Number of flu cases higher than expected

Auckland City Hospital. File photo / Natalie Slade

The number of patients admitted to hospital with influenza is higher than previously thought, an important New Zealand study on the disease has found.

The Auckland and Counties Manukau district health boards, two universities and a state-owned science institute are doing a series of studies on influenza, funded by United States Government health authorities. The findings from the first year of the research were made public at Auckland City Hospital this afternoon.

Lead researcher Dr Sue Huang, of the NZ Institute of Environmental Science and Research, said, "We were surprised that the incidence of hospitalised patients with influenza-related severe acute respiratory infections is much higher than we previously thought, particularly in very young children ... and those over 65 years."

From May to August, 1370 patients were recorded with severe acute respiratory infections at the two DHBs' hospitals, including 38 who were admitted to an intensive care unit. Seven died.

Infants under 1 year had the highest age-group rate of hospitalisation for influenza, at 229 cases per 100,000 children in that age range. This was followed by people aged 80 or more, at 129 cases per 100,000. For those aged 65 to 79, the rate was 81 per 100,000 and for children aged 1 to 4 it was 55 per 100,000.

"Influenza causes more illness each year than any other vaccine-preventable illness," said Dr Huang. "This study highlights the importance of tracking the viruses which cause influenza across the population so we can better protect against them through the right vaccines, prevent their spread and care for those who do get the flu."

In June and July, A (H1N1) swine flu pandemic viruses were the main kind of influenza in Auckland, but A(H3N2) viruses came to predominate in August and September. The latter kinds dominated the winter flu season for the rest of New Zealand.

A (H3N2) viruses tend to cause a greater disease burden and substantial hospitalisation rates, particularly among the very old and very young. An influenza outbreak in Christchurch during the winter was dominated by those viruses and led to the city's Public Hospital setting up an isolation ward and cancelling elective surgery for a week.

The aims of the five-year, US-funded research - which is based on beefed up influenza surveillance systems - include guiding better influenza vaccine design and the targeting of vaccination strategies for population sub-groups.