2015 HSC Section 1 Book of Articles

United States but only a nonsig- ni fi cant reduction in con fi rmed hos- pitalization events in children aged , 1 year. In contrast, a recent study from the United States showed a sus- tained decrease in hospitalizations for pneumonia in children and a de- crease in people . 65 years old, pos- sibly a herd effect. 26 Our use of a discharge diagnosis of pneumonia coded as bacterial pneumonia as an endpoint was motivated by the dif fi - culty of establishing an etiological diagnosis of pneumonia, especially in small children. Interestingly, we observed an in- creasing incidence of admissions to the hospital for pneumonia among children , 2 years and from 5 to , 18 years old before vaccine introduction, from 2003 to 2007 (Fig 1). The reason for this increase is unclear, but nat- ural fl uctuations caused by expansion of certain pneumococcal serotypes or clones might have contributed. A similar increase in 2004 to 2006 was seen in a national time trend (1997 to 2008) study on hospitalizations for pneumonia among children in England. 9 This might have led to an underes- timation of the real effect of the PCV vaccination, because we did not cal- culate expected rates assuming a continued increasing trend and com- paring those with the observed rates, as was done in other studies. 27 Previous in fl uenza virus infection has been shown to increase the risk of developing pneumococcal pneumo- nia. 28,29 Recent data from the United States showed excess risk of pneu- mococcal pneumonia during the H1N1 in fl uenza pandemic in 2009. 30 In our study we observed only an increase in hospitalizations for pneumonia, coded as bacterial pneumonia, in children aged 2 to , 5 years during this pandemic. There was a high cov- erage rate (50% of children aged 6 months to 2 years, 70% of children TABLE 2 Trend Analysis of Monthly Hospitalizations by Discharge Diagnosis per 100 000 Person-Years by Age Group in Stockholm County, Sweden, 2003 – 2012 Disease (Age Group) RR (95% CI) a P Disease (Age Group) RR (95% CI) P Disease (Age Group) RR (95% CI) P Sinusitis (0 – 2 y) Sinusitis (2 – 5 y) Sinusitis (5 – 18 y) Preintervention trend 1.00 (0.992 – 1.017) .47 Preintervention trend 1.01 (0.991 – 1.029) .32 Preintervention trend 1.01 (0.996 – 1.029) .13 Change in level 0.52 (0.265 – 1.014) .055 Change in level 1.026 (0.434 – 2.425) .95 Change in level 0.77 (0.381 – 1.561) .47 Postintervention trend 0.976 (0.957 – 0.996) .018 Postintervention trend 0.969 (0.949 – 0.990) .004 Postintervention trend 1.00 (0.985 – 1.014) .96 Pneumonia (0 – 2 y) Pneumonia (2 – 5 y) Pneumonia (5 – 18 y) Preintervention trend 1.01 (1.004 – 1.017) .001 Preintervention trend 1.004 (0.997 – 1.011) .29 Preintervention trend 1.01 (1.007 – 1.022) , .001 Change in level 0.65 (0.494 – 0.856) .002 Change in level 0.90 (0.647 – 1.255) .54 Change in level 0.50 (0.357 – 0.70) , .001 Postintervention trend 0.996 (0.990 – 1.001) .13 Postintervention trend 0.992 (0.985 – 0.999) .02 Postintervention trend 1.01 (1.005 – 1.019) .001 Pyelonephritis (0 – 2 y) Pyelonephritis (2 – 5 y) Pyelonephritis (5 – 18 y) Preintervention trend 1.00 (0.994 – 1.007) .99 Preintervention trend 1.00 (0.990 – 1.016) .67 Preintervention trend 1.003 (0.992 – 1.014) .64 Change in level 1.09 (0.809 – 1.474) .57 Change in level 0.974 (0.551 – 1.740) .93 Change in level 1.123 (0.681 – 1.849) .65 Postintervention trend 1.00 (0.994 – 1.006) .90 Postintervention trend 0.999 (0.987 – 1.010) .82 Postintervention trend 1.003 (0.994 – 1.013) .52

a When referring to the preintervention and postintervention trend, the RR expresses the month-to-month change in hospitalization rates. For the change in level it expresses comparison of the hospitalization rate of the fi rst postintervention month to the last preintervention one.

PEDIATRICS Volume 134, Number 6, December 2014

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