Intensive checking for influenza at two hospitals has shown that the number of patients with the potentially severe infection is around one-third greater than was previously thought.
The finding from a United States Government-funded study at Auckland City and Middlemore Hospitals has implications for public health programmes, especially for babies and the elderly, among whom the disparities were largest.
Lead researcher Dr Sue Huang, of the Institute of Environmental Science and Research, said when releasing selected data from the 2012 autumn/winter flu season that traditionally there was no standardised definition for identifying suspected influenza in hospital. It was based on whether a doctor thought flu was likely, followed by laboratory tests.
But for the study, patients with a history of a high fever, cough, onset of symptoms within the preceding week and needing to be admitted to hospital were defined as having a severe acute respiratory infection, of whom the majority were tested for influenza and other respiratory viruses.
The study found the highest age-based rate of influenza hospital admissions was among infants aged less than 1 year, at 229 cases for every 100,000 children.
This contrasts with a rate of around 150 per 100,000 using the traditional coding method.
Dr Huang said it was a surprise to find the incidence of hospital patients with influenza-related severe acute respiratory infection was much higher than thought, especially in the very young and the elderly.
Middlemore paediatrician Dr Adrian Trenholme said the research's standardised and intensive testing had shown that young children with respiratory illness often had more than one kind of micro-organism causing harm.
"We have very high rates of disadvantaged kids admitted with respiratory and chest infections. We now understand the viruses causing that; part of that is influenza.
"We know that under-4s are vulnerable. Proving it in our local population makes us focus on viruses we can deal with. Part of that is immunisation strategies. What are parents with a vulnerable infant in their family doing? Are they immunised? And we can talk about hand-washing and other simple protective measures."
Other study findings were that from May to mid-October, some 2100 cases of severe acute respiratory infection (SARI) were reported in the two hospitals - 3.4 per cent of admissions. Of the 1500 cases tested, 22 per cent had influenza viruses. Ninety-three people with a SARI were admitted to an intensive care unit and 14 died.
The research is one of a series of studies - the Southern Hemisphere Influenza Vaccine Effectiveness Research and Surveillance (Shivers) project - planned to run for five years and funded by the US Centres for Disease Control and Prevention. The aims include guiding better vaccine design and developing targeted vaccination strategies for population sub-groups.By Martin Johnston Email Martin