Many people who have died of H1N1 flu in the United States have also had bacterial infections, health officials reported on Wednesday.

A study of 77 patients who died of the new pandemic H1N1 virus showed 29 percent of them had so called bacterial co-infections, the U.S. Centers for Disease Control and Prevention reported.

About half of these had Streptococcus pneumoniae, which can be prevented with a vaccine, the CDC said. It said doctors may be missing these infections in people severely ill with flu.

The CDC has already reported that H1N1, declared a pandemic in June, has become more active as weather cools and schools reopened after summer breaks. Cases are reported in all 50 states and it is still circulating globally.

H1N1 is not any more deadly than seasonal influenza so far but it attacks a younger age group than seasonal flu does and because virtually the entire population lacks immunity, it can infect far more people at once than seasonal flu usually does.

"The findings in this report indicate that, as during previous influenza pandemics, bacterial pneumonia is contributing to deaths associated with pandemic H1N1," the team of experts at the CDC and state health departments reported.

"Our influenza season is off to a fast start and unfortunately there will be more cases of bacterial infections in people suffering from influenza," CDC epidemiologist Dr. Matthew Moore, who helped organize the study, added in a statement.

The report noted in previous pandemics — in 1968, 1957 and 1918 — many of the patients who died were also infected with S. pneumoniae, Haemophilus influenzae, Staphylococcus aureus and group A Streptococcus, which causes rheumatic fever and "strep throat".

The CDC team noted that at first it did not appear that people who were seriously ill with swine flu or who died of it had secondary infections but doctors may have missed them.

"Routine clinical tests used to identify bacterial infections among patients with pneumonia do not detect many of these infections," the CDC team reported.

Five of the patients who died, including a 9-year-old and an 11-year-old, had infections with the so-called superbug methicillin-resistant S. aureus or MRSA. None of the seven children who died had reported medical conditions that should put them at special risk of flu complications, although one was obese and one had Down syndrome.

The researchers cautioned that the patients whose cases were studied may not represent the nation as a whole. But like most of the victims of swine flu, they were young, with a median age of 31 and ranging from 2 months to 56 years.

Moore said people getting flu vaccinations should also ask about getting a pneumococcal vaccine.

Wyeth's Prevnar is part of the routine series of immunizations that children should get, and Merck and Co. makes a vaccine against so-called pneumococcal bacteria that is available for adults, mostly those over 65. Merck also makes an Hib vaccine, although there is no vaccine to prevent group A streptococcal infections or MRSA.

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