Individual and environmental characteristics related to influenza-like illness among children: a school-based case-control study.

To investigate individual and environmental characteristics associated with the manifestation of influenza-like illness (ILI), a survey enrolling 509 schoolchildren was conducted after an epidemic peak in the 1988/89 season, and then a case-control study was designed according to the information concerning illness onset and usual life-style. Case series were defined as those with fever greater than or equal to 38 degrees C due to acute respiratory illness, subsequent absenteeism and medical consultation during the epidemic peak; mild-ILI (MILI; fever greater than or equal to 38 degrees C and less than 39 degrees C) and severe-ILI (SILI; fever greater than or equal to 39 degrees C). Control series were defined as those with no symptoms (NS) during the same period. An adjusted odds ratio (OR) was obtained using an unconditional logistic regression model for MILI (55) or SILI (33) and NS (204) groups among respondents (508). For MILI, increased risk was observed for easily inflamed tonsils (OR, 3.2; 95% CI, 1.6-6.4), with decreased risks for higher school grade (0.4, 0.2-0.8) and frequent intake of milk or dairy products (0.5, 0.3-1.0), but no significant association was determined for vaccination (0.6, 0.3-1.2). For SILI, there was an increased risk for easily inflamed tonsils (3.3, 1.4-7.9); and decreased risks for the higher grades (0.1, 0.1-0.3); use of a kerosene or gas heater with indoor exhaust (0.4, 0.2-0.9); and vaccination (0.3, 0.1-0.7). The OR of vaccinees for SILI constantly decreased regardless of grade and easily inflamed tonsils.