[Seasonal changes of invasive pneumococcal disease in children and association with day care attendance].

To determine seasonal changes in the incidence of invasive pneumococcal disease (IPD) in children, we retrospectively analyzed 69 children with 72 episodes of IPD, admitted to a regional center in Kobe, Japan, between July 1994 and June 2011. IPD episodes involved occult bacteremia (n = 48), pneumonia (n = 10), meningitis (n = 10), periorbital cellulitis (n = 3), and mastoiditis (n = 1), including 3 cases of two IPD recurrences. We analyzed 5 IPD-associated factors previously documented in Europe and North Amrica with inconsistent results--1) age at onset, 2) sibling number, 3) preschool sibling number, 4) subjects' day care attendance, and 5) siblings' day care attendance. We collected information on these factors by reviewing medical charts or contacting subjects' parents or guardians by telephone. IPD peaked bimodally in April and May (n = 21) and in November and December (n = 20), decreasing prominently between July and September (n = 8). Subjects with IPD attending day care formed a significantly higher propotion during April and May than did those developing IPD during other months: 12/21 [57.1%] vs. 12/51 [23.5%], odds ratio 4.3, 95% confidence interval, 1.5-12.8; p = 0.006. Combined day care attendance among subjects with IPD and/or their siblings also differed significantly between these two groups: 17/21 [80.9%] vs. 27/51 [52.9%], odds ratio 3.8, 95% confidence interval, 1.1-12.8; p = 0.027. Not significant differences were seen in age at onset, sibling number, or preschool sibling number. In contrast, however children with IPD onset during November and December showed no significant difference in association with any of the 5 factors, compared to children with IPD onset in other months. Our findings showed a bimodal peak in IPD in children, the first and highest of which occurred in April and May and was significantly associated with day care attendance by those with IPD and/or their siblings. This first peak may, however, be related to circumstances in Japan, where preschool children usually enter day care center or kindergarten in April.