were 3.6 times more likely to die of influenza than those of European or other ethnicity, while Pacific people were 2.4 times more likely to die during a typical flu season.
Those living in the most deprived 20 per cent of neighbourhoods were almost twice as likely to die of influenza compared with those living in the least deprived areas.
Men were also more vulnerable, with males aged 65-79 years almost two times more likely to die of influenza than females.
"Future work will update these estimates for the post-pandemic period, but we would expect the findings to be broadly similar," said study author Dr Trang Khieu, who carried out the work as part of her PhD.
Her study was based on a 15-year period, from 1994 to 2008, before the last influenza pandemic, which took place over 2009 and 2010.
It used modelling to estimate influenza deaths in each population group - an approach that was necessary because only about one in 23 New Zealand influenza deaths were recognised, confirmed and recorded on death certificates.
This modelling showed that the largest numbers of flu deaths (37 per cent) were circulatory conditions such as heart attacks and strokes.
In most instances, flu would not even be suspected as the cause, particularly in cases of sudden death.
The crude influenza mortality rate found in the study was 13.5 per 100,000 for all causes.
This rate was similar to those estimated for other high income countries, though none have reported the distribution of influenza mortality risk by ethnicity and socioeconomic position.
All of these studies show that increasing age was the single most important risk factor for influenza mortality.
In New Zealand, 86 per cent of deaths occurred in those 65 years of age and over, a rate of 90.3 per 100,000.
"This research illustrates how the borderline between infectious diseases and non-communicable diseases, such as heart disease and cancer, is not particularly firm," said study co-author Professor Michael Baker, also of Otago University.
"Infectious diseases like influenza can precipitate sometimes fatal events such as heart attacks, and chronic conditions such as cardiovascular disease make us more vulnerable to infections."
These results show that it is important to target flu vaccination and other interventions to the most vulnerable groups, in particular Maori and Pacific people and men aged 65-79 years and those living in the most deprived areas."
The study has been published in the Journal of Infection.