BackgroundThe incidence of severe bacterial infections (SBIs) in infants aged ≤90 days is thought to have decreased because of widespread vaccination programs. However, relevant epidemiological data in Japan are scarce. Materials and methodsThis observational, single-center study investigated the epidemiology of fever in infants aged ≤90 days. SBI was defined as the presence of meningitis, urinary tract infections (UTIs), or bacteremia. Invasive bacterial infection (IBI) was defined as the presence of meningitis, bacteremic UTI, or bacteremia. We determined the incidence of UTIs, bacteremia, meningitis, SBIs, and IBIs in the following three age groups: 0-28, 29-60, and 61-90 days. We subsequently calculated the relative incidence for the groups aged 29-60 and 61-90 days, using the group aged 0-28 days as the reference group. ResultsHerein, 58, 124, and 166 infants were included in the 0-28 days, 29-60 days, and 61-90 days age groups, respectively. Of the total number of patients, 15.5%, 8.9%, and 16.9% in the 0-28 days, 29-60 days, and 61-90 days age groups, respectively, were diagnosed with SBI. The relative incidences were 1 for the 0-28 days group (reference group), 0.67 for the 29-60 days group (95% confidence interval [CI], 0.39-1.15), and 1.08 for the 61-90 days group (95% CI, 0.58-2.00). Of the total number of patients, 10.3%, 3.2%, and 0.6% in the 0-28 days, 29-60 days, and 61-90 days age groups, respectively, were diagnosed with IBI. Relative incidences were 1 (reference group), 0.50 (95% CI, 0.29-0.88), and 0.28 (95% CI, 0.19-0.41) for the 0-28 days, 29-60 days, and 61-90 days age groups, respectively. All cases of IBI were caused by Group B streptococcus (GBS), except for two cases of bacteremia, which were caused by Haemophilus influenzae. ConclusionThe incidence of SBI was similar in the 0-28 days and 61-90 days age groups. However, the incidence of IBI decreased with increasing age. The incidence of UTIs was highest in the 61-90 days age group, and that of meningitis and bacteremia decreased with increasing age.