Objective: We aimed to assess the effects of outdoor time in preventing incident myopia among primary school students and evaluate its differences among different grades to provide evidence for policy formulation related to myopia prevention. Methods: This study is a cohort study. We investigated 6 046 grade 1 to 5 students in 9 provinces, Jiangsu, Shanghai, Fujian, Shanxi, Henan, Hunan, Gansu, Chongqing, and Guangxi. In 2019, we measured their myopia on site. In 2020, we did a follow-up visit on those students to detect the myopia incidence rate. Information regarding outdoor time and myopia-related behaviors were obtained from a questionnaire within one week of visual acuity measurement in 2020. The chi-square test and Cochran-Armitage trend test compared the differences between groups. The Cox proportional hazards risk model was used to test the relationship between outdoor time and myopia. Results: In 2020, the overall myopia incidence rate of grade 1 to 5 students in the baseline was 27.5%; while 23.0% in grades 1 and 2 students and 31.7% in grades 3 to 5 students, respectively. After controlling for covariates, for students in grade 1 to 2, those with ≥1 hour of outdoor time per day were at 0.76 (95%CI: 0.62-0.93, P=0.008) times risk of being myopia than that of students with <1 hour of outdoor time per day; while for students in grades 3 to 5, an average of ≥3 hours of outdoor time per day was required to have a significant protective effect on myopia. The students with ≥3 hours of outdoor time per day were less likely to be myopia (OR=0.75, 95%CI: 0.61-0.93, P=0.007) than those students with <3 hours of outdoor time per day. Conclusions: For grade 1 and 2 students, 1 hour of outdoor time per day could reduce the incidence of myopia, whereas for grade 3 to 5 students, 3 hours of outdoor time per day could effectively reduce the incidence of myopia. Therefore, the recommendations for outdoor time as myopia prevention should be different for different grades. The higher the grade is, the more outdoor time should be spent to reduce myopia incidence. Moreover, other factors that affect myopia's incidence should be also paid attention to, and a comprehensive approach should be adopted to prevent and control the incidence of myopia.
目的： 探索户外时间对小学生近视发生的影响，并评估其在不同年级学生中的差异，为预防近视相关政策制定提供依据。 方法： 本研究基于队列研究设计，在2019年调查了江苏、上海、福建、山西、河南、湖南、甘肃、重庆、广西9个省（自治区、直辖市）6 046名1~5年级小学生，现场对其近视情况进行检查，并在2020年对该群体再次进行随访，现场检测近视发生情况，并在2020年视力测量后一周内对近视相关行为进行问卷调查。采用χ2检验、Cochran-Armitage趋势检验比较组间差异，采用Cox比例风险回归模型检验户外时间与近视发生的关系。 结果： 在2020年，基线时为1~5年级的小学生近视发生率为27.5%，其中，1~2年级小学生近视发生率为23.0%，3~5年级小学生近视发生率为31.7%。控制协变量后，在1~2年级小学生中，平均每天户外时间≥1 h学生近视发生风险是平均每天户外时间<1 h学生的0.76倍（95%CI：0.62~0.93，P=0.008）；而对于3~5年级小学生，平均每天户外时间≥3 h才对近视发生有显著保护作用，其近视发生风险是平均每天户外时间<3 h学生的0.75倍（95%CI：0.61~0.93，P=0.007）。 结论： 1~2年级小学生平均每天达到1 h户外时间可以降低近视发生率，而3~5年级小学生平均每天需达到3 h户外时间可能才会有效降低近视发生率。因此，针对不同年级小学生预防近视的户外时间推荐应有差异，年级越高，户外时间相应增加，降低其近视发生率。但同时，还要关注其他因素对近视的作用，采用综合手段防控近视的发生。.