Since the PCV7 early childhood vaccine for bacterial pneumonia was introduced in the United States in 2000, scientists have found a 70 percent increase in the rate of a pneumonia complication called empyema, a serious and sometimes life-threatening infection in a cavity between the lung and chest wall.

This dramatic rise in the incidence of empyema may partly be because the PCV7 eliminates certain types of pneumococcus, which provides an opportunity for more virulent and antibiotic-resistant types of bacteria to take its place.

Among children younger than 5, the rate of empyema has increased 100 percent, from 3.8 per 100,000 in 1997 to 7.6 per 100,000 in 2006.