Mysteries of the Flu

There is a joke among flu researchers: "If you've seen one flu season, you've seen…one flu season." The translation, for those not up on epidemiological humor: the joke is wry commentary on the unpredictable nature of the flu virus. Every year it looks different, and every strain follows its own pattern. This is not just a quirk that frustrates scientists—it's the reason new strains like H1N1 are impossible to anticipate and fully prepare for.

"I know less about influenza today than I did 10 years ago," quips Dr. Michael Osterholm, director of the Minnesota Center of Excellence for Influenza Research and Surveillance and a former adviser to the U.S. Department of Health and Human Services. "Every stone we've turned over, we get more questions than we do answers."

The flu returns every season and the world periodically experiences catastrophic pandemics, but epidemiologists still do not understand why some strains evolve to infect people and others do not; they are not entirely sure about how the flu is transmitted; nor do they understand why some patients become fatally ill while others develop minimal symptoms. As a result, when a new strain shows up—like H1N1—they often have little information to fall back on, and the lessons of previous pandemics are only somewhat helpful. While epidemiologists are still putting together a complete picture of H1N1, for example, its most striking difference with the seasonal flu is that the elderly are not the most vulnerable population. And when H1N1 does cause serious illness, patients develop different complications (that are more difficult to treat) than those with seasonal flu. "It's a very different death," says Osterholm.

The Centers for Disease Control currently maintains six different categories of flu-surveillance programs, but has rolled out new measures this year in order to monitor H1N1's most worrying features. The backbone of its routine surveillance systems is not designed to count individual flu cases, but rather to get general indicators of how widespread the flu is and which strains are in circulation. Uncertainty about the fast-moving H1N1 prompted the CDC to begin asking state health departments to report the number of hospitalizations and deaths caused by influenza, and it is still adjusting the methods for calculating the disease's impact. Even the methods for counting the number of those who died of H1N1 is uncertain: on Tuesday, The New York Times reported that the CDC will revise its estimates of H1N1 deaths to 4,000 from 1,200. This revised figure is the result of a new calculation that encompasses fatal cases confirmed via lab tests to have been caused by H1N1, as well as hospital reports of deaths that "appear to have been brought on" by the flu.

Influenza's chimerical nature makes it a moving target for researchers, says Dr. Allison Aiello, assistant professor of epidemiology at the University of Michigan. (NEWSWEEK reached Aiello by phone at home, where she and her husband are both recovering from H1N1 under self-imposed quarantine to protect their infant child.) "Even if we had [complete] seasonal flu data from the past, it wouldn't necessary be helpful for a new strain" of influenza, she explains.