New details on H1N1

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The H1N1 influenza virus that brought the world to the brink of a pandemic is sure to spawn reports in scholarly journals for years to come. The first take from epidemiologists at the U.S. Centers for Disease Control and Prevention was published today in the New England Journal of Medicine.

The report traces the origins of the swine-origin flu from the first coughs of a 9-year-old Imperial County girl on March 28 – the earliest-known onset among U.S. cases – through May 5. In that time, 643 cases have been confirmed in 41 states, and two patients have died.

Copious media coverage has already conveyed the big ideas: The novel flu has its roots in strains that have been circulating in pigs, and the victims have tended to be much younger than those who fall ill to the typical seasonal influenza that strikes each year. But the report slices and dices the data in new ways.

Among the findings:

-- Patients range in age from 3 months to 81 years, with 60% age 18 or younger and only 5% above the age of 50.

-- Fever, cough and sore throat were the most common clinical symptoms, but, unlike with seasonal flu, approximately 38% of patients also reported diarrhea and/or vomiting.

-- Spread of the virus appears to have peaked around April 25 and dropped dramatically by the end of the month.

- - The states that haven’t reported a confirmed case of H1N1 flu are Alaska, Montana, Wyoming, North Dakota, South Dakota, Arkansas, Mississippi, West Virginia and Vermont, along with the District of Columbia.

-- 49 virus samples from 13 states have been genetically sequenced, and were 99% to 100% identical.

-- Six of the genes (PB2, PB1, PA, HA, NP and NS) were “similar” to a swine-bird-human flu strain that has been occasionally found in humans over the last several years. The other two genes (NA and M) “were most closely related to” a swine flu virus found in European and Asian pigs.

-- The HA gene has already been mutating – samples from the 49 patients differed by as many as four amino acids.

-- The H1N1 outbreak strain is so “antigenically distinct” from the typical human H1N1 flu that the latest seasonal flu vaccine “is not anticipated to provide protection.”

For more coverage of H1N1 in the New England Journal of Medicine, check out the H1N1 Influenza Center here.

-- Karen Kaplan

Graphic courtesy of New England Journal of Medicine