The BPCE study is a school-based cohort study for three consecutive years. Students aged 6–12 years from 8 primary schools in Pinggu District of Beijing, China were prospectively selected to evaluate the visual acuity, refractive status and ocular. In the present manuscript, we aim to report the baseline characteristics for the 3-year study, and further information will be reviewed yearly.

The present project has been initiated by the Beijing Centers for Disease Control and Prevention (BJCDC) as a public health program designed to establish refraction development data in Beijing city. The investigation will be annually conducted in primary schools from January 2021 to December 2023. The present project conforms to the tenets of the Declaration of Helsinki. Ethics approval was obtained from the institutional review board of the Peking University People's Hospital (2021PHB322-001). After the study purposes and procedures were explained to the parents or legal guardians in detail during a school seminar, written informed consent was obtained.

The schools were selected due to 85% students in these schools promised to follow up the study in the 3 years. Once the schools were selected, 2–3 classes in every grade were randomly selected and all students in the classes were recruited to ensure at least 80 students in every grade in the study. Of them, 4,942 (91.7%) signed informed consent forms, and 4,806 (89.1%) children successfully completed all biometry examinations and cycloplegic refraction examinations and were eligible for analysis.

The distance VA was estimated using Early Treatment Diabetic Retinopathy Study Tumbling E charts (Precision Vision, La Salle, IL, USA). Ocular biometry was measured before pupil dilation with non-contact partial-coherence laser interferometry (IOLMaster 500; Carl Zeiss Meditec, Oberkochen, Germany) (13).

The non-cycloplegia autorefraction was measured first. Then, cycloplegia was induced with 2 drops of 1% cyclopentolate, administered 5 min apart, with a third drop administered 20 min later. The cycloplegia and pupil dilation were evaluated after an additional 15 min. Refraction was performed with a desktop autorefractor (KR8800; Topcon Corp., Tokyo, Japan) in participants with their pupils dilated to at least 6 mm, and when their pupillary light reflex was absent. The data on the spherical and cylindrical power and axis were extracted from the device.

The trained and certified examiners (three optometrists, two public health physicians, and one ophthalmologist) were from Peking University People's Hospital. The examinations were performed according to standard measurement procedures. All the participants with uncorrected visual acuity (VA >0.0 logMAR) received recommendations to be referred to eye care practitioners for further detailed ophthalmic examinations and evaluations.

Uncorrected VA higher than 0.0 logMAR was defined as referral vision. The spherical equivalent refraction (SER) was calculated as the sphere power (D) + 0.5 × cylinder power (D). Myopia was defined as an SER of −0.50 D or less, emmetropia was defined as −0.50 D < SER < +0.50 D, mild hyperopia was defined as +0.50 D ≤ SER < +2.00 D, and mild-to-high hyperopia was defined as an SER of ≥+2.00D.

Because there was high correlation between the right and left eyes of the same individual, only the biometry parameters from right eyes were presented. The corneal radius (CR) was calculated as the mean of the flattest and steepest corneal power (CP) and was converted using the formula: corneal power (D) = 0.3375/CR (mm) × 1,000; Guo et al. (13) the AL-to-CR ratio (AL/CR) was computed as the AL in millimeters divided by the CR in millimeters. The lens power was calculated based on the modified Bennett-Rabbetts formula (14).

Analysis of variance (ANOVA) was used to compare the biometric measurements and Chi - square were used to compare the prevalence of refractive error across different ages. Paired samples T-tests were performed to explore the difference of SER pre- and post- cycloplegia. Non-parametric tests were used to compare the prevalence of refractive error between pre- and post- cycloplegia. Multiple regression analysis was performed to explore the associations between the refraction and ocular biometry. All analyses were performed using IBM SPSS Statistics 26.0 software. A P-value of 0.05 or less was considered statistically significant.

There are variations in the prevalence of myopia between different regions and ethnic groups (15, 16). As reported by Zhou and colleagues, it has been universally accepted that Chinese schoolchildren have a higher prevalence of myopia at a given age in population-based epidemiology study, and highly related to less time spend out doors and intense education (17). However, in view of the current situation in exposure to electronic screens, intensive early education, and especially the lockdowns caused by COVID-19, the distribution of emmetropia, ametropia and the prevalence of myopia need to be investigated further. Pang P and colleagues reported (18) that the overall myopia prevalence under non-cycloplegia refraction after COVID-19 was 42.82, 75.25, and 82.57% in elementary school, junior high school and senior high school, respectively. Restricted outdoor time and increased screen time are contributors to increased myopia during quarantine.

Evidence from the present BPCE study showed a substantially higher prevalence of myopia, increasing by 10–15% across all age groups comparing to recently reported results, (6, 19–21) and this possibly reflects the trends for the earlier development of myopia in Beijing Pinggu District children. Particularly surprising is that, in 9- and 10-year-olds showing the most prominent increases. This may be due to the characteristics of Chinese education, which in Grade 3 to Grade 5, the schoolwork pressure suddenly increased, and students began to endure intense out-of-class activities, such as English and Mathematical Olympiad, etc. However, as reported by several studies, the non-cycloplegic refraction is inaccurate for studying the refractive error in children, especially those under 12 years of age (22, 23). In the present study, a greater discrepancy between the cycloplegia and non-cycloplegia autorefraction results was found. As shown in Table 1, the detection rate of myopia and emmetropia under cycloplegic treatment decreased by 22.3 percent points and 15.7 percent points compared with that before cycloplegia, respectively, while the prevalence of hyperopia increased. After cycloplegia, 42.6% of myopic children became emmetropia or hyperopia. Cycloplegia have a greater effects for younger children than those older children. Among 6 years old children, 91.3% of myopic children became emmetropia or hyperopia, but only 26.5% among those in 12 years old remained myopic. So the prevalence of myopia before and after cycloplegia became gradually closer to each other as age increased. In Anyang study, it was also suggested Chinese children with myopia may not be accurately diagnosed. So the prevalence of myopia before and after cycloplegia became gradually closer to each other as age increased.

It is widely acknowledged that differences between non-cycloplegic and cycloplegic refractions were more marked at younger ages (22–25). The non-cycloplegic condition includes a strong accommodative component, especially in younger children with dark irises. Inadequate cycloplegia can lead to a marked increase in the prevalence of myopia that is reported. In the present study, compared to cycloplegia data, the non-cycloplegic SER was −0.76 D (range from −1.06 D to −0.47 D), which is more myopic than the cycloplegic refraction. The prevalence of myopia of non-cycloplegis is significantly increased compared to non-cycloplegia SER, and the difference ranges from 23.6% at 6 years old to 13.9% at 12 years old. With this strict condition of cycloplegic eye drop administration, it is concluded that the prevalence in primary school children is substantially increased (11) and is higher than that in other cities in China, such as in Guangzhou, compared to the same age group (26, 27).

The present finding is consistent with Liu et al.'s (27) report that without cycloplegia, autorefraction tends to overestimate the SER in children with myopia and is negatively correlated with age. For an accurate diagnosis and effective myopia management strategies, cycloplegia should be used for children in primary school students between 6 and 12 years of age according to the present study.

The eye grows rapidly during early childhood and then grows slowly during the later stages (28). It has been reported that each 0.1 mm in the eye elongation corresponds to an approximately 0.20–0.25 D myopic change (29, 30). Consistent with previously reported articles, the AL increased with age across all of the refractive error groups, but there were variations among the different refraction statuses and different age groups. For the emmetropia and hyperopia groups, the average eye growth rate was 0.15 mm/year from 6 to 12 years of age, while the rate was approximately 0.19 mm/year in the myopia group, which was faster than that in non-myopic students. It is more interesting that, if the 6-year-old group was excluded, and if only the age 7- to 12-year-old group of students were analyzed, the eye growth rate in myopic children was ~0.22 mm/year, which is faster than normal emmetropazation condition, indicating the shift to myopia development.

We found the same results as previously reported: the ACD deepened and the lens power decreased with increasing age among 6- to 12-year-old children. The AL-to-CR ratio is supposed to be a synthetic indicator to determine the refractive state of the human eye, and no <3.0 has been considered as an indicator for the onset of myopia (13). Our present study results showed that a higher AL-to-CR ratio correlated with myopia prevalence and a negative refractive error status, which explained 72.4% of the SER variance. In the present study, the average AL-to-CR ratio was 3.12 in myopia children, and the increment was 0.16 from 6 to 12 years old in myopia group. However, the increase of the AL-to-CR ratios in emmetropia group and hyperopia group were 0.06 and 0.08, respectively. In addition, when including AL, CR and LP in prediction computation model, they can explain 86.4% of the variance in the SER, in which LP and AL was found to be inversely associated with the SER.

There were several limitations in the present study, including that there was no direct measurement data of the lens thickness and lens power, although they have been calculated through validated forums. In addition, there were no records about the parental history, duration of near work, screen use, or outdoor activity time. The above information will be collected through questionnaires during subsequent screening studies. Despite the limitations, the relative changes imply that these are valuable indicators for evaluating refraction and ocular biometry parameters.