Obesity may aggravate flu.

Most research on infections has been conducted in animals and people of normal weight. With an obesity epidemic under way, will existing data enable scientists to predict how people of above-average weight will respond to infectious agents? Fat chance, at least for flu, according to a study in mice.

A recent trial by Melinda Beck's team at the University of North Carolina at Chapel Hill indicated that heavy people don't derive the expected protection from flu vaccines. So, her group raised 140 female mice for 22 weeks, feeding half of them normal diets and half of them extremely fattening fare. Then, the researchers infected all the animals with influenza virus.

Most lean mice developed mild disease, and only 4 percent died. The obese mice, in contrast, got extremely sick, and 40 percent of them died, said Beck at the Experimental Biology meeting in San Diego early this month.

The fat animals' poor outcomes reflected severely impaired immunity, says Beck. Compared with their leaner cohorts, the heavy animals produced lower quantities of infection-fighting agents. Also, it took obese mice twice as long--6 days--to rev up production of inflammatory compounds important in fighting the virus.

Finally, the researchers extracted from the animals' spleens natural killer cells, the special forces for ridding the body of infected cells. In the test tube, killer cells from the obese animals were only 50 percent as effective as were cells from the lean mice.

Such data suggest that ostensibly healthy, but obese people might benefit from being added to priority lists for vaccinations, Beck says. They may need tailored vaccines or simply bigger doses of flu vaccines than lean people get, she adds.--J.R.