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Swine flu may be mildest pandemic ever, researchers say

December 8, 2009 |  4:00 pm

Pig The first comprehensive study of pandemic H1N1 influenza from April to the end of July indicates that the pandemic may be the mildest ever, assuming that the virus doesn't mutate during the winter and come back stronger than before. The analysis suggests that the swine flu virus might directly cause as many as 45,000 deaths in the United States by the end of winter but that the most likely figure is somewhere between 10,000 and 15,000 deaths. In a worst-case scenario, the virus would kill no more than 45,000 people, well below earlier estimates that suggested as many as 90,000 could die in the pandemic.

A typical flu season is associated with about 35,000 deaths. In 1957, the Asian flu pandemic killed about 70,000 Americans.

Making accurate projections about the course of a pandemic is difficult, both because the virus itself is so unpredictable and because it is hard to obtain accurate estimates of the total number of people infected. In the new study, a team led by epidemiologist Mark Lipsitch of the Harvard School of Public Health studied data collected in New York City and Milwaukee, which have some of the most effective surveillance programs available. They reported their results in the journal PLoS Medicine.

Their best estimate was that during the April-to-July period, 1.44% of patients with swine flu symptoms were hospitalized, 0.239% required intensive care or mechanical ventilation and 0.048% died. The team concluded that about 15% of the population would ultimately be infected by the virus. The previous worst-case scenario estimating that 90,000 could die was based on the virus infecting 30% of the population.

But the virus can still be exceedingly lethal. Another study led by Dr. James R. Gill of the New York City Office of Chief Medical Examiner studied the lungs of 34 people who died of swine flu. They reported in the Archives of Pathology & Laboratory Medicine that the virus did most of its damage in the upper airway -- the trachea and bronchial tubes -- like seasonal flu. But unlike seasonal flu, the virus also damaged tissues deep in the lungs. That damage paved the way for bacterial infections, which were found in more than half the victims.

They also found that 91% of those who died suffered from underlying medical conditions, such as heart disease and respiratory illnesses like asthma, and 72% of those who died were obese.

The virus is also particularly lethal in children with sickle cell disease, according to researchers from the Johns Hopkins Children's Center. Dr. John J. Strouse and his colleagues reported Monday at a New Orleans meeting of the American Society of Hematology that sickle cell children who were infected with swine flu were three times more likely than those infected with seasonal flu to develop acute chest syndrome, marked by inflammation of the lungs, reduced oxygen capacity and shortness of breath. That is a leading cause of death among such patients. They were also five times more likely to end up on a ventilator and more likely to need a blood transfusion.

The same team reported earlier this year that sickle cell children who were infected with seasonal flu were 80 times more likely than other children to be hospitalized. The findings emphasize the critical need for such children to be vaccinated against both forms of the flu, Strouse said.

-- Thomas H. Maugh II