During the social distancing period, the incidence of obesity was higher in boys than in girls. The obesity rate in girls who visited the growth clinic for early breast budding during routine follow-ups did not increase.

The novel coronavirus disease outbreak occurred in December 2019 in Wuhan, Hubei province, China [1,2]. It was named coronavirus disease 2019 (COVID-19) [3]. This disease started as a local epidemic that eventually reached pandemic proportions, as declared by the World Health Organization on March 11, 2020 [4]. On January 20, 2020, the first confirmed case of COVID-19 was registered in Korea [2]. That day, the Korea Centers for Disease Control and Prevention (KCDC) raised the alert level from 'blue' (level 1) to 'yellow' (level 2) out of the country's 4-level national crisis management system [5]. On January 27, 2020, the KCDC raised the alert level to 'orange' (level 3). As the number of COVID-19 cases continued to increase in the country, the KCDC raised the alert level to 'red' (level 4) on February 23, 2020, prompting the Ministry of Education to postpone the beginning of the new school year [6].

As part of the response strategy to the COVID-19 pandemic, the population was advised to embrace social distancing, a powerful means of reducing the transmission of infectious diseases. However, maintaining social distance for an extended period reduces its effectiveness, and the socioeconomic impact is enormous [7]. More than 5 million students and their families stayed at home because all schools were closed until May 2020 [8].

Due to COVID-19, the term "quarantine-15" has become popular and refers to the weight gain associated with the rapid reduction in physical activities and increased food consumption. Many social problems have also emerged due to this degradation of body appearance [9]. After May 2020, we noticed a new trend of weight gain among most children who visited our growth clinic due to puberty evaluation or short stature. Parents and children complained of decreased activities because of school closure.

In this study, we aimed to determine whether childhood obesity has increased after school closure. We compared the obesity rate of new outpatient patients visiting the growth clinic before and after social distancing. In addition, we compared the body mass index (BMI) between the 2 groups: (1) girls who were diagnosed with precocious puberty and on gonadotropin releasing-hormone agonist (GnRHa) therapy and (2) girls who were regularly followed up in the growth clinic for breast budding without a GnRHa therapy, either because their bone age was lower than their chronological age or because they had a peak luteinizing hormone level below 5 mIU/mL after a GnRHa stimulation test.

In this study, we compared the percentage of obese patients among new outpatients who visited the growth clinic before and after social distancing due to COVID-19. There was no significant difference in the proportion of overweight or obese patients among the girls (23.3% vs. 31.4%, P=0.328). However, the proportion of overweight or obese patients among the boys increased significantly between the 2 periods (23.3% vs. 45.8%, P=0.045).

A possible reason for this sex difference in the obesity rate is that boys are likely to be more physically active than girls [11]; thus, the effect of reduced physical activity due to social distancing may be greater in boys than in girls. Therefore, we can assume that high normal weight or overweight boys were more likely to transition to obesity than girls. Additionally, the percentages of the chief complaint were different between boys and girls. We enrolled boys who visited the growth clinic due to short stature (n=18, 60%) or precocious puberty (n=12, 40%) and girls who visited due to short stature (n=21, 25%), or early breast budding (n=62, 75%). This means that our findings do not fully reflect the obesity level of all children and adolescents.

Although the difference between chronological age and bone age in boys who visited the clinic after social distancing was not statistically significant, there was a slight increase. This was probably due to the increased rate of obesity [12]. For both boys and girls, BMI (SDS) tended to increase in patients with a lower chronological age (r=-0.12 and P=0.03) and higher Tanner stage (r=0.18 and P=0.002).

In this study, girls who visited the growth clinic due to early breast budding were stratified into 2 groups according to their GnRHa treatment status. We compared the changes in height, weight, and bone age of these girls between 2019 and 2020. The BMI of girls who received GnRHa treatment increased from 18.7±2.2 in 2019 to 20.6±2.9 in 2020. In contrast, the BMI of the girls in the control group showed no significant difference between 2019 and 2020.

There is some evidence that over weight or obesity is associated with precocious puberty [13,14]. Previous studies on the effects of GnRHa on BMI have controversial findings. Some studies have reported a small or no difference in BMI [15,16], while other studies have shown a higher BMI after GnRHa therapy [17,18]. In this study, no significant change in BMI was observed in the first 6 months after GnRHa administration. However, after 6 months of GnRHa, a gradual increase in BMI was observed, and this trend was maintained during the social distancing period. Therefore, based on our findings, the impact of social distancing on childhood obesity is not clear. In addition, the absence of BMI differences in the control group (no GnRHa treatment) between 2019 and 2020 could be due to regular hospital visits for weight control.

Mask wearing due to COVID-19 contributed to a reduction in the number of patients who visited the pediatric clinic because of viral illness (21,509 patients in 2019 vs. 14,816 patients in 2020). However, the number of children visiting the growth clinic for short stature, precocious puberty, and obesity increased (7,022 patients in 2019 vs. 7,495 patients in 2020). Therefore, we can assume that a reduction in children's physical activity due to COVID-19 has a significant impact on their growth and obesity.

As of December 2020, several studies on the effect of decreased physical activity due to COVID-19 on endocrine metabolic diseases such as diabetes and thyroid diseases in adults have been conducted [19-21]. Similarly, studies on endocrine diseases in children and adolescents have also examined the effects of social distancing due to COVID-19, but the research is still limited compared to adults. Besides, more published studies focused on changes in incidences and glycemic control in children and adolescents during periods of social distancing [22,23]. In response to the limited number of studies, we evaluated the changes in the incidence of obesity in children and adolescents following the adoption of behavioral changes due to COVID-19. We noted concerns by parents of children visiting the growth clinic about their children's weight gain since the closure of the schools.

In prior studies, the subjects were children and adolescents who visited hospitals for treatment, which indirectly suggests that obesity could increase even in normal children and adolescents although the findings were not generalizable.

However, in this study, we targeted children who visited a pediatric growth clinic. Therefore, we could compare the anthropometric changes of relatively healthy children with those from previous studies because the purpose of hospital visits was to check the changes in height and weight and puberty.

Therefore, our results could be more representative of healthy children and their obesity rates. Furthermore, consistent with previous studies, approximately 70% of our study subjects were normoweight, and about 30% were overweight or obese [17,24].

However, our study has several limitations. First, there were relatively few male outpatients and controls. Second, although the study subjects were healthy children who visited the hospital intentionally, the increase in obesity was possibly lower than the actual change because of selection bias in the social distancing effects analysis. Third, only BMI was measured using a retrospective chart analysis; therefore, we could not monitor the changes in other metabolic syndrome parameters. Fourth, we could not obtain the BMIs before 2019. Finally we enrolled patients who visited a single center.

Therefore, a large-scale study involving the national population is needed. However, the annual examination of students was not conducted this year due to COVID-19, and this could limit future research. Recent reports of successful vaccine development indicate p ossible elimination of COVID-19, but the population should prepare for "post-pandemic" childhood and adolescent health problems. In this regard, an urgent national study on the changes in the incidence of obesity and metabolic syndrome due to decreased physical activity among children and adolescents is warranted.

In conclusion, the percentage of overweight or obese children who visited the growth clinic during the social distancing period compared to the same period in the previous year increased significantly in boys. Girls who received GnRHa had higher BMI and bone age during the social distancing period compared to the same period in the previous year, and the BMI did not differ in the control group (no GnRHa treatment) during the same period. Therefore, obesity in boys with relatively much decreased activity should be controlled. Importantly, routine follow-ups through regular hospital visits are recommended for successful control of obesity. Furthermore, national health examinations to evaluate obesity of children are necessary during the social distancing period.