One of the most systematic looks yet at the swine flu pandemic confirms that it is at worst only a little more serious than an average flu season and could well be a good deal milder, researchers said on this week.
They analyzed data from Milwaukee and New York, two U.S. cities that have kept detailed tabs on outbreaks of H1N1, to calculate a likely mortality rate of 0.048 percent.
"That is, about 1 in 2,000 people who had symptoms of pandemic H1N1 infection died," Dr. Marc Lipsitch of Harvard University and colleagues wrote.
Probably 1.44 percent of patients with H1N1 who were sick enough to have symptoms were hospitalized, and 0.24 percent required intensive care, they added.
The findings, published in PLoS Medicine, a Public Library of Science journal, should be reassuring to public health officials and policymakers who worry that a flu pandemic could kill millions and worsen the global recession.
They do not, however, guarantee that H1N1 will not worsen, or that some other, stronger, strain of flu will not emerge.
"We have estimated ... that approximately 1.44 percent of symptomatic pandemic H1N1 patients during the spring in the United States were hospitalized; 0.239 percent required intensive care or mechanical ventilation; and 0.048 percent died," Lipsitch and colleagues wrote.
Using a different method and New York City data only, they said they calculated a much lower death rate of 0.007 percent.
Health experts agree it is impossible to count precisely how many people have been sickened by H1N1, which was declared a pandemic in June.
Few people are tested, tests are inaccurate and many people only have mild illness. So careful projections give a more accurate picture of a pandemic than actual counts of confirmed illnesses and deaths.
Lipsitch specializes in these sorts of calculations and a global estimate he did in September gave similar projections.
One open question is how many people have actually been infected. The U.S. Centers for Disease Control and Prevention estimated in November that number was 22 million Americans.
Lipsitch's team calculated a potential range of 7,800 to 29,000 deaths.
This compares to seasonal flu, which kills 36,000 people a year and puts 200,000 into the hospital.
"To date, symptomatic attack rates seem to be far lower than 25 percent in both the completed Southern Hemisphere winter epidemic and the autumn epidemic in progress in the United States," the researchers added in their report, published at http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fjournal.pmed.1000207.