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Mental health
Original research
Impact of loneliness on suicidal ideation during the COVID-19 pandemic: findings from a cross-sectional online survey in Japan
  1. Hirokazu Tachikawa1,
  2. Midori Matsushima2,
  3. Haruhiko Midorikawa3,
  4. Miyuki Aiba4,
  5. Ryo Okubo5,
  6. Takahiro Tabuchi6
  1. 1Department of Disaster and Community Psychiatry, Institute of Medicine, University of Tsukuba, Tsukuba, Ibaraki, Japan
  2. 2Faculty of Humanities and Social Sciences, University of Tsukuba, Tsukuba, Ibaraki, Japan
  3. 3Department of Psychiatry, University of Tsukuba Hospital, Tsukuba, Ibaraki, Japan
  4. 4Faculty of Human Science, Toyo Gakuen University, Bunkyo-ku, Tokyo, Japan
  5. 5Department of Neuropsychiatry, National Hospital Organization Obihiro Hospital, Obihiro, Hokkaido, Japan
  6. 6Cancer Control Center, Osaka International Cancer Institute, Osaka, Japan
  1. Correspondence to Dr Hirokazu Tachikawa; tachikawa{at}md.tsukuba.ac.jp

Abstract

Objectives We aim to reveal how loneliness relates to suicidal ideation following the onset of the COVID-19 pandemic.

Design Cross-sectional online survey.

Setting Community cohort study in Japan.

Participants The second wave of a large web-based survey, Japan COVID-19 and Society Internet Survey, was conducted in February 2021, and we analysed the data of 6436 men and 5380 women who were aged 20–59 years.

Main outcome measures The prevalence ratios (PRs) of suicidal ideation due to loneliness, depression, social isolation and decline in income during the pandemic and other sociodemographic and economic information were adjusted in the analysis.

Statistical methods Estimations were conducted by separating a male and female sample. The survey weight (inverse probability weighting) was applied for analyses, and a Poisson regression model was used with all the potential confounders adjected.

Results Overall, 15.1% of male and 16.3% of female participants were found to have had suicidal ideation during the COVID-19 pandemic. Among them, 23% of male and 20% of female participants experienced suicidal ideation for the first time. The results of the Poisson regression suggested that those who were feeling lonely had higher PRs for suicidal ideation (4.83 for men (95% CI, 3.87 to 6.16) and 6.19 for women (95% CI, 4.77 to 8.45)). The relationship between loneliness and suicidal ideation remained robust even after adjusting for depression, although there were declines in PRs. Additionally, the results showed that those who were lonely, and continued to feel lonely during the pandemic, had the highest PRs of suicidal ideation.

Conclusion Loneliness had both direct and indirect effects on suicidal ideation mediated through depression. Those who felt lonelier during the pandemic had the highest risk of suicidal ideation. It is necessary to adopt national measures focused on providing psychological support to people who feel lonely to prevent them from taking their own lives.

  • Covid-19
  • Suicide & self-harm
  • Public health

Data availability statement

Data are available upon reasonable request.

http://creativecommons.org/licenses/by-nc/4.0/

This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.

https://doi.org/10.1136/bmjopen-2022-063363

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    STRENGTHS AND LIMITATIONS OF THIS STUDY

    • This study used data of the Japan COVID-19 and Society Internet Survey, a large web-based survey in Japan, which was conducted during the third wave of the pandemic, in 2021.

    • The strength of this study is that it revealed how loneliness during COVID-19 affects suicidal ideation by using a large sample.

    • A limitation of the study is the possible sampling and self-reporting bias due to the web-based survey used.

    Introduction

    A significant number of studies have documented the prevalence of mental health issues including fear, anxiety, distress, depression and loneliness during the global COVID-19 pandemic.1 Elevated suicidal ideation was found in various countries.2–7 In Japan, a 16% increase in suicide rates from July to November 2020, followed by an immediate decline in the number of suicides (from February to June 2021), provided the first statistical evidence of a severe mental health crisis induced by the pandemic.8 According to the National Police Agency statistics, the total number of suicide victims in 2020 was 21 081, an increase of 912 (4.5%) from 2019—the first increase in 11 years.9

    There have been several issues caused by COVID-19 that have led to eventual suicidal ideation. Risk factors include fear of contracting COVID-19, high risk of infection, financial strain and lower social support, worsening chronic physical illness, presence of a psychiatric disorder, increased psychological stress, loss of employment and loneliness.10 11 Among the risk factors, loneliness has received special attention from researchers worldwide because it has been exacerbated by infection control measures including quarantine, lockdown and social distancing in social settings, which has hampered various levels of human communication and relationships.12–14 In the USA, the prevalence of self-reported loneliness significantly and continuously increased during the 3 months of lockdown restrictions between April and June 2020, a finding that was associated with increased depression and suicidal ideation.2 15 A longitudinal study in Brazil revealed that loneliness was consistently associated with the incidence of suicidal ideation, while other variables, such as living alone, not leaving home and social distancing, were not.16

    One of the most important issues is the differences and relationship between loneliness and social isolation. Loneliness is the subjective evaluation and feelings of an unpleasant experience of quantitative and/or qualitative deficiencies in a person’s network of social relationships,17 and this frequently occurs when there is a gap between one’s ideal and actual number and depth of social networks. In contrast, social isolation is the objective evaluation of a state of being socially disconnected from a formal and informal social network. Although there is a distinct difference between loneliness and social isolation, they are interconnected as socially isolated people are more likely to feel lonely than non-isolated people. To date, studies of loneliness conceptualised loneliness as multidimensional phenomenon with mainly three components: state of deprivation, length of deprivation and emotion towards the condition. The state of deprivation is feeling an absence of an intimate attachment and feelings of emptiness or abandonment. The length is one’s perception of the state of loneliness as either being hopeless and never ending or as changeable and treatable. The third component, emotion towards the condition, involves different types of emotional aspects, such as sorrow, and feelings of shame, guilt, frustration and desperation.18 Given the above definition and conceptualisation of loneliness, this study treats loneliness as a different concept from social isolation.

    Another important distinction is between loneliness and depression. It is worth noting that 95% of people who committed suicide during the pandemic were diagnosed with mental health disorders, including depression.19 20 While Cacioppo et al21 claim that loneliness and depression are intimately related but distinct, there is a need for further studies examining the relationship between loneliness, depression and suicidal ideation, with a particular focus on the association between suicidal ideation and depression exacerbated by loneliness. To this end, the authors have chosen to undertake an in-depth exploration of the relationship between loneliness, depression and suicidal ideation following the onset of the COVID-19 pandemic, when many people were at risk of suddenly being forced to be lonely at the same time.

    Hence, the authors aim to examine how loneliness, as a result of the COVID-19 pandemic, directly or indirectly impacts suicidal ideation and employ analytical models that focus on the effects of loneliness and depression on suicidal ideation. Many literatures strived to understand the motives and mechanisms of suicide and have shown its complexity and difficulties of identifying a clear path. The recent influential work by Joiner,22 ‘the interpersonal theory of suicide’, can provide some guidance. This theory claims that suicidal desire emerges when individuals face simultaneous feelings of perceived burdensomeness and thwarted belongingness and that near lethal or lethal suicidal behaviour occurs in the presence of suicidal desire and a capability for suicide. Thwarted belongingness comprises loneliness and an absence of reciprocal relationships, and the deterioration of a sense of belonging particularly matters in the present society where individuals are living with the prolonged COVID-19 pandemic. Based on the interpersonal theory of suicide and our experience with the COVID-19 pandemic, we hypothesise that loneliness, due to the pandemic, both directly and indirectly increases suicidal ideation. The results have implications for COVID-19-related policy measures as well as other crisis we may face in the future and contribute to a deeper understanding of the determinants of suicidal ideation.

    Methods

    Procedures

    We used data from the second wave of the Japan COVID-19 and Society Internet Survey (JACSIS). The JACSIS was conducted as a population-based online questionnaire survey approaching the panellists of Rakuten Insight, which holds approximately 2.2 million panellists aged 15 or above (a detailed study design is documented in Miyawaki et al23). To date, three waves of surveys have been conducted, and we used data from the second wave of the survey conducted between 8 and 26 February 2021. Of the 28 000 participants in the first survey, a total of 24 059 participants responded to the questionnaire. For the second survey, we recruited 1941 new participants and a total of 26 000 samples were obtained.

    Exclusion criteria were established to maintain the quality of data. First, the following responses with discrepancies and/or artificial/unnatural responses were excluded: (1) invalid responses to ‘please choose the second alternative from the bottom’ (ie, panellists who failed to select the second from last alternative from the five options available);i (2) positive responses to all questions related to illegal drug use; and (3) positive responses to all questions regarding 16 underlying chronic diseases. After this screening, 23 142 observations were left (men: 11 766 and women: 11 376). We also limited the observation samples to 20–59 years old, which led the number of observations to 7537 men and 7138 women. Finally, we excluded the observations, which gave the answer as ‘do not want to answer/do not know’ to their last year’s annual income. As a result, 6436 men and 5380 women were included in this study. For presenting our study, we have completed Strengthening the Reporting of Observational Studies in Epidemiology checklist to ensure our manuscript presents relevant information.

    Measures

    Suicidal ideation was measured by the question, ‘have you ever felt that you wanted to die since April 2020?’ The responses were chosen from: ‘1. Yes, for the first time’, ‘2. Yes, even before April 2020’ and ‘3. No’. Using this, we created two variables: ‘presence of suicidal ideation (1=if the respondent chose either 1 or 2 from the afore-mentioned choices, and 0 otherwise)’ as having suicidal ideation following the onset of the COVID-19 pandemic and ‘onset of suicidal ideation (1=if the respondent chose 1 from the afore-mentioned choices, and 0 if the respondent chose 3)’ as having suicidal ideation for the first time following the onset of the COVID-19 pandemic.

    Two indicators were used to identify a person who was suffering from loneliness during the COVID-19 pandemic and a person who felt lonely after the onset of the pandemic. The first indicator was the University of California, Los Angeles Loneliness Scale (V.3), Short Form 3-item (UCLA-LS3-SF-3). The UCLA is a loneliness scale developed by Russel24 and has been revised three times. We used the latest version’s short form 3-item. Arimoto and Tadaka25 validated Japanese versions for this. Respondents answered three questions on a 5-point scale. The scores for these answers were added up, and the total scores ranged from 3 to 15. Next, we followed Yamada et al26 and set three tiers. The first tier was ‘no loneliness’ (if the total score was 3), the second was ‘mild loneliness’ (if the total score ranged from 4 to 5) and the third was ‘moderate-to-severe loneliness’ (if the total score ranged from 6 to 15). Then, we made a binary variable to identify respondents who were suffering from ‘moderate-to-severe loneliness’.

    The second indicator was created using the UCLA-LS3-SF-3 and the JACSIS’s original question that asks the respondent, ‘do you think you feel loneliness more frequently recently than you did before the onset of the COVID-19 pandemic?’ The answer to this question was obtained using a 5-point scale ranging from 1 (never) to 5 (always). Based on these two questions, we identified three groups. The first group reported ‘moderate-to-severe loneliness’ as per the UCLA-LS3-SF-3 and ‘feeling lonely more often following the onset of COVID-19’ (if the answer was either 4 or 5 in JACSIS’s original question). The second group reported ‘moderate-to-severe loneliness’ and ‘not feeling lonely more often following the onset of COVID-19’ (if the answer was either 1, 2 or 3). The third category was categorised as either ‘none’ or suffering ‘mild loneliness’.

    Depression was proxied by the Kessler Psychological Distress Scale (K6), a 6-item questionnaire developed for screening mood and anxiety disorders, which evaluates mood and anxiety experienced in the last 30 days on a 5-point scale.27 The total K6 scores ranged from 0 to 24. We used the Japanese version of the K6.28 Referring to Furukawa’s study, we adopted K6≥13 as the cut-off value indicating a state of severe psychological status and mood disorders. Although the K6 was originally developed to measure psychological distress, depression was commonly screened by the K6 in Japan.29

    Covariates were variables specific to the pandemic, and demographic and socio-economic status were considered risk factors for suicidal ideation. The variables included ‘social isolation’, ‘being infected by COVID-19 (respondents, respondents’ friends, family and/or coworkers)’ and ‘financial insecurity’. Social isolation was measured by answers to the following questions: ‘who have you met or talked to more than once in the past 2 weeks?’ and ‘who have you communicated with using online tools more than once in the past 2 weeks?’ We identified a socially isolated person when the respondent answered ‘none’ to both questions. Income decline due to the COVID-19 pandemic was captured by the question, ‘regarding the following items, how have they changed in the past month as compared with a year ago?’, with income included in the list of items. From this question, we created a binary variable: 1 if decreased and 0 otherwise. The experience of a shortage of money to buy daily necessities was captured by the question, ‘have you ever experienced a shortage of money to buy daily necessities since April 2020?’ We created a binary variable of 1 for yes and 0 for no.

    As for demographic and socioeconomic status, we included household income, educational attainment, employment status, marital status, number of household members, age (measured un decades) and presence of psychiatric and physical illnesses. We also included the log number of COVID-19 positive cases in their residing prefectures. The descriptions of each variable are shown in table 1.

    View this table:
    Table 1

    Participant demographics

    Statistical analysis

    Descriptive statistics were reported in terms of number and percentage (for binary variables), mean and standard deviation (for scale variables). All estimations were conducted by separating the sample by sex and adjusting for potential confounders. The survey weight was applied for analyses, and a Poisson regression model with a robust error variance was used. We used a Poisson regression model rather than logistic regression due to a relatively minor incidence of suicidal ideation and for a better interpretation of the OR as relative risk leads to potential exaggeration.30 Also, by using a robust error variance, we can avoid overestimation of relative risk when the Poisson regression is applied to binominal data.31 Regarding survey weight, we used inverse probability weighting to account for potential non-response bias and calculated national estimates.

    We first estimated how loneliness affected suicidal ideation. In this estimation, loneliness was captured in two ways as mentioned above: moderate-to-severe loneliness (panel A) and worsened loneliness during the pandemic (panel B). In panel A, we calculated the prevalence ratio (PR) of suicidal ideation among those who were in a moderate or severe state of loneliness. In panel B, we calculated PRs among those who felt more isolated with moderate-to-severe states of loneliness during the pandemic and those who did not feel more isolated and yet experienced moderate-to-severe loneliness compared with people who experienced none or a mild state of loneliness.

    The second estimation was performed to investigate how loneliness affected the onset of suicidal ideation during the pandemic. In this estimation, the dependent variable was the onset of suicidal ideation, and those who had suicidal ideation before April 2020 were omitted from the study sample. With regard to loneliness, we used two different variables: moderate-to-severe loneliness (panel C) and worsened loneliness during the pandemic (panel D), similar to the first estimation.

    For both analyses, we ran two models: with the K6 scores as covariates (model 1) and without (model 2), to examine whether loneliness directly or indirectly affected suicidal ideation. The PRs of loneliness in model 1 were regarded as direct effects of loneliness, and differentials obtained by subtracting those in model 2 from model 1 were regarded as indirect effects. A schematic diagram of the mediation model of loneliness, depression and suicidal ideation is shown in figure 1 for clarity. Direct and indirect effects of loneliness were calculated for each PR on the presence and onset of suicidal ideation.

    Figure 1
    Figure 1

    Schematic diagram of mediation model of loneliness, depression, and suicide ideation.

    All data analyses were performed using STATA MP V.15.

    Patient and public involvement

    Patients or the public were not involved in the design, conduct, reporting or dissemination plans of our research.

    Results

    Participant demographics

    As presented in table 1, 15.12% of male and 16.30% of female participants, respectively, had suicidal ideation. Among them, 22.82% of male and 19.75% of female participants, respectively, experienced suicidal ideation for the first time during the COVID-19 pandemic. This translates to 3.45% and 3.22% of the whole study sample, n, respectively. Regarding loneliness, nearly 50% of participants (of both sexes) were categorised as experiencing moderate-to-severe loneliness as per the UCLA-LS3-SF-3, and 9.88% of males and 10.76% of females felt more isolated during the pandemic. The percentage of participants (of both sexes) above the cut-off of the K6 was 15.00%. Among the participants, a higher percentage of men became COVID-19 positive, and the percentage of participants who experienced an income decline and a shortage of money to buy daily necessities were nearly identical in both sexes.

    Loneliness and presence of suicidal ideation

    Table 2 shows the PRs of loneliness and worsened loneliness with other covariates on the presence of suicidal ideation (results of covariates are not shown here for brevity. Full results are available in online supplemental file 1). In panel A, the PRs were 4.83 for men and 6.19 for women, and they decreased to 3.60 and 4.33, respectively, with the K6 being controlled for. In panel B, the PRs were highest for those who were feeling more isolated since the pandemic began: 6.5 for men and 8.86 for women (model 1) and 3.98 for men and 4.86 for women with the K6 (model 2). PRs were lower for those who were not feeling more isolated but in a state of moderate-to-severe loneliness: 4.41 for men and 5.44 for women with the K6 as covariates and 3.55 for men and 4.24 for women without the K6. PRs of having suicidal ideation among people with the K6 over 13 were 2.3 and 2.22 in men and 2.75 and 2.62 in women.

    View this table:
    Table 2

    Regression models showing associations between loneliness and presence of suicidal ideation

    The social isolation variable showed no statistically significant relationship with the PRs. Other COVID-19-related variables were significantly associated with suicidal ideation. The PRs of ‘being infected by COVID-19’ were 1.60 for men and 1.29 for women. The PRs of ‘income declines due to the COVID-19 pandemic’ were approximately 1.22 for men and 1.18 for women. The PRs of ‘shortage of money to buy daily necessities’ were 1.78 for men and 1.48 for women.

    Loneliness and onset of suicidal ideation since the outbreak of the pandemic

    Table 3 shows the PRs of loneliness and worsened loneliness with other covariates on the onset of suicidal ideation during the pandemic with all the other covariates being adjusted. (Results of covariates are not shown here for brevity. Full results are available in online supplemental file 2.) In panel C, the PRs were 4.53 for men and 11.37 for women and decreased to 2.60 and 4.96 for men and women, respectively, with the K6 being controlled. In panel D, the PRs were high for those who were feeling more isolated since the onset of the pandemic: 9.89 for men and 28.52 for women and 4.79 for men and 9.72 for women with the K6. The ones who were not feeling more isolated but were in a state of moderate and severe loneliness also had higher PRs, yet the magnitudes were much smaller: 3.02 for men and 6.33 for women and 2.23 for men and 3.96 for women with the K6. PRs among those with K6 scores over 13 were 3.89 and 2.97 for men and 7.32 and 5.20 for women. PRs of social isolation were only significant in one column.

    View this table:
    Table 3

    Regression models showing associations between loneliness and onset of suicidal ideation

    Other COVID-19-related variables were significantly associated with suicidal ideation. PRs of ‘being infected by COVID-19’ were 2.11 for men and 1.36 for women. PRs of ‘income declines due to the COVID-19 pandemic’ were 1.55 for men and 1.57 for women. PRs of ‘shortage of money to buy daily necessities’ were 3.53 for men and 3.27 for women.

    Direct and indirect effect of loneliness on suicidal ideation

    Table 4 shows the comparison of PRs of loneliness mediated by depression to analyse the relationships between loneliness, depression and suicidal ideation in four situations (panels A–D) more clearly. In panel A (effect of loneliness on the presence of suicidal ideation), PR was higher as a direct effect of loneliness than an indirect effect in both sexes. In panel B (effect of worsened loneliness on the presence of suicidal ideation), PR as a direct effect was also higher than the indirect effect, as in panel A. In panel C (effect of loneliness on onset of suicidal ideation), PR as a direct effect was higher than the indirect effect in men, but PR as an indirect effect was higher than as a direct effect in women. In panel D (effect of worsened loneliness on onset of suicidal ideation), PR as an indirect effect was higher than as a direct effect in both sexes.

    View this table:
    Table 4

    Comparisons of controlled prevalence ratios of loneliness mediated by depression

    Discussion

    This is the first study to reveal the effect of loneliness on suicidal ideation in Japan during the COVID-19 pandemic. The strengths of this study are twofold: a large web-based survey encompassing the Japanese population was used and both direct and indirect effects of loneliness were investigated. The latter strength is also a novel addition to the study of suicide. In the prepandemic era, various negative consequences of loneliness were reported on mental and physical health, including suicidal behaviour,32–38 and we assume that elevated levels of loneliness during the pandemic do contribute to an increase in the prevalence of suicidal ideation. Our analyses have shown the evidence of elevated suicidal ideation following the onset of the pandemic. Moreover, loneliness was a strong predictor of suicidal ideation, which is consistent with findings of studies from other countries, such as those of Antonelli-Salgado et al16 and Gratz et al’s15 studies. There were three important findings in our study.

    First, the prevalence of suicidal ideation following the outbreak of the pandemic was higher than that reported previously in Japan. Although a direct comparison is not possible, a recent nationwide survey by the Japanese government revealed that the prevalence of suicidal ideation during the year before the survey, which was the equivalent period as that of our survey, was 6.6% for men and 11.7% for women.39 A suicide awareness survey by the Nippon Foundation,40 which was conducted at the same time as our study, reported that the prevalence of suicidal ideation during the past year was 5.6% in men and 7.3% in women. In our survey, more than 15.1% of male participants and 16.3% of female participants reported experiencing suicidal ideation since the pandemic began. Moreover, 3.5% of men and 3.2% of women among the general population reported experiencing suicidal ideation for the first time following the onset of the pandemic. Thus, the results suggest that the prevalence of suicidal ideation among Japanese people may have increased due to the pandemic.

    Second, the relationship between loneliness and suicidal ideation in both men and women remained strong and consistent across all model specifications, even after general sociodemographic risk factors and COVID-19-related risk factors were considered. This result has two important implications. First, loneliness is both a direct and an indirect predictor of suicidal ideation. As shown, when depression (K6) was adjusted for, the PRs of loneliness declined yet still held a significant magnitude. Other variables were not influenced by the inclusion of the variable K6, suggesting that the effect of loneliness on suicidal ideation is either direct or mediated by depression. Second, this result was obtained even when other confounders did not show consistent results across model specifications (see online supplemental files 12). For instance, COVID-19 patients were not always statistically significant among women. In addition, low-income level, which was consistently found to be one of the greatest risk factors, did not show a statistically significant relationship or consistent PRs in the analysis of the onset of suicidal ideation in both sexes.

    Finally, the unique dataset comparing PRs of direct and indirect effects of loneliness on the presence or onset of suicidal ideation enabled us to examine the relationship between worsened loneliness and the onset of suicidal ideation during the pandemic. The results suggest that worsened loneliness (increased isolation during the pandemic) had a greater impact on both the presence and onset of suicidal ideation than loneliness before the pandemic. Moreover, in the context of suicidal ideation, loneliness as well as worsened loneliness have a direct effect, rather than an indirect effect, and for the onset of suicidal ideation, they impact it as an indirect effect mediated by depression rather than a direct effect. These relationships could be interpreted as follows: ‘chronic loneliness’ makes people have suicidal ideation directly and ‘acute loneliness’ makes people have suicidal ideation both directly and indirectly via depression. For females, the PRs were approximately twofold of their male counterparts in the analysis of the onset of suicidal ideation when the reference was set as ‘none or mild’ as a state of loneliness. These findings suggest that COVID-19-induced loneliness increases the risk of the onset of suicidal ideation, especially in women.

    This explains the significant increase in suicide rates among Japanese women in 2020. COVID-19 may have forced every person to be isolated and experience acute loneliness. It is interesting, however, that a recent follow-up study of the UK Biobank cohort41 revealed that living alone increased the risk of suicide in men, and not in women—a finding not explained by subjective loneliness. Reconsidering our findings, loneliness does not simply affect suicide, but its chronic or acute state is important. Sudden loneliness may lead to the onset of suicidal ideation via depression, especially in women.

    The policy implications are clear from the above discussion. At the clinical setting, loneliness should be cared and treated, and awareness campaigns and gatekeeper trainings are needed. This study’s findings create awareness regarding the need for providing urgent emotional support and mental healthcare to people struggling with loneliness and depression following the onset of the pandemic in order to prevent attempted suicides. For the last decade, since the enforcement of the Basic Act on Suicide Countermeasures in 2006,42 suicide measures in Japan have been specifically developed to tackle social problems, including job loss and economic hardship, but the importance of loneliness is often overlooked. Politicians should take mental health measures into account for suicide prevention, as recommended by the United Nations and the COVID-19 Suicide Prevention Research Collaboration.43 44

    Limitations

    Although our study makes meaningful contributions, the limitations of the study should be considered when interpreting the results. First, since the sample was collected through a web survey, it does not reflect the demographic distribution of the general population in Japan. Therefore, we used sampling weights using external, nationally representative data to adjust for potential bias in the demographic distribution of the collected sample. However, there may be residual bias as those who participated in the survey tended to be familiar with the internet and social networking platforms, which possibly led to an underestimated prevalence of social isolation. In a similar vein, sample exclusion may have caused some bias to our estimation. In our case, the largest drop of the number of observations was caused due to no response to income question. To explore the potential bias to our estimation, we have run an alternative regression model by including a dummy variable of ‘no response’. The results were nearly identical with PRs of loneliness to suicidal ideation for all the specifications. This implies that these biases do not severely impact on the estimation in our study. Second, we used the UCLA-LS3-SF-3 and an original one-item question to examine loneliness during the pandemic. However, there are a few doubts as to whether these scales really evaluate loneliness itself rather than isolation. Future studies must consider various forms of assessments to evaluate loneliness more accurately. Third, several risk factors for suicide and suicidal ideations, such as family history, alcohol and drug misuse, were not listed as confounders in this study. This should be addressed in future research. Fourth, since this study used a cross-sectional design, we could not confirm the direction of causality. For example, following the onset of the pandemic, people suffering from depression became socially isolated and felt lonely; in such cases, the mediating variable might be loneliness rather than depression. Longitudinal panel data should be collected for future analyses to determine the causality among loneliness, psychological distress, and suicidal ideation.

    Conclusion

    The results of this study revealed a high prevalence of suicidal ideation during the COVID-19 pandemic in Japan. Loneliness had both direct and indirect effects on suicidal ideation mediated through depression, and acute loneliness during the pandemic presented the highest risk for the onset of suicidal ideation, especially in women. Thus, intimate communication as well as a cognitive-behavioural approach for loneliness should be included in suicide prevention programme, especially for women. Also, awareness campaigns and gatekeeper trainings are needed to create awareness that people struggling with loneliness in the aftermath of the COVID-19 pandemic and those experiencing depression need urgent mental healthcare to prevent attempts at suicide. The results have strong implications regarding the development of policy measures to protect people from both the spread of infection and mental illness in the containment of a major pandemic.

    Data availability statement

    Data are available upon reasonable request.

    Ethics statements

    Patient consent for publication

    Ethics approval

    This study involves human participants. The research ethics committee of the Osaka International Cancer Institute (Approval No. 20084). Participants gave informed consent to participate in the study before taking part.

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    Supplementary materials

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    Footnotes

    • Twitter @TakahiroTabuchi

    • Contributors TT managed whole survey. TT, HM, MA and RO designed data collection and developed the study concept. MM and HT wrote the statistical analysis plan, cleaned and analysed the data and drafted the paper. TT, HM, MA and RO provided feedback for the paper. MM and HT have revised and finalised the manuscript. MM, HT and TT are the guarantors.

    • Funding This study was funded by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grants (Grant Nos. 18H03062, 19K22788 and 21H04856), a JSPS Grant-in-Aid for Young Scientists (Grant No.19K20171), the Japan Health Research Promotion Bureau Research Fund (2020-B-09) and the Research Institute of Science and Technology for Society of Japan Science and Technology Agency (JST-RISTEX), 'SOLVE for SDGs: Social Isolation & Loneliness (FY2021-)' (Grant No. JPMJRX21K2). The sponsors had no role in data collection and interpretation of our data. Findings and conclusions of this article are the sole responsibility of the authors and do not represent the official views of the research funders.

    • Competing interests None declared.

    • Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.

    • Provenance and peer review Not commissioned; externally peer reviewed.

    • Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.

    • ↵This question was included in the survey to identify systematic respondent inattention.