BACKGROUND: Social isolation has been associated with increased risk of coronary heart disease and stroke. However, it is unclear whether the associations differ between fatal and non-fatal events or by the type of isolation (living alone or having few social contacts). We aimed to examine these associations in two large UK prospective cohorts. METHODS: Million Women Study and UK Biobank participants without previous coronary heart disease or stroke who provided data in median year 2010 (IQR 2009-2011) on social contacts were included in this prospective analysis. Participants were followed up to median year 2017 (2017-2017) by electronic linkage to national hospital and death records. Risk ratios (RRs) were calculated using Cox regression for first coronary heart disease and stroke event (overall, and separately for hospital admission as the first event and for death without an associated hospital admission as the first event) by three levels of social isolation (based on living alone, contact with family or friends, and group participation) adjusted for age, sex, study, region, deprivation, smoking, alcohol intake, body-mass index, physical activity, and self-rated health. FINDINGS: 938 558 participants were included in our analyses (mean age 63 years [SD 9]): 481 946 participants from the Million Women Study (mean age 68 years ) and 456 612 participants (mean age 57 years ) from UK Biobank. During a mean follow-up period of 7 years (2), 42 402 first coronary heart disease events (of which 1834 were fatal without an associated hospital admission) and 19 999 first stroke events (of which 529 were fatal without an associated hospital admission) occurred. Little, if any, association was found between social isolation and hospital admission for a first coronary heart disease or stroke event (combined RR for both studies 1·01 [95% CI 0·98-1·04] for coronary heart disease and 1·13 [1·08-1·18] for stroke, when comparing the most isolated group with the least isolated group). However, the risk of death without an associated hospital admission was substantially higher in the most isolated group than the least isolated group for coronary heart disease (1·86 [1·63-2·12]) and stroke (1·91 [1·48-2·46]). For coronary heart disease or stroke death as the first event, RRs were substantially higher (test for heterogeneity, p=0·002) for participants living alone versus those not living alone (1·60 [1·46-1·75]) than for those with fewer versus more contact with family, friends, or groups (1·27 [1·16-1·38]). These findings did not differ greatly between studies, or by self-rated health. INTERPRETATION: Social isolation seems to have little direct effect on the risk of developing a first coronary heart disease or stroke. By contrast, social isolation substantially increases the risk that the first such event is fatal before reaching hospital, particularly among people who live alone, perhaps because of the absence of immediate help in responding to an acute heart attack or stroke. FUNDING: UK Medical Research Council, Cancer Research UK.
Lancet Public Health