The main strain causing flu illnesses in the U.S. is resistant to the antiviral drug Tamiflu, a government report released Monday shows.

As of Feb. 19, researchers found 264 of 268, or 98.5 per cent, of influenza A viruses tested were resistant to oseltamivir, or Tamiflu, researchers reported in Monday's online issue of the Journal of the American Medical Association (JAMA).

Last flu season, 19 per cent of H1N1 viruses tested were resistant to the drug, said Dr. Nila Dharan of the Centers for Disease Control and Prevention in Atlanta and her colleagues.

In December, the CDC first warned doctors that they were noticing resistant to Tamiflu among H1N1 strains.

At the time, the CDC advised giving Tamiflu with another anti-viral drug, Relenza, also called zanamivir, or a generic drug called rimantadine.

For the study, the researchers interviewed 99 patients. They found 30 per cent of them were vaccinated against the flu, but came down with it anyway.

Many of the deaths occured in children who had other medical problems, the study's authors said.

The findings emphasize the need to develop new anti-viral drugs and tests to quickly distinguish what strains of flu are causing illness, they added.

100% resistance in Canada

In an editorial accompanying the study, Dr. David Weinstock of the Dana-Farber Cancer Institute and Dr. Gianna Zuccotti of Brigham and Women's Hospital, both in Boston, said that understanding of influenza has improved.

"However, the global dissemination of oseltamivir-resistant influenza came as a great surprise," they wrote.

"For now, the best tools to mitigate influenza infection are tried-and-true— vaccination, social distancing, handwashing, and common sense."

In Canada, all 81 samples of H1N1 tested this flu season by the National Microbiology Laboratory were also resistant to the drug. The samples were from British Columbia, Alberta, Saskatchewan, Ontario, Quebec, Nova Scotia and Prince Edward Island.

"The identification of resistant influenza samples poses no immediate threat to public health, as antimicrobial resistance continues to be a reality that will require ongoing monitoring and further study," said Andrew McDermott, a spokesperson for the Public Health Agency of Canada in Ottawa.

"Common antivirals such as oseltamivir are not widely used in Canada to treat seasonal flu, and influenza can be effectively prevented through immunization and other prevention measures like handwashing, covering coughs and staying home when sick," he added in an email to CBC News.

Isolating patients recommended

A second study appearing in the same issue warned that Tamiflu-resistant virus posed a risk to patients with weakened immune systems who are hospitalized.

The Dutch researchers recommended isolating patients to avoid hospital outbreaks, but noted their conclusions were based on a small number of subjects.

A third flu study, in the same JAMA online issue, concluded that the flu shot beats the nasal form of the vaccine for reducing influenza- and pneumonia-related trips to the doctor among adults.

The researchers looked at more than one million military personnel between 2004 and 2007.