New Zealand has an average of 401 influenza-associated deaths each year according to estimates published for the first time. This is an average annual mortality rate of 10.6 per 100,000 population.

A University of Otago, Wellington study estimating influenza-associated deaths from 1990 to 2008 found that the majority of deaths (86%) occurred in those aged 65 years and over.

Lead author Dr Tara Kessaram says the results show the large public health impact of influenza, particularly among the elderly, many of whom will have underlying conditions that are triggered or made worse by influenza.

"Although gets a lot of justified attention, we need to keep remembering the huge toll from seasonal influenza, which comes around each year," Dr Kessaram says.

University of Otago, Wellington's modelling shows that estimated mortality varied widely from year to year, ranging from 31 deaths in 1991 to 898 deaths in 2003. These differences reflect a number of factors including the virulence of the dominant circulating strain, Dr Kessaram says.

High mortality years tended to be those dominated by the influenza A(H3N2) subtype, she says. Despite these year to year variations, the overall mortality rate did not appear to show any downward trend over the study period.

Co-author Professor Michael Baker says statistical modelling is needed to estimate the impact of influenza as most of those who die as a result of are killed by other illnesses that are triggered by influenza infection. These illnesses include bacterial pneumonia and heart attacks, with three quarters of influenza-related deaths being for respiratory and circulatory diseases.

"The estimated number of influenza deaths over the study period was 17 times higher than the number recorded in standard mortality statistics," Professor Baker says.

Seasonal influenza probably remains New Zealand's biggest single infectious disease killer, accounting for 1.6% of all medical deaths recorded over the 1990 to 2008 period, he says.

"These findings reinforce the need to continue efforts to develop and implement effective measures to prevent influenza and reduce the harm it causes to vulnerable groups. Seasonal influenza vaccination is free and recommended for pregnant women, those aged 65 years and over, those under 65 with specified medical conditions, and children under five years with a history of significant respiratory illness."

The study has been published online in the international journal Influenza and Other Respiratory Viruses.