UC Davis researchers say that while the incidence of bacterial pneumonia has declined since the introduction of an early childhood vaccine nearly a decade ago, there has been dramatic increase of a relatively uncommon but nevertheless serious and sometimes life-threatening complication.

The data from the comprehensive, national study show a 70 percent increase in the incidence of empyema, an infection in the cavity between the lung and the chest wall, compared with a 50 percent drop in hospitalizations from pneumonia due to pneumococcus and a decrease in all bacterial pneumonias.

The study did not directly examine the reason for the increased incidence of empyema. But the researchers conjectured that the PCV7 vaccine, by eliminating certain types of pneumococcus, is creating opportunity for other bacteria that the vaccine does not target.

These other serotypes are responsible for less pneumonia overall, but, when they do occur, they may lead to more complicated pneumonias such as empyema.

In addition, the incidence of empyema caused by staphylococcus, another bacterial cause of pneumonia not addressed by pneumococcal vaccines, appears to be increasing. This increase in staphylococcal empyema may be due to more virulent and antibiotic-resistant forms of staphylococcus, such as methicillin-resistant staphylococcus aureus, or MRSA.

PCV7 came out in 2000 in an effort to protect infants and children from the seven serotypes, or strains of microorganisms, most commonly responsible for serious infections from Streptococcus pneumoniae, which is responsible for most community-acquired bacterial pneumonias.

Lead author Su-Ting T. Li and co-author Daniel Tancredi, both assistant professors in the Department of Pediatrics at UC Davis Children’s Hospital, said parents should continue to immunize their children with the pneumonia vaccine because it has proven highly effective in combating invasive pneumococcal disease.

“They should certainly immunize their kids because the fraction of pneumonia that tends to result in empyema is still low, so there is a huge benefit from immunization,” Tancredi said.

But parents should be watchful for complications from the types of bacterial pneumonia not covered by the vaccine. Each year, empyema causes significant morbidity with prolonged hospitalizations and multiple invasive procedures.

Li said an expanded vaccine that would cover the serotypes associated with empyema is in the works and is expected to be approved and distributed within a few years.

The researchers launched the study after learning of conflicting regional data on empyema incidence since the introduction of PCV7. Since its introduction, empyema incidence was reported to have increased by 88 percent in Utah and 400 percent in California. Texas, on the other hand, reported a 55 percent decrease. The incidence in Quebec, Canada, reportedly remained unchanged.

The study used the nationally representative Kids Inpatient Database, or KID, to estimate the total annual empyema hospitalizations of children 18 and under in the United States.

The researchers took KID hospitalization counts from 1997, 2000, 2003 and 2006, and, using Census Bureau data, converted the counts into annual incidence rates per 100,000 children. The researchers then compared the findings to incidence rates from 1997 and later to gauge the impact of the PCV7 vaccine on hospitalizations.

For 2006, the researchers estimated 2,898 empyema-associated hospitalizations of children 18 and under in the United States. This converted to a hospitalization rate of 3.7 per 100,000, an increase of almost 70 percent over the 1997 empyema hospitalization rate of 2.2 per 100,000.

The incidence of complicated pneumonia, which includes empyema, pleural effusion, or bacterial pneumonia requiring a chest tube or decortication, went up 45 percent to 5.5 per 100,000.

While the incidence of hospitalization associated with complications from pneumonia went up, the incidence of bacterial pneumonia hospitalization went down 13 percent to 244.3 per 100,000. The incidence of invasive pneumococcal disease — pneumonia, sepsis or meningitis caused by Streptococcus pneumoniae — decreased 50 percent to 6.3 per 100,000.

The researchers also found that since the introduction of the PCV7 vaccine, the average age of children with empyema went down. In 2006, the mean age for children hospitalized for empyema decreased to just above 6 years, compared with a mean age of slightly more than 7 in 1997. For children younger than 5, the incidence of empyema hospitalization went up 100 percent, from 3.8 per 100,000 in 1997 to 7.6 per 100,000 in 2006.

In empyema infection, purulence, or pus, develops in pockets in the pleural space between the outer surface of the lung and chest wall. The pus pockets can impede breathing by pressing against the lungs.

Early intervention is key to decreasing mortality. Interventions range from antibiotics to minimally invasive drainage with a catheter, to the riskier open surgical removal of the layer of pus restricting lung expansion.

Phyllis Brown is a senior public information officer for the UC Davis Health System.