**1. Introduction**

The outbreak and spread of the coronavirus disease 2019 (COVID-19) pandemic have greatly impacted education systems worldwide, with more than 190 countries/territories closing schools partially or in full during peak months, affecting at least 1.5 billion schoolage children [1]. In order to minimize learning disruptions and to resume proper functioning of educational activities, more than 60 educational systems have elected to partially re-open schools by offering online remote instruction and supplementary digital learning modalities [2]. While replacing print textbooks with e-learning arrangements and swapping in-person classroom teaching with online video conferencing provides reasonable and timely solutions to deal with challenges of pandemic-led school closures, the adverse vision consequences associated with these ad hoc emergency and crisis arrangements may be substantial [3], especially considering new eye-use routines during the pandemic and its consequent behavioral implications for young children whose sensory function is going through critical development [4]. Notably, the current global increase in myopia, which is

**Citation:** Liu, J.; Li, B.; Chen, Q.; Dang, J. Student Health Implications of School Closures during the COVID-19 Pandemic: New Evidence on the Association of e-Learning, Outdoor Exercise, and Myopia. *Healthcare* **2021**, *9*, 500. https:// doi.org/10.3390/healthcare9050500

Academic Editor: Manoj Sharma

Received: 19 April 2021 Accepted: 22 April 2021 Published: 23 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

a major factor contributing to irreversible blindness, has become a leading public health concern. By 2050, it is projected that 50% of the world population will be affected [5], which underscores the need to investigate the relevant risk factors, as well as potential mitigation strategies to combat this public health challenge.

In this regard, it is hypothesized that the reduction of outdoor activity due to school closures and home confinement, coupled with the overlapping increase in e-learning digital screen use as a result of remote e-learning arrangements, presents critical vision development risk factors that could propel higher myopia incidence and progression by re-shaping daily physical and learning behaviors. On the one hand, intensive use of digital screens for extended periods of time can have detrimental effects on children's vision health development. Particularly, since eye development occurs throughout early stages of life, in vivo studies have suggested that prolonged near-vision stimulation can result in premature hyperopic defocus, which triggers compensating axial myopic eye growth and refractive vision development [6]. On the other hand, reduction in outdoor activities due to pandemic-led social-distancing measures and closure of public venues may present another less favorable environmental factor influencing young children's vision health. In this regard, prior research has found outdoor playtime was associated with better uncorrected visual acuity [7], especially because outdoor lighting is categorically less fluorescent than indoors [8].

Notwithstanding, the potential vision health risks propagated by new norms in instructional and learning arrangements amidst the ongoing COVID-19 pandemic will likely add to an already serious global youth vision crisis [9]. Importantly, early myopia (nearsightedness) onset and progression among young children are especially concerning, not only because of its widespread prevalence and difficulty in proper mitigation applications [10] but also due to critically associated risks of lasting vision impairment [11]. In particular, excessive axial elongation of the eye associated with early myopia increases later-life risks of vision disease complications including macular degeneration, posterior staphyloma, retinal detachment, cataract, and glaucoma that could lead to blindness [12]. More worryingly, myopia onset is becoming increasingly prevalent among young children, particularly for girls in higher grades and in urban areas [13–15]. Studies have also shown that the earlier children become myopic, the more likely they are to develop high myopia, and the worse the prognosis [16].

From a physiological perspective, in vivo studies employing infant monkeys have suggested that extended relative peripheral hyperopic defocus stimulation can alter central refractive development [17], which could likely induce compensating axial myopic eye growth and premature refractive vision development [18]. For instance, it has been shown using inflammatory markers in mice that blue light emitted from computer screens has potentially harmful effects on the retinal pigment epithelium, which may result in axial elongation and development of pathological myopia [19]. In addition, outdoor eye use under natural light is commonly associated with increased depth of focus and reduced eye strain, which are inversely related to axial elongation [20]. In studies employing in vivo models, visual experiments have shown that poor lighting conditions can lead to excessive vitreous chamber lengthening, such that low-light level can result in axial elongation and refractive myopic excursions, while high-light level is associated with lower rates of form-deprivation myopia [21].

Critically, while existing studies have independently examined near-vision electronic use, outdoor activity, and lighting condition, the interrelated relationship among the three have not been assessed in conjunction, and their relative risks are unknown. In this study, we examined the association between digital screen use, outdoor activity, lighting condition, and myopia development among school-age children in China during the COVID-19 pandemic.
