A doctor administers a flu shot

A new U.S. study published in the journal "The Lancet" says worldwide resistance to drugs to treat influenza is increasing.  The finding could be bad news amid fears of a possible global flu pandemic.

Drugs known as adamantanes have been used for the past 30 years to treat influenza.  Over time, and with repeated use, the drugs have become less effective in fighting flu viruses.

In the largest study looking at resistance to two drugs, amantadine and rimantadine, researchers at the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, found that resistance among circulating flu virus strains increased from 0.4 percent between 1994 and 1995 to just over 12 percent between 2003 and 2004.

Investigators analyzed gene mutation among 7,000 influenza A strains, and found that 60 percent of the resistant strains isolated since 2003 came from Asia. In some Asian countries, 70 percent of the isolates contained mutations that made them resistant to adamantanes.

Currently, there is concern about a particularly lethal strain of avian flu, known as H5N1, that has been transmitted to a small number of humans from infected birds.  Public health officials worry that the virus, which has resulted in some deaths, could cause a global pandemic. But so far, experts say avian flu does not appear to be easily transmitted from person to person. 

Spokesman Tom Skinner of the Centers for Disease Control (CDC) says luckily, new anti-viral drugs have come along to treat influenza.

"And what this paper in the Lancet really attests to is the need for us to have really good surveillance for the emergence of drug resistant influenza, and the emergence of novel strains or new strains of influenza, so that we can move quickly to prevent the spread of these influenza viruses when they occur," said Mr.Skinner.

But Mr. Skinner says it's premature to say whether amantadine and rimantadine would have no role to play in an avian flu pandemic since no one can predict what it might look like, or when or where it might strike.