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US swine flu cases dissected

May 8, 2009 By Natasha Gilbert This article courtesy of Nature News.

An up-to-date analysis suggests the young may be particularly susceptible.

The latest analysis of the first 642 confirmed human cases of swine flu in the United States has revealed some clues about the distribution of the virus across the country, the susceptibility of different age groups and the disease's symptoms.

The study, conducted by the Centers for Disease Control and Prevention (CDC) in Atlanta, tracked the virus, officially known as influenza A (H1N1), from 15 April to 15 May, over which time it spread across 41 states. In New York City, 80 cases have been confirmed — the second-highest number of incidents out of the 41 states, and no cases were reported in the District of Columbia, the study says (see map below).

The study, led by Fatimah Dawood, with the US Epidemic Intelligence Service at the CDC, was published yesterday in The New England Journal of Medicine1.

The largest proportion of patients (60%) infected with the virus were 18 years of age or younger, and only 5% were over 50 years old. The study puts forward several suggestions for this observation, including that children and young adults may be more susceptible to infection than older people or that transmission to older people has been slower because they mix with fewer people.

Clinical data was obtained for between 295 and 397 patients. The study reports that the most common symptoms were fever and coughing, which is similar to the symptoms of seasonal flu. Out of 394 patients, 371 suffered from fever, and out of 397 patients, 365 had a cough. But other symptoms included diarrhoea, which was exhibited by 82 of 323 patients, and vomiting, which was found in 74 of 295 patients. Neither of these symptoms are typical of seasonal influenza.

Transmission question

The study says that the mode through which the virus spreads between humans is still unknown. Because many patients suffered with diarrhoea, the potential for the virus to be spread through faeces should be considered and investigated, although the study says it is suspected that the virus may be transmitted through the airborne droplets produced by sneezing.

"Until further data are available, all potential routes of transmission and sources of viral shedding should be considered," it says.

Out of 399 patients for whom information was available, 36 required hospitalization. Out of the 22 patients for whom hospital data was available, 8 required admission to intensive care units, 4 patients had respiratory failure requiring mechanical ventilation and 7 reported having travelled to Mexico within the 7 days before the onset of their illness. As of 5 May, 18 out of the 22 patients have recovered. Two remain critically ill — a 23 month old child and a 30 year old woman, both of whom were healthy prior to infection. Two others died of the illness.

As of today, the CDC reports 896 cases of the virus in the United States, with 2 deaths, and 2,384 cases worldwide, with 44 deaths from the virus in total.

John McCauley, a virologist at the UK's National Institute for Medical Research, told Nature News that the figures on age distribution are one of the most important factors to look at. If the 50-plus age group experiences milder symptoms, that could suggest that they may have built up some resistance to other strains of swine flu that have been circulating since the 1950s.

But he adds, "There may be other reasons for the peculiar age distributions. If you are older you may come into contact with fewer people.


  1. Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team N. Engl. J. Med. doi:10.1056/NEJMoa0903810 (2009).


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