July 6 (Bloomberg) -- Swine flu, which has spread to 114 countries since April, may be at least 40 times less lethal than an earlier estimate, researchers in New Zealand said.

The probability of dying from the pandemic flu strain may be 1 in 10,000 cases and possibly as low as 1 in 100,000, doctors at the University of Otago in Wellington found. They applied four methods to calculate the proportion of fatal cases, and compared the results with an earlier estimate based on data from Mexico of 4 in 1,000.

Fatalities would be more common if the new A/H1N1 virus mainly affected the elderly, for whom flu usually poses the greatest risk, the scientists said in a study published in the journal Eurosurveillance last week. Gauging the pandemic’s ability to kill and cause serious disease is a key measure in assessing and determining how to respond to the threat, said co- author Michael Baker, an associate professor of public health at the university.

“The implications are potentially huge in terms of where you focus your effort responding to this,” Baker said in a July 2 telephone interview. “One of the reasons why this influenza pandemic might be having a very low case fatality is simply because it is infecting mostly younger people.”

The pandemic bug appears to be as contagious as seasonal influenza, and is spreading faster among people aged 10 to 45 years, according to the World Health Organization. The severity of the disease ranges from mild symptoms such as sore throat and muscle ache to severe illnesses including pneumonia that can result in death, WHO said.

‘Spread Is Unstoppable’

“Once a fully fit pandemic virus emerges, its further international spread is unstoppable,” WHO Director-General Margaret Chan told a meeting on the flu pandemic in Cancun, Mexico, last week. “Influenza pandemics are remarkable events because they spread throughout a world population that is either largely or entirely susceptible to infection.”

The Geneva-based agency has recorded 89,921 cases worldwide, including 382 deaths, since the virus was discovered in North America in mid-April. Dividing reported infections by the number of deaths isn’t a true reflection of the fatality rate because once community-wide transmission occurs, infections become too numerous to be accurately counted, Baker said.

Doctors treating young people with severe viral pneumonia may see the pandemic as being “pretty nasty,” he said. “From the population point of view, it’s actually not looking quite so bad.”

Annual Epidemics

Influenza epidemics occur yearly during autumn and winter in temperate regions and throughout the year in some tropical countries. They result in 3 million to 5 million cases of severe illness and about 250,000 to 500,000 deaths, mostly in people older than 65 years, according to WHO.

The pandemic virus is spreading in the Northern Hemisphere summer, especially among younger people who have no natural immunity to it. Its ability to infect more people might increase the total number of annual flu deaths by 1.4 million, the World Bank in Washington said in a report last month.

“This is behaving very much like the first wave of previous pandemics, which cause widespread morbidity but very little mortality,” Baker said. “The pandemic mainly does its damage because it infects a lot more people and also because of the shift to the younger age groups, and deaths are much more significant in those age groups.”

To contact the reporter on this story: Jason Gale in Singapore at j.gale@bloomberg.net.

Last Updated: July 6, 2009 01:00 EDT