An electron-microscope image shows a culture of the H1N1 influenza A virus from a California patient in April. The pandemic strain of virus has been 'amazingly stable,' the World Health Organization says.An electron-microscope image shows a culture of the H1N1 influenza A virus from a California patient in April. The pandemic strain of virus has been 'amazingly stable,' the World Health Organization says. (Reuters/C. S. Goldsmith and A. Balish/Centers for Disease Control/Handout)

A new estimate of the number of Americans killed by swine flu since April is about four times higher than previous estimates.

About 4,000 Americans have been killed by the H1N1 flu, including 540 children, the U.S. Centers for Disease Control and Prevention said Thursday.

The updated estimates don't mean the H1N1 influenza A virus responsible for the swine flu pandemic is more severe. However, they better reflect the true toll, based on more data on flu cases and deaths, the agency said.

"We think the 540 number is a better estimate for the big picture that we are getting out there" on pediatric deaths, Dr. Anne Schuchat, director of CDC's Center for Immunization and Respiratory Diseases, told reporters.

Federal public health officials believe they have a "good handle" on how many severe H1N1 cases there are in Canada and say there won't be a fourfold increase as in the U.S., said Dr. David Butler-Jones, Canada's chief public health officer.

Use Tamiflu prudently

A doctor from Quebec City has warned physicians to be cautious when prescribing Tamiflu to prevent illness in people exposed to the H1N1 virus but not infected with it.

Dr. Guy Boivin of Laval University analyzed North America's first reported case of a Tamiflu-resistant H1N1 virus in the New England Journal of Medicine on Wednesday.

A man from Quebec who had chronic lung disease was given Tamiflu as a prophylactic or preventive measure after his teenage son, who had asthma, contracted H1N1 and was treated with the drug.

A prophylactic dose is one pill a day. A treatment dose is two pills a day.

Seasonal flu usually affects more older people than children while H1N1 is doing the reverse. About 82 children in the U.S. die during an average flu season in the U.S.

"What we are seeing in 2009 is unprecedented," Schuchat said, noting this seems to be the worst flu season in the country since 1997, when the current measuring system began.

Seasonal flu shots

Getting a seasonal flu shot last year did not increase or decrease a person's risk of contracting the H1N1 influenza A virus this year, a new U.S. study suggests.

Researchers at the U.S. Centers for Disease Control and Prevention (CDC) looked at 356 adults in eight states who received flu shots last fall and adults with laboratory-confirmed swine flu in May and June.

"Current evidence from this study and other studies does not suggest that seasonal influenza vaccination either decreases of increases the risk for acquiring pandemic H1N1 illness," the team concluded in Thursday's issue of Morbidity and Mortality Weekly Report.

An editorial note published with the report said the weight of evidence doesn't support the unpublished findings of Canadian researchers who found that the seasonal 2008-09 flu shot was associated with a 1.5 to two-fold greater risk of contracting the pandemic virus and needing medical attention. Those findings were based on a series of five studies from four provinces.

Several other studies looking into the links between seasonal influenza and H1N1 using more rigorous study designs and methods are underway in the U.S. and other countries, the CDC said.

The U.S. agency now estimates about 98,000 people have been hospitalized in the first six months of the swine flu epidemic, including 36,000 children.

The estimates are based on an extrapolation of detailed data from 10 states, rather than a count of actual H1N1 cases.

The previous estimate on U.S. flu deaths was 1,200.

During an average flu season, about 36,000 Americans die and 200,000 are hospitalized, with 90 per cent of deaths and hospitalizations among people over 65.

Early antiviral therapy key for H1N1: WHO

To avoid deaths and serious complications from H1N1, the World Health Organization advised Thursday that pregnant women, young children and people with an underlying risk factor should start to take an antiviral at the first hint of contracting the virus.

Antiviral medicines can prevent H1N1 flu deaths and should be given quickly to those in high-risk groups or to an otherwise healthy person whose fever lasts more than three days or has trouble breathing, the United Nations health agency said.

People with risk factors such as severe obesity and a weakened immune system should also seek early treatment.

"The window of opportunity is very narrow to reverse the progression of the disease," Nikki Shindo of WHO's global influenza program told journalists in a teleconference from Geneva. "The medicine needs to be administered before the virus destroys the lungs."

In most cases, people have a regular bout of flu without the need for drugs or hospital treatment. In rare cases, people infected with the pandemic strain on H1N1 influenza A virus can die within a week.

Outbreaks in Eastern Europe

It is the start of the flu season in the Northern Hemisphere, but WHO is already receiving reports from Ukraine, Afghanistan and Mongolia of hospitals and clinics being overwhelmed by pandemic flu.

WHO recently shipped antivirals to Afghanistan, Mongolia, Belarus and Ukraine, and will soon send more supplies to Azerbaijan and Kyrgyzstan in the hopes that early treatment of at-risk patients could help ease the strain on health-care systems.

The virus has been "amazingly stable," Shinto said.

She also warned against taking antivirals as a preventive step, which could lead to the virus becoming resistant. Tamiflu and other antivirals should not be bought over the internet for the same reason, Shindo added.

At least 6,000 people have died since the start of the outbreak last spring, according to WHO.

With files from The Associated Press