Data are no. (%) of subjects, unless otherwise indicated.

Abbreviations: A(H1N1)pdm09, 2009 pandemic influenza A(H1N1) virus; BMI, body mass index; EIR, electronic immunization record (includes electronic medical record, employee health record, and state immunization registry); IIV3/4, inactivated influenza vaccine, trivalent/quadrivalent; IQR, interquartile range; LAIV4, live attenuated influenza vaccine, quadrivalent.

^a The χ² statistic was used to assess differences between the numbers of persons with influenza virus–negative and influenza virus–positive test results, with respect to the distributions of site, age group, sex, race/ethnicity, presence of any high-risk condition, vaccination status, interval from illness onset to enrollment, general health status, self or household exposure to smoke, and number of children ages <12 years in the household. The Wilcoxon–Mann–Whitney test was used to assess differences with respect to the distribution of the current health assessment. A P value of <.05 is statistically significant.

^b Data were missing for 10 enrollees.

^c Defined as a body mass index of ≥40, calculated as the weight in kilograms divided by the height in meters squared. Data were missing for 312 enrollees.

^d Vaccination status (≥14 days prior to illness onset) was determined by a combination of EIR and plausible off-site self-report or report by the parent or guardian. Persons aged ≥50 years with unknown vaccine type were assumed to have received inactivated vaccine (Figure 1).

^e Vaccination status (≥14 days prior to illness onset) determined by EIR.

^f Data were missing for 4 enrollees.

^g Data were missing for 3 enrollees.

^h Possible values range from 1 (the worst) to 100 (the best). Data were missing for 6 enrollees.