beijing is one of the largest cities in china and has 18 districts and a population 
 of > 20 million persons . although there is considerable variation in district size 
 and a greater population density in urban areas , health care is accessible for 
 residents in all districts . during the pandemic period , the notifiable disease 
 surveillance system ( ndss ) fifty - five collaborating 
 laboratories covering all hospitals were authorized to conduct confirmation testing 
 for influenza a(h1n1)pdm09 virus ( 3 ) . case - patients were students for whom diagnosis was confirmed during october 1 , 
 2009january 31 , 2010 . we randomly selected 3 urban and 3 rural districts from the 18 
 districts and listed all case - patients < 18 years of age . controls were matched with case - patients at a ratio of 2:1 by sex and age (  1 
 year ) and were recruited from the same school and grade but from different parallel 
 classes than case - patients . the survey was conducted as a face - to - face interview by trained investigators from 
 the centers for disease control and prevention in beijing by using a standardized 
 questionnaire . data entry and statistical analysis were conducted by using epidata software version 
 3.1 ( www.epidata.dk/download.php ) and spss version 16.0 ( ibm , armank , ny , 
 usa ) . bivariate and multivariate conditional logistic regression analyses were used 
 to determine risk factors associated with infection . all variables with p<0.05 in 
 bivariate analysis were included in multivariate analysis . backward conditional logistic 
 regression was conducted by removing variables with p>0.10 , and statistical 
 significance was defined as p<0.05 ( technical appendix ) . a total of 304 case - patients and 608 controls were recruited from either primary or 
 middle schools . age range was 618 years for case - patients and 619 
 years for controls ( median age 13 years for both groups ) . these factors were vaccination history , eye rubbing , handwashing immediately after 
 sneezing , handwashing after lessons in communal classrooms , sleep time per day , 
 participation in outdoor activities after class , population density of classrooms , 
 classroom ventilation , mode of transportation to and from school , and participation 
 in clustered social activities after school ( table 
 1 ) . * bivariate conditional logistic regression was used to generate p values . or , 
 odds ratio . multivariate analysis showed that in addition to vaccination , a series of 
 environmental and behavioral factors were associated with reducing the risk for 
 influenza a(h1n1)pdm09 . these factors included provision of classroom space 
 > 1.6 m / student , participation in outdoor 
 activities after school , decreased interval of classroom ventilation , immediate 
 handwashing after sneezing , having more sleep time ( > 7 
 h / day ) , and use of open modes of travel ( walking , bicycle , and motorcycle ) ( table 2 ) . backward conditional logistic regression was conducted by removing 
 variables with p>0.10 , and 8 variables remained in the final regression 
 model . all statistical tests were 2-sided , and significance was defined as 
 p<0.05 . the statistic for each variable we found several variables that determined whether students would have influenza 
 a(h1n1)pdm09 . these factors were vaccination , classroom space , outdoor activities , 
 classroom ventilation , handwashing , sleep time , and modes of travel . vaccination against influenza a(h1n1)pdm09 was more common among controls than 
 case - patients , suggesting its potential value of protection . limited knowledge and misconceptions 
 regarding vaccination safety were contributing risk factors ( 46 ) . because transmission modes for this virus appeared to be similar to those for 
 seasonal influenza viruses , involving close , unprotected contact with respiratory 
 droplets ( 7 ) , we found that 
 environmental issues appeared to be protective . when the interval of classroom 
 ventilation exceeded 1 h , air renewal was determined to be inadequate , increasing 
 potential risk for infection . these findings are consistent with those of other 
 studies , which reported that influenza can spread in a confined space with 
 insufficient air flow and that clustering of students within classrooms or during 
 after - school activities can facilitate transmission of infectious diseases ( 8,9 ) . when 
 available classroom space per student was < 1.6 m , there was a greater 
 chance that students having influenza a(h1n1)pdm09 would have close contact with 
 healthy classmates , who would be at higher risk of acquiring this disease . although 
 other studies reported that transmission rates of influenza a(h1n1)pdm09 were not 
 increased by close and frequent contact with other persons on public transportation , 
 we advocate use of open modes of transportation for travel to and from school , and 
 self - protection measures when using closed modes of transportation ( 10 ) . for instance , because 
 wearing of face masks is easily applicable and has been shown to be protective , it 
 tended to be a preventative measure for students who use closed transport systems 
 ( 11 ) . school closure has been identified as a protective measure for controlling influenza 
 pandemics ( 12 ) . some 
 students after school closure continued to participate in clustered social 
 activities , thereby having potentially increased their risk for contact with 
 patients with influenza a(h1n1)pdm09 outside the school environment . thus , after 
 school closure , avoidance of large gatherings and clustered social activities may 
 further reduce infection among students . some variables that we analyzed were not risk factors ( vaccination with pneumococcal 
 vaccine , drug prophylaxis [ using traditional chinese medicine ] , some handwashing 
 habits ( e.g. , duration of handwashing ) , and sharing of tableware with classmates . case - patients were recruited into the study 
 38 months after receiving a confirmed diagnosis . controls were not subjected 
 to laboratory testing , and some asymptomatic infected students may have been 
 misclassified as controls , resulting in underestimation of odd ratios of certain 
 risk factors and overestimation of odd ratios of certain protective factors . we did 
 not include face mask use in the analysis because it was difficult to accurately 
 categorize wearing face masks , given the large variety of face masks in different 
 sizes and varying tightness in use during the pandemic in beijing , and because we 
 had no data for time , place , or duration of face mask use . in conclusion , thus , it is essential to increase 
 awareness regarding severity of influenza a(h1n1)pdm09 to improve knowledge of the 
 protective effect of influenza vaccine and to promote use of nonpharmaceutical 
 interventions among school - age children . methods used to determine risk factors for influenza a(h1n1)pdm09 among 
 students , beijing , china .
to identify risk factors associated with influenza a(h1n1)pdm09 among students in 
 beijing , china , we conducted a case  control study . 
 participants ( 304 
 case - patients and 608 controls , age range 619 years ) were interviewed by 
 using a standardized questionnaire . 
 we found that in addition to vaccination , 
 nonpharmaceutical interventions appeared to be protective .