Background Trade union membership has massively declined in economically advanced economies. In addition, considerable variation exists between indicators of unionization, including trade union membership or presence rates. However, trade union membership and collective bargaining rates have recently risen in the UK. Yet, studies on the association between collective bargaining and workers’ mental health are sparse and non-existent in the current pandemic context. This study examines whether the impact of the COVID-19 pandemic on UK workers’ mental health is moderated by the presence of trade unions within their industrial sector.

Methods We used UK Household Longitudinal Study (Understanding Society) data from adults in paid employment, who participated in at least: one pre-COVID-19 Wave (9 or 10/11) and one COVID-19 wave. Primary outcome was probable psychological distress, defined by ‘caseness’ (score≥4) in the General Health Questionnaire-12 (GHQ-12). In model 1, exposure was trade union presence in workplaces, interacting with a variable splitting time between the pre and pandemic periods. In model 2, industry was added to the exposure in model 1 (3-way interaction). We fitted mixed-effects logistic regression models, adjusting for age, gender, ethnicity, UK country of residence and dummy variables for Understanding Society waves.

Results Our final sample included 69,348 observations across 9,075 individuals. Pre-pandemic, odds of GHQ-12 caseness were almost 11% (OR:1.11 95%CI:0.98,1.26) higher for workers in unionised compared to non-unionised workplaces; however post-pandemic there was no difference between the two groups. Examining changes over the pre- to pandemic period, showed that odds of GHQ-12 caseness for those in non-unionised workplaces increased by 42% (OR:1.42 95%CI:1.23,1.64), whereas for those in unionised workplaces odds increased by 28% (OR:1.28 95%CI:1.11,1.47). Overall, industrial classification did not modify the relationship between trade union presence and GHQ-12 caseness, except for workers in Manufacturing (OR:0.61 95%CI:0.40,0.95) where we found a protective effect but the opposite stands for those in Transportation and Storage (OR:1.89 95%CI:1.18,3.03). Pre-pandemic, odds of GHQ-12 caseness for education sector workers were considerably higher in unionised workplaces (OR:1.70 95%CI:1.15,2.53) but declined during the pandemic period (OR:0.62 95%CI:1.05,2.47).

Conclusion Trade union presence appears to have a protective effect for worker mental health during the pandemic. However, apart from Manufacturing, Transportation and Storage and Education, there is no evidence that type of industry modifies this effect. Therefore, policies should be designed to encourage and facilitate trade union presence across industries as they are likely to mitigate adverse mental health effects in times of extreme uncertainty.