A powder inhaler that works against both types was approved by federal regulators in July.
The pill, to be marketed as Tamiflu, helped reduce the duration and severity of flu symptoms by about half in 80 unvaccinated adults who were voluntarily infected with flu virus, researchers reported in Wednesday's Journal of the American Medical Association.
The prescription drug also helped prevent infections in unvaccinated adults who took the drug before being exposed to the virus, said researchers led by Dr. Frederick G. Hayden of the University of Virginia. Only eight of 21 subjects who took the drug before exposure got infected (38 percent), compared with eight of 12 who didn't take the drug (67 percent).
"These drugs are not a substitute for vaccine. They're an adjunct, a supplement, in terms of the protection they can provide," Hayden said Monday.
Vaccination is always preferred, but some people can't take vaccines because of medical conditions or allergies, many people don't get vaccinated and vaccination does not prevent all flu, he noted.
Tamiflu (generically called oseltamivir) and the powder inhaler, Relenza (zanamivir), are in a new class of medicines that cripple the chemical process used by flu viruses to spread from cell to cell. Unlike older drugs, they work against both A and B viruses.
The two older flu medicines, Symmetrel (amantadine) and Flumadine (rimantadine) have a different mechanism of action. Though they are both available as pills, they only work against flu virus A, which accounts for about two-thirds of the estimated 20 million U.S. flu cases yearly, usually the most severe.
An expert not involved in the new study said the new medicines also have the advantage of affecting flu virus later in its life cycle, so are less likely to cause genetic mutations that lead to drug resistance.
But the expert, Dr. Ben Z. Katz, an infectious disease specialist at Children's Memorial Hospital in Chicago, said the study is limited by having been done only on healthy adults infected in a laboratory, and results might be different with flu in the real world.
Hayden said that subsequent studies have since shown Tamiflu effective with few side effects. He said the drug is to be considered for approval later this month by the Food and Drug Administration. As a matter of policy, the FDA does not comment on its schedule for considering drug approvals.
The research was funded by the drug's manufacturer, F. Hoffman LaRoche Ltd., and by the National Institutes of Health.
No price has been set for Tamiflu, according to Hoffman-LaRoche. Relenza costs $37 wholesale for a five-day course of treatment.