This part included questions on sex, age, education attainment, apprenticeship, and county of residence. The company, seniority within the company, working rates, and employment as BD in other companies were also addressed.

BDs were asked to check off all health problems experienced at least once a month. The 13 proposed health problems were abnormal fatigue, shoulder or neck muscle pain, upper limb muscle pain, lower limb muscle pain, back pain, headaches, stomach pain, stress, anxiety, irritability, sleep disorders, appetite, or digestion problems, and hypersudation. Drivers were also asked to answer with yes or no to the following questions: “Have you had to miss work for health reasons in the past year?” coded as sick leave, “Have you had a work-related accident with time off work in 2021?” coded as an accident, and “Do you always drive while in full control?” coded as driving in unfit conditions. If the answer was “no” to the last question, we asked to indicate the reasons.

BDs were asked to grade twelve working conditions on a scale from 1-not tedious to 4-very tedious. The work conditions proposed were night work (from 10 p.m.), evening work (from 6 p.m.), Sunday work, difficult traffic conditions, driving time over 4 hours, a working day over 10 h, delayed traffic, aggressive customers, aggressiveness of other road users, traffic disruptions (accidents/deliverymen parking), cyclists’ unfair behavior, and long periods without access to toilets.

The fourth part concerned SARS-CoV-2 management and its consequences on health and working conditions. These data are described in details elsewhere [45].

Sleep disorders, sick leaves, and accidents were significantly associated with the geographic region of BDs’ residence. Sleep disorders increased in Zurich [OR = 3.19 (1.31, 7.72)] compared to Espace Mitteland. In addition, sick leaves increased in Lake Geneva Region [OR = 1.63 (1.01, 2.61)]. Accidents were less frequently reported in Lake Geneva Region [OR = 0.33 (0.15, 0.74)] compared to Espace Mitteland.

Shoulder or neck muscle pain increased with the presence of upper limb muscular pain [OR = 5.58 (3.13, 9.95)], headaches [OR = 1.89 (1.27, 2.81)], and with the driving period over 4 h [OR = 1.43 (1.12, 1.81)].

Sleep disorders and sick leaves increased with the presence of anxiety [OR = 1.87 (1.11, 3.16, and OR = 1.65 (1.00, 2.73), respectively]. The prevalence of self-reported sleep disorders and driving accidents was associated with sick leaves [OR = 1.64 (1.11, 2.40), and OR = 2.16 (1.26, 3.72), respectively]. Sleep disorders were less prevalent among BDs affiliated with union 3 versus union 1 [OR = 0.47 (0.26, 0.86)] but more prevalent among BDs with superior school level versus mandatory school level [OR = 1.72 (1.02, 2.88)]. Their prevalence of sleep disorders increased in the presence of stomach pain [OR = 2.02 (1.14, 3.58)], abnormal fatigue [OR = 2.41 (1.63, 3.55)], and back pain [OR = 1.49 (1.02, 2.17)].

Driving accidents were three-fold more frequent among BDs who made an apprenticeship of another occupation before becoming BD than BDs who did not [OR = 3.00 (1.42, 6.36)]. BDs driving regional lines, or a mix of regional and urban lines reported fewer accidents [OR = 0.37 (0.16, 0.88), and OR = 0.53 (0.28, 0.99), respectively].

Sick leave prevalence increased with seniority [1.03 per additional year (1.00, 1.05)], the presence of sleep disorders [1.71 (1.17, 2.50)], and the accident reporting in the last 12 months [OR = 2.16 (1.01, 1.62)].

In most cases, when adding the SARS-CoV-2 consequences on working conditions in the multivariate models, the associations found in the first multivariate model remained and some new ones appeared (Supplementary Tables S4–S7). The prevalence of sleep disorders, sick leaves, and accidents increased with SARS-CoV-2 crisis consequences on physical or mental health [OR = 1.83 (1.11, 3.00), OR = 1.82 (1.14, 2.91), and OR = 3.05 (1.51, 6.14), respectively]. In addition, sleep disorders increased with seniority [1.03 per additional year (1.01, 1.06)]. Shoulder or neck muscle pain was more prevalent among female BDs versus male BDs [OR = 1.81 (1.04, 3.17)].

In the explanatory model of shoulder or neck muscle pain, the associated working condition was the driving shift longer than 4 h, which seems perfectly plausible, considering all the other factors as independent. While a working day longer than 10 h was also associated with this health outcome, with a similar effect size as >4-h driving shift in the univariate analysis, in the adjusted models, this association disappeared because of collinearity with the long driving shifts. The same is true for the effect size of anxiety and stress, which are similar and statistically significant in the univariate analysis, but change in the fully adjusted models. Besides these working conditions, the reporting of the upper limb muscular pain and headache also explained the presence of shoulder or neck muscle pain, with quite high ORs (Supplementary Table S1). Furthermore, a two-fold increased risk of shoulder or neck muscle pain associated with female sex is another important finding, considering the growing feminization of the bus driver profession. For a long time, all bus drivers were men, and the vehicles were designed according to their stature. Presently, there are four times as many female BDs as in 2010 and the buses are not adapted to their, usually smaller, stature. This finding should be considered in the design of new vehicles. Finally, an observed association between shoulder or neck muscle pain and passengers’ aggressiveness in the fully adjusted deserves a cautious interpretation. In the univariate model, a statistically non-significant OR of 1.10 was observed. In the adjusted model, it first decreased to 0.79, then to 0.73, which became significant, but more as a statistical artifact than a meaningful finding. While it can be a risk factor for the outcomes considered in this study, when controlled for all the other variables identified in the DAG, it disappears and/or changes direction likely due to an over-adjustment. Considering passengers or other road users’ aggressiveness as protective against shoulder or neck muscle pain would be completely misleading, especially because this effect is observed only when adjusting for SARS-CoV-2 impacts. It is thus likely, that the measures taken against the spread of SARS-CoV-2 had a protective effect on the bus driver who was thus isolated from the unkind passengers and protected from their aggression.

An important finding was the association of seniority with an increased prevalence of sleep disorders. We expected the cumulated exposure to various hazards to increase with increasing seniority, which in turn increases the risk of having sleep disorders. Studies have shown an increase in mental health issues, including stress, due to the SARS-CoV-2 pandemic [25, 50]. As known, stress could deteriorate sleep quality [51]. Sleep disorders were three times more frequent in Zurich than in the Espace Mitteland. This could be explained by different working conditions or different sensitivities to sleep deprivation. To be noted, union 1 did a prevention campaign focused on sleep disorders at one of the companies corresponding to one-third of the participants living in Zurich. This might explain the association of a higher prevalence of sleep disorders with affiliation with union 3, as BDs part of union 3 might be more indulgent or less sensitive to this problem.

In the explanatory model of sick leaves, the associated working conditions were the working day over 10 h and the driving shift longer than 4 h, which seems perfectly plausible as this corresponds to longer exposure to hazards during a day. The association of anxiety and stress, which were statistically significant and led to an excess risk of sick leaves in univariate analysis, changed in the fully adjusted models. Indeed, stress usually negatively impacts health, increasing the risk of sleep disorders or stomach pain [51, 52]. However, in our study, anxiety, which is highly correlated with stress, explains better the sick leaves, anxious BDs tend to have more sick leaves. The cumulated exposure to various hazards increases with increasing seniority, which in turn increases the risk of having a disease and being on sick leave. The association of sick leaves with SARS-CoV-2 impact on physical or mental health was expected as bus drivers were at higher risk to contract the disease [24, 26] and the pandemic was linked to an increase in mental disorders in essential workers [25, 49, 50]. Among unions, Lake Geneva Region was known to have a problem with high absenteeism. Unions explained that due to the shortage of BDs [7, 11] and to maintain the service, bus companies had to make the present staff work, even if it meant eliminating days of rest. This results in an overload of work that could lead to employees taking a couple of sick leave days because they were no longer able to work.

Urban service was associated with excess risk of driving accidents. The working environment is different for urban and regional routes. Urban traffic is often considered more stressful with heavier traffic, more bicycles, more pedestrians, more disruptions on the road, and a tighter schedule. All these elements can be stressors and cause traffic accidents. In contrast, regional service generally has less traffic, fewer disruptions, and a looser schedule with more break time at the terminus. In addition, according to many drivers and unions, passenger behavior is also different between the two types of services. In the cities, people are usually more stressed and aggressive than outside the cities [53].

In both univariate and multivariate logistic regression models (Supplementary Table S4) the increased prevalence of driving accidents was associated with apprenticeship. To understand this association, it is important to understand the Swiss educational system, composed of three levels: compulsory (mandatory school), upper secondary (apprenticeship or general education), and tertiary (universities or college). During mandatory school, depending on their school grades, students are divided into sectors, either extended requirements or basic-requirement. After mandatory school, students from the basic-requirement sector do an apprenticeship, while the others can either continue their study or do an apprenticeship. Overall, in Switzerland, two-thirds of young people choose to do an apprenticeship [54, 55]. The BD profession is not an apprenticeship in Switzerland, but a training acquired in employment in the bus company for 6 months to 1 year. To start the BD training, it is mandatory to have an apprenticeship or at least 2 years of professional experience. All Swiss BDs had previous work experience, which might have no link with driving such as cook or jurist. In addition, apprenticeships are more frequent for occupations that are more physically demanding and exposed than occupations accessible only through higher education.

Contrary to before, in the univariate models, stress had a stronger effect than anxiety, and after adjustment, stress kept an excess risk (OR>1) while anxiety lost its effect when adding stress to the model. This seems plausible as stress is a known cause of driving accidents as it might lead to loss of focus and induce fatigue [56]. Fatigue is another known cause of driving accidents [31], we found that fatigue led to an excess risk (OR>1) of accidents in our univariate model. Moreover, the impacts of the SARS-CoV-2 pandemic on health and sick leave were partly related to mental health and fatigue, both of which were risk factors for accidents [31, 56].