The evolution of workplace risk for Covid‐19 in Canadian healthcare workers and its relation to vaccination: A nested case‐referent study

Abstract

Background During the early months of the Covid‐19 pandemic, studies demonstrated that healthcare workers (HCWs) were at increased risk of infection. Few modifiable risks were identified. It is largely unknown how these evolved over time. Methods A prospective case‐referent study was established and nested within a cohort study of Canadian HCWs. Cases of Covid‐19, confirmed by polymerase chain reaction, were matched with up to four referents on job, province, gender, and date of first vaccination. Cases and referents completed a questionnaire reporting exposures and experiences in the 21 days before case date. Participants were recruited from October 2020 to March 2022. Workplace factors were examined by mixed‐effects logistic regression allowing for competing exposures. A sensitivity analysis was limited to those for whom family/community transmission seemed unlikely. Results 533 cases were matched with 1697 referents. Among unvaccinated HCWs, the risk of infection was increased if they worked hands‐on with patients with Covid‐19, on a ward designated for care of infected patients, or handled objects used by infected patients. Sensitivity analysis identified work in residential institutions and geriatric wards as high risk for unvaccinated HCWs. Later, with almost universal HCW vaccination, risk from working with infected patients was much reduced but cases were more likely than referents to report being unable to access an N95 mask or that decontaminated N95 masks were reused. Conclusions These results suggest that, after a rocky start, the risks of Covid‐19 infection from work in health care are now largely contained in Canada but with need for continued vigilance.

Excess risk of Covid-19 infection and mental distress in healthcare workers during successive pandemic waves. Analysis of matched cohorts of healthcare workers and community referents in Alberta, Canada

“…An excess risk in HCW relative to the community during the Omicron wave has been reported from Hong Kong, comparing infection rates in staff from a single hospital to that in the general population [16], but not more widely. Our earlier work has shown that while risk of work-related Covid-19 decreased after vaccination it did not disappear completely [6] and it seems feasible that, with a surge in infection in the population, HCWs were again at greater risk, as suggested here. The increase in risk of a diagnosis of a stress/adjustment reaction or a depressive condition from the start of the pandemic to the fourth wave (to late fall 2021) is unlikely to be due to chance.…”

Determinants of Sickness Absence Duration After Mild COVID-19 in a Prospective Cohort of Canadian Healthcare Workers

“…Participants were recruited early in the pandemic and completed the following four online questionnaires: phase 1 in spring/summer of 2020, phase 2 in fall of 2020, phase 3 in spring of 2021, and phase 4 in spring of 2022. Information on positive tests for COVID-19 was collected in the questionnaire completed at each phase and, for a nested case-referent study, 15 by self-reports between questionnaires. From December 2020, when vaccination against COVID-19 became available to Canadian healthcare workers, information on dates and type of vaccine was collected alongside data on infection.…”

Cohort profile: Recruitment and retention in a prospective cohort of Canadian health care workers during the Covid-19 pandemic

“…Both cases and referents were asked, in the days immediately following case infection, to complete a detailed exposure questionnaire about experiences and exposures in the days leading up to the case's positive test. More details are given in Cherry et al [3].…”

“…Of 542 cases eligible for the case-referent study,534 (99%) completed the case-referent questionnaire. Among 1815 selected referents 1697 (93.5%) completed [3].…”

“…In the case-referent study it was evident that unvaccinated HCW, in the early phase of the pandemic were at greater risk of infection if they worked hands-on with patients with Covid-19, on a ward designated for care of infected patients, or handled objects used by infected patients. Later in the pandemic, with almost universal HCW vaccination, risk from working with infected patients was much reduced [3].…”