Table 2.
Marginal Structural Model of Variables Associated With Incident Coronavirus Disease 2019a
| Self-Reported Time-Varying Variable | Adjusted HR (95% CI) | P Value |
|---|---|---|
| COVID-19 exposure at work | ||
| No exposure | Reference | .01 |
| Low riskb | 0.8 (.4–1.7) | |
| High riskb | 2.5 (1.3–4.8) | |
| Time spent in healthcare workplace | ||
| <10 h/wk | Reference | .68 |
| 10–20 h/wk | 1.9 (.5–7.9) | |
| 21–30 h/wk | 2.5 (.7–8.9) | |
| 31–40 h/wk | 2.2 (.7–7.0) | |
| >40 h/wk | 2.0 (.6–6.6) | |
| Time spent providing direct patient-facing care | ||
| <10 h/wk | 0.5 (.2–.9) | .21 |
| 10–20 h/wk | 0.6 (.3–1.3) | |
| 21–30 h/wk | 0.9 (.5–1.7) | |
| 31–40 h/wk | Reference | |
| >40 h/wk | 0.9 (.4–2.0) | |
| Direct contact outside the workplace with someone who tested positive for COVID-19 (yes vs no) | 8.1 (3.8–17.5) | <.001 |
| Extent of avoidance of people who live outside your home when not at work (all/most of the time vs some/none) | 1.0 (.6–1.6) | .91 |
| Mask adherence when not at work (all of the time vs most/some/never) | 0.8 (.5–1.6) | .59 |
| Average daily no. of new COVID-19 cases in 6 Bay Area counties in 14-d period before visit (1-unit increase per 10 000 cases) | 1.3 (.97–1.8) | .08 |
Abbreviations: CI, confidence interval; COVID-19, coronavirus disease 2019; HR, hazard ratio.
For each marginal structural model, we estimated the inverse probability of treatment weights in which exposure probability was estimated for each participant conditioning on fixed variables (age, sex, race, ethnicity, household size, role, and work category) and the time-varying variables shown in the table.
Low-risk exposure at work included interacting with a patient who had COVID-19, with no reported breach in personal protective equipment (PPE) or other safety protocols. High-risk exposure at work was defined as ever interacting with a patient with COVID-19 without wearing full personal protective equipment (PPE) or while having a breach in PPE (eg, tears or accidental removal).