Background: Subthreshold depression could be a significant precursor to and a risk factor for major depression. However, reliable estimates of the prevalence and its contribution to developing major depression under different terminologies depicting subthreshold depression have to be established.Methods: By searching PubMed and Web of Science using predefined inclusion criteria, we included 1 129 969 individuals from 113 studies conducted. The prevalence estimates were calculated using the random effect model. The incidence risk ratio (IRR) was estimated by measuring the ratio of individuals with subthreshold depression who developed major depression compared to that of non-depressed individuals from 19 studies (88, 882 individuals).Results: No significant difference in the prevalence among the different terminologies depicting subthreshold depression (Q = 1.96, p = 0.5801) was found. By pooling the prevalence estimates of subthreshold depression in 113 studies, we obtained a summary prevalence of 11.02% [95% confidence interval (CI) 9.78–12.33%]. The youth group had the highest prevalence (14.17%, 95% CI 8.82–20.55%), followed by the elderly group (12.95%, 95% CI 11.41-14.58%) and the adult group (8.92%, 95% CI 7.51–10.45%). Further analysis of 19 studies' incidence rates showed individuals with subthreshold depression had an increased risk of developing major depression (IRR = 2.95, 95% CI 2.33–3.73), and the term minor depression showed the highest IRR compared with other terms (IRR = 3.97, 95% CI 3.17–4.96).Conclusions: Depression could be a spectrum disorder, with subthreshold depression being a significant precursor to and a risk factor for major depression. Proactive management of subthreshold depression could be effective for managing the increasing prevalence of major depression.

Since the first case of coronavirus disease 2019 (COVID-19) was reported in Wuhan, China,¹ severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was identified as the causative virus for the global pandemic.² On March 11, 2020, the World Health Organization declared that the COVID-19 outbreak was a pandemic. Widespread outbreaks of COVID-19 were associated with psychological distress and symptoms of mental illness.³ Previous studies have observed moderate to severe psychological distress symptoms, such as depression and anxiety, among the populations in areas severely affected by the COVID-19 pandemic.^4,5 Moreover, compounding factors, such as high mortality rate, shortage of medical protective materials, social isolation, and media information overload, could be associated with further adverse mental health outcomes during the pandemic.⁶ To date, the increasing attention toward on mental health status has shown that subsyndromal mental health problems are a common response to the COVID-19 pandemic.³ For example, a study by Wang et al⁷ found moderate to severe depressive symptoms in 16.5% of the general population in China, while moderate to severe anxiety symptoms were observed in 28.8% of the population and moderate to severe stress symptoms were observed in 8.1% of the population. Although many studies have focused on the general population,⁷ the mental health status of specific populations during the COVID-19 outbreak is attracting an increasing amount of attention. For example, Chen et al⁸ found that there was a negative association between internet-related behaviors and psychological distress among children in China during the COVID-19 pandemic. However, the power to determine the impacts of this unprecedented pandemic on the mental health status of adolescents is limited by the small sample size of previous studies. In addition, it is unclear what potential factors may be associated with risk of or protection from adverse outcomes in adolescents’ mental health status, and this lack of knowledge might impede the use of proper psychological interventions for adolescents.

Transcranial direct current stimulation (tDCS) is a promising nonpharmacological intervention for treating depression. We aimed to provide an updated meta-analysis assessing the anti-depressant efficacy of tDCS. We searched the literature from the first available date to 30 December 2020 to identify relevant randomized controlled trials (RCTs). 27 RCTs (N = 1204 patients, 653 in active tDCS and 551 in sham tDCS) were included. Active tDCS was superior to sham tDCS (g = 0.46, 95 % CI 0.15-0.76) in modulating depressive symptoms measured by depression rating scales. Active tDCS was also superior to sham tDCS in reducing response and remission rates, but these differences did not reach statistically significant levels (ORresponse = 1.75, 95 % CI 0.85-3.58; ORremission = 1.29, 95 % CI 0.59-2.83). The two groups had comparable dropout rates (OR = 1.28, 95 % CI 0.62-1.64). For treatments of depressive episodes, tDCS may be efficacious. Specific tDCS parameters (e.g., a 2-mA stimulation current and 30-min sessions) and clinical characteristics (e.g., antidepressant-free) may augment the treatment efficacy of tDCS.