CDC: Swine flu sickened 57 million Americans 

About 57 million Americans, or slightly over 18 percent of the population, have contracted pandemic H1N1 influenza since April, federal public health officials said Friday.

About 11,690 people have died from the viral infection, also known as swine flu. Nearly 260,000 have been hospitalized. Adults ages 18 to 64 have been hit much harder than either children or the elderly. They account for 58 percent of the infections and hospitalizations and 76 percent of the deaths.

This demographic profile of the H1N1 pandemic is markedly different from that of seasonal flu, in which 60 percent of hospitalizations and 90 percent of flu-related deaths occur in people 65 years and older. Epidemiologists believe the pandemic virus is distantly related to a strain that circulated more than 40 years ago. People alive then who were infected with that strain might have partial immunity to the new one.

New estimates, released by the Centers for Disease Control and Prevention, cover the period from the swine flu's emergence in Mexico and Southern California in late April through mid-January. In addition to effects of the H1N1 virus so far, the estimate clearly shows the new strain is in retreat. Only 2 million cases, and just more than 500 deaths, occurred between Dec. 11 (the date of the last estimate) and Jan. 16.

A CDC survey released last month found that about 70 million Americans have been vaccinated against the virus. That means that potentially 127 million of the nation's 309 million residents -- 41 percent of the population -- might now be immune.

The percentage will be lower, however, if many people who got pandemic flu shots in the past few months were already immune because they'd unknowingly contracted the virus before that.

The fraction of infections that caused no symptoms -- a crucial variable in calculating "population immunity" -- is not known. A recent study from France estimated that only about 20 percent of people with evidence of H1N1 infection in their blood stream went to the doctor, suggesting that a large fraction of swine flu cases are extremely mild.

There have been two waves of H1N1 virus -- a small one in late spring and a large one in the fall, peaking in late October. How much of the population is now immune to the virus -- either through infection or vaccination -- is one of the variables that determines the likelihood of a third wave this winter. Public officials have not ruled out that possibility.

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A daily newspaper covering San Francisco, San Mateo County and serving Alameda, Marin and Santa Clara counties.
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