Falling Influence: Influenza fighters have limited effects

The most readily available drugs against influenza have abruptly declined in effectiveness in the past decade, and flu vaccines offer elderly people only modest protection against illness, according to two new analyses.

The findings lay bare limitations of both countermeasures, sounding a discouraging note as flu season draws near in the Northern Hemisphere.

One study finds that viruses are becoming resistant to drugs known as M2 blockers or adamantines. Those drugs, which impede the flu virus from copying itself, have been used to treat and prevent infections, mainly in the elderly, for nearly 4 decades.

“We can see resistance increasing to this class of drugs,” says virologist Rick A. Bright of the Centers for Disease Control and Prevention (CDC) in Atlanta. “Fortunately, we have other drugs, like Tamiflu.”

But Tamiflu, the trade name for oseltamivir, and its chemical relative zanamivir (Relenza) are patented, more expensive than M2 blockers, and manufactured by just one company each.

Bright and his colleagues conducted genetic testing on influenza viruses that had been collected from some 7,000 infected people around the world since 1994. Of viruses isolated in the 1994–1995 flu season, 0.4 percent had mutations conferring resistance to the M2 blockers. By the 2003–2004 season, 12.3 percent of all isolates carried resistance mutations.

Much of the increase has come since 2002, particularly in China and Hong Kong. There, 70 percent or more of isolates were resistant to M2 blockers in 2004.

The M2 blocker amantadine is widely used and available without a prescription in some countries, including China, note Yi Guan and Honglin Chen, both of the University of Hong Kong and Shantou University Medical College in China. The pair’s comments and the two new studies appear in an upcoming Lancet.

In June, news reports stated that China’s agricultural ministry has encouraged farmers to use amantadine on flu-infected poultry. Such use could increase viruses’ drug resistance, Guan and Chen say.

“What is needed is to improve control over the distribution of the adamantine drugs, particularly in developing and southeast Asian countries,” they propose, “so that these cheap and easily administered drugs can continue to play a part in our influenza control strategies.”

In the second new report, epidemiologist Tom Jefferson and his colleagues at the nonprofit Cochrane Vaccines Field in Alessandria, Italy, compiled data from 64 earlier studies in people 65 years or older.

Influenza vaccines reduced flulike illnesses by no more than 23 percent in long-term care facilities such as nursing homes.

Nevertheless, in these facilities, the vaccines prevented 46 percent of pneumonia cases, which can be a complication of influenza. Hospitalizations and flu- or pneumonia-related deaths dropped by similar amounts in those settings.

Outside such facilities, immunizations didn’t cut the number of flulike illnesses or pneumonia cases but did reduce their severity: These vaccines blocked up to 26 percent of hospitalizations and 42 percent of deaths.

Only a minority of flulike illnesses stem from influenza viruses, Jefferson says. The rest result from other pathogens or unknown causes.

“The public is told: Vaccinate yourselves and you’ll avoid [flulike illnesses]. That is not so. You’ll avoid a proportion of these” illnesses only, says Jefferson.

Says CDC spokesperson Tom Skinner, “We’re going to continue to recommend [that] high-risk groups get vaccinated every year.”

More Stories from Science News on Health & Medicine