A study of swine flu has found a key characteristic is that many of those who become extremely sick have no underlying health problems.
Seasonal influenza exacts its heaviest toll on the elderly and those with severe medical conditions.
By contrast, critical illness due to swine flu was commonest in infants and middle-aged people, said the leader of the Australia-New Zealand study, Clinical Associate Professor Steve Webb, of Royal Perth Hospital.
Pregnant women, the obese, and Pacific Island, Maori and Aboriginal patients were particularly affected, said Dr Webb.
"Overall, about one-third of patients admitted to an intensive care unit because of swine flu had no underlying health problems."
The three-month study by Australia's Monash University of Australian and New Zealand intensive care units, conducted at the height of the pandemic, found 722 swine flu patients were admitted to ICUs, 96 of them in New Zealand.
At the peak of the pandemic, up to 20 per cent of ICU beds were occupied by swine flu patients, says the study report, published in the New England Journal of Medicine, a leading international journal, based in the United States.
Dr Colin McArthur, clinical director of the adults' ICU at Auckland City Hospital, said the impact of the virus on intensive care units was dramatic.
"... the surge of patients with H1N1 influenza [swine flu] placed substantial strain on staff and resources.
"The most severely affected patients had pneumonia affecting both lungs that was caused by the virus. The number of patients and the long stays in ICU required by some patients stretched the resources of many hospitals in both countries."
Professor Rinaldo Bellomo, director of intensive care research at Austin Health in Melbourne, said the findings would assist other countries to make better preparations for influenza.
"... many countries in the Northern Hemisphere will benefit from the lessons we have learned.
"Fortunately a vaccine will soon be available to help prevent infection with swine flu and its complications and this study will also assist countries in deciding how best to protect their populations."
The New Zealand Ministry of Health said on Wednesday that four people were in hospital with confirmed swine flu or its complications.
Based on the number of flu-like cases seen by "sentinel" general practices, the weekly rate of influenza now appears to be the usual level for this time of year and well down on the peak in July.
The death toll from swine flu stood at 18 on Wednesday, unchanged from last week, the ministry said. Some deaths were still being investigated by coroners.