COMMENTARY

Bacteremia and the Pneumococcal Vaccines

William T. Basco, Jr, MD, MS

Disclosures

July 17, 2017

The Decline in Pneumococcal Bacteremia Among Infants

Before the introduction of the conjugated pneumococcal vaccine, the incidence of bacteremia among febrile children 3-36 months of age was 1%-4%.[1] More recent studies have found that the bacteremia incidence in this same age group of infants who received the pneumococcal conjugate vaccine is < 1%. In fact, most of the studies have demonstrated the bacteremia frequency to be even lower, at < 0.5%.[1]

Greenhow and colleagues[1] evaluated changes in pneumococcal bacteremia rates among children aged 3-36 months with fever using data from Kaiser Permanente of Northern California. They analyzed data from the calendar years 1998 through 2014, and divided this study period into three eras:

  • Before the seven-valent pneumococcal conjugate vaccine (PCV7) (September 1998-March 2000);

  • The period when children received PCV7,but before the availability of the 13-valent pneumococcal conjugate vaccine (PCV13) (April 2000-May 2010); and

  • The period after the introduction of PCV13 (June 2010-August 2014).

Pneumococcal coverage was high among the children enrolled in the plan, so it should be emphasized that this study was done in an immunized population of children.

The investigators queried the electronic medical records to identify children from whom a blood culture was obtained, focusing on children without chronic medical or immunocompromising conditions. During the 16-year study period, an average of 98,000 children were enrolled annually in the plan.

More than 57,000 blood cultures were included in the study, 58% of which were obtained in outpatient clinics, 36% in emergency departments, and 6% during inpatient admissions. A pathogen was grown in 1% of these cultures, and 2% grew an organism that was later deemed to be a contaminant. The number of blood cultures obtained declined by 68% during the study years.

The incidence of bacteremia cases declined from 97 per 100,000 children/year before the introduction of PCV13 to 21 per 100,000 children/year afterward. Pneumococcal bacteremia declined even more dramatically, from 74.5 per 100,000 children/year to 3.5 per 100,000 children/year.

The annual incidence of bacteremia remained relatively constant for the other isolates, including Escherichia coli, which decreased from 9.4 to 8.4 per 100,000 children/year—a nonsignificant change. Salmonella and Staphylococcus aureus varied between 3.1 and 4.6 per 100,000 children/year. The incidence of Neisseria meningitides declined from 2.5 to 0.2 per 100,000 children/year, but the confidence intervals overlapped.

Although the absolute frequency of bacteremia from nonpneumococcal sources declined or remained constant during the study, the relative contributions of E coli, Salmonella, and Staphylococcus aureus increased over time. For example, after the introduction of PCV13, E coli isolates comprised 39%, Salmonella comprised 21%, and Staphylococcus aureus comprised 17% of pathogens.

The study authors concluded that bacteremia in healthy children aged 3-36 months who received the pneumococcal vaccine is rare.

Viewpoint

During the years after the introduction of PCV13, 76% of the pathogens were isolated from patients who had an identifiable source of infection on exam or laboratory testing. This is a higher proportion than observed during the prevaccine era, but it still leaves a large number of patients with bacteremia without an identifiable source.

These data emphasize how difficult it is to identify the very small number of otherwise healthy children with fever who have bacteremia. With only 27 cases per 100,000 febrile children/year, it truly is a challenge to find the needle in the haystack.

It's also important to remember that this was a vaccinated population, so using these percentages to assess risk in an unvaccinated toddler would not be appropriate.

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