For each study that we identified through the systematic review, we critically reviewed the quality of each manuscript. Based on published recommendations for measuring quality of epidemiologic studies of pneumonia [15] , we assessed quality using the following six criteria: (1) geographic location was reported, (2) study was conducted for a period of at least one year or multiples of one year to account for seasonal factors, (3) site of case detection or surveillance location was reported, (4) age and population size of cohort of at least 50 cases were reported, (5) quality assurance and monitoring methods were employed to assure that data was complete and high quality, and (6) a clearly defined case definition (e.g., not based solely on clinical diagnosis) was used and reported. These six criteria were selected to represent basic and essential indicators of epidemiologic study quality. Each criteria was dichotomous (1 = reported and 0 = not reported); the sum of all reported criteria yielded the manuscript's overall quality criteria score (0 to 6; Tables S2 and S3). At least one coauthor who read Chinese as a first language (X.G., W.L.) reviewed each of the studies to evaluate these six criteria; evaluations were then validated during direct discussions with an English-speaking epidemiologists (B.J.S., A.T.F., S.J.O., A.L.C.). We included all studies in this review to report the differences in study quality.