Many people who have died of swine flu infections 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.

The CDC has already reported that H1N1, declared a pandemic in June, has become more active as weather cools and schools return from summer breaks. Cases are reported in all 50 states.

“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 said in a statement.

“It’s really important for people, especially those at high risk for the serious complications from influenza, to check with their provider when they get their influenza vaccine about being vaccinated against pneumococcus.”

Wyeth’s Prevnar is part of the routine series of immunizations that children should get, and another vaccine against so-called pneumococcal bacteria is available for elderly adults.

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, the CDC said.

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

WASHINGTON (Reuters)