Many people who've died from swine flu also have been infected with pneumonia bacteria, underscoring the need for vaccination against that bug, too, federal health officials said Wednesday.

Of 77 people who died from complications of the H1N1 strain between May 1 and Aug. 20, 22, or nearly 30 percent, also had bacterial co-infections, including some caused by pneumococcus, according to a Centers for Disease Control and Prevention report.

Seven of the victims were children aged 15 and younger, the report showed.

Those infections likely worsened the illness and contributed to the deaths, six of which occurred in previously healthy people with no known medical conditions, the data showed. CDC officials urged people at high risk for flu complications to check with doctors about getting a pneumonia vaccine in addition to the recommended seasonal and H1N1 flu shots.

It's those bad pneumonia infections that are the complicating factor," said Dr. William Schaffner, an infectious disease expert and director of preventive medicine at Vanderbilt University.

Influenza can cause viral pneumonia, but it can also weaken the system enough to allow opportunistic bacteria to surge, resulting in separate, potentially deadly infections, said Dr. Gregory Poland, an infectious disease expert at the Mayo Clinic.