**2. Methods**

#### *2.1. Study Subjects*

Schools in China were closed between January and May, 2020 due to the COVID-19 pandemic. An anonymous online survey was conducted to collect school-age children's responses regarding their background information, time use, and vision condition during this period of pandemic-led school closure. The questionnaire was distributed from 12 to 18 May 2020 via a nationally known education press, which solicited respondents from 29 provinces and autonomous regions. Completing the questionnaire takes about 10–15 min online. The inclusion criteria for participants were as follows: (1) literate and can understand the questionnaire; (2) enrolled in primary, lower-secondary, or upper-secondary schools; (3) voluntary participation; (4) submitted only one response using the same IP address; (5) whose guardian has submitted informed consent. A total of 3405 respondents from 1st to 12th grade satisfied the study's inclusion criteria. This study was approved by the Institutional Review Board of Shaanxi Normal University, and the study was conducted according to the World Medical Association Declaration of Helsinki.
