Objectives To describe the mental health gap between those who live alone and those who live with others, and to examine whether the COVID-19 pandemic had an impact on this gap.Design Ten population based prospective cohort studies, and a retrospective descriptive cohort study based on electronic health records (EHRs).Setting UK Longitudinal population-based surveys (LPS), and primary and secondary care records within the OpenSAFELY-TPP database.Participants Participants from the LPS were included if they had information on living status in early 2020, valid data on mental ill-health at the closest pre-pandemic assessment and at least once during the pandemic, and valid data on a key minimum set of covariates. The EHR dataset included 16 million adults registered with primary care practices in England using TPP SystmOne software on 1st February 2020, with at least three months of registration, valid address data, and living in households of lt;16 people.Main outcome measures In the LPS, self-reported survey measures of psychological distress and life satisfaction were assessed in the nearest pre-pandemic sweep and three periods during the pandemic: April-June 2020, July-October 2020, and November 2020-March 2021. In the EHR analyses, outcomes were morbidity codes recorded in primary or secondary care between March 2018 and January 2022 reflecting the diagnoses of depression, self-harm, anxiety, obsessive compulsive disorder, eating disorders, and severe mental illnesses.Results The LPS consisted of 37,544 participants (15.2 and we found greater psychological distress (SMD: 0.09 (95 0.04, 0.14) and lower life satisfaction (SMD: -0.22 (95 -0.30, -0.15) in those living alone pre-pandemic, and the gap between the two groups stayed similar after the onset of the pandemic. In the EHR analysis of almost 16 million records (21.4, codes indicating mental health conditions were more common in those who lived alone compared to those who lived with others (e.g., depression 26 and severe mental illness 58 cases more per 100,000). Recording of mental health conditions fell during the pandemic for common mental health disorders and the gap between the two groups narrowed.Conclusions Multiple sources of data indicate that those who live alone experience greater levels of common and severe mental illnesses, and lower life satisfaction. During the pandemic this gap in need remained, however, there was a narrowing of the gap in service use, suggesting greater barriers to healthcare access for those who live alone.What is already known on the topic?Households with one individual are an increasing demographic, comprising over a quarter of all households in the UK in 2021. However, the mental health gap between those who live alone compared to those who live with others is not well described and even less is known about the relative gaps in need and healthcare-seeking and access. The pandemic and associated restrictive measures further increased the likelihood of isolation for this group, which may have impacted mental health.What this study adds?We present comprehensive evidence from both population-based surveys and electronic health records regarding the greater levels of mental health symptoms and in recorded diagnoses for common (anxiety, depression) and less common (OCD, eating disorders, SMIs) mental health conditions for people living alone compared to those living with others.Our analyses indicate that mental health conditions are more common among those who live alone compared to those who live with others. Although levels of reported distress increased for both groups during the pandemic, healthcare-seeking dropped in both groups, and the rates of healthcare-seeking among those who live alone converged with those who live with others for common mental health conditions. This suggests greater barriers for treatment access among those that live alone.The findings have implications for mental health service planning and efforts to reduce barriers to treatment access, especially for individuals who live on their own.Competing Interest StatementThe authors have declared no competing interest.Funding StatementThis work was funded by UK Research and Innovation (UKRI) (COV0076,MR/V015737/1), the Longitudinal Health and Wellbeing strand of the National Core Studies programme (MC PC 20030: MC PC 20059: COV-LT-0009). EH was funded by an NIHR post-doctoral fellowship (PDF-2016-09-029). DMP was funded by an MRC fellowship (MR/W02148X/1). MG, RJSh and SVK acknowledge funding from the Medical Research Council (MC UU 00022/2) and the Scottish Government Chief Scientist Office (SPHSU17). RJSh additional acknowledges funding from Health Data Research UK (SS005). SVK additionally acknowledges funding from a NRS Senior Clinical Fellowship (SCAF/15/02). RM is supported by Barts Charity (MGU0504). This research will also use data assets made available as part of the Data and Connectivity National Core Study, led by Health Data Research UK in partnership with the Office for National Statistics and funded by UK Research and Innovation (grant ref MC PC 20058). In addition, the OpenSAFELY Platform is supported by grants from the Wellcome Trust (222097/Z/20/Z), MRC (MR/V015757/1, MC PC-20059, MR/W016729/1), NIHR (NIHR135559), and Health Data Research UK (HDRUK2021.000, 2021.0157).Author DeclarationsI confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.YesThe details of the IRB/oversight body that provided approval or exemption for the research described are given below:Generation Scotland (GS) obtained ethical approval from the East of Scotland Committee on Medical Research Ethics (on behalf of the National Health Service). Reference number 20/ES/0021. For the EHR analysis via the OpenSAFELY platform, we received ethics approval to conduct the data linkage and analyses by the London City amp; East Research Ethics Committee on the 2nd of April 2020 (REC reference 20/LO/0651) and LSHTM Ethics Board (ref 21863). No further ethical or research governance approval was required by the University of Oxford but copies of the approval documents were reviewed and held on record. All waves of TwinsUK (TwinsUK) have received ethical approval associated with TwinsUK Biobank (19/NW/0187), TwinsUK (EC04/015) or Healthy Ageing Twin Study (HATS) (07/H0802/84) studies from NHS Research Ethics Committees at the Department of Twin Research and Genetic Epidemiology, Kings College London.I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.YesI understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).YesI have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.YesData for MCS (SN 8682), NS (SN 5545), BCS70 (SN 8547), NCDS (SN 6137) and all four COVID-19 surveys (SN 8658) are available through the UK Data Service. NSHD data are available on request to the NSHD Data Sharing Committee. Interested researchers can apply to access the NSHD data via a standard application procedure. Data requests should be submitted to mrclha.swiftinfoatucl.ac.uk; further details can be found at http://www.nshd.mrc.ac.uk/data.aspx. doi:10.5522/NSHD/Q101; doi:10.5522/NSHD/Q10.ALSPAC data is available to researchers through an online proposal system. Information regarding access can be found on the ALSPAC website (http://www.bristol.ac.uk/media-library/sites/alspac/documents/researchers/data-access/ALSPAC_Access_Policy.pdf).Understanding Society (USoc) data are available through the UK Data Service (SN 6614 and SN 8644).English Longitudinal Study of Aging (ELSA) data are available through the UK Data Service (SN 8688 and 5050).Access to Generation Scotland data is approved by the Generation Scotland Access Committee. See https://www.ed.ac.uk/generation-scotland/for-researchers/access or email accessatgenerationscotland.org for further details.The TwinsUK Resource Executive Committee (TREC) oversees management, data sharing and collaborations involving the TwinsUK registry (for further details see https://twinsuk.ac.uk/resources-for-researchers/access-our-data/).All EHR data were linked, stored and analysed securely within the OpenSAFELY platform https://opensafely.org/. Data include pseudonymized data such as coded diagnoses, medications and physiological parameters. No free text data are included. All code is shared openly for review and re-use under MIT open license ([https://github.com/opensafely/lone_households]). Detailed pseudonymised patient data is potentially re-identifiable and therefore not shared. We rapidly delivered the OpenSAFELY data analysis platform without prior funding to deliver timely analyses on urgent research questions in the context of the global Covid-19 health emergency: now that the platform is established we are developing a formal process for external users to request access in collaboration with NHS England, details of this process are available at OpenSAFELY.org.