Severe acute respiratory coronavirus virus 2 (SARS-CoV-2) transmission occurs primarily via close contact. ¹ There is some debate about the relative contribution of larger or smaller respiratory particles to this short-range transmission. ^{Reference Klompas, Baker and Rheen2} Whether surgical masks or N95 respirators are used by healthcare workers (HCWs) testing patients with suspected coronavirus disease 2019 (COVID-19) varies across different institutions.

In March 2020, COVID-19 assessment centers (CACs) were established throughout Ontario, Canada, and virtually all ambulatory nasopharyngeal testing for SARS-CoV-2 in the Province was conducted at these locations. Because COVID-19 is most infectious immediately prior to and shortly after the onset of symptoms, those cases diagnosed in the CACs, in which the median time from symptom onset to testing is ∼2–4 days, are more infectious compared to patients seen later in their disease course. ^{Reference Paul, Daneman and Brown3} , ^{Reference Johansson, Quandelacy and Kada4} We evaluated the risk of occupational COVID-19, and we assessed the effectiveness of the control measures implemented to protect HCWs in this high-risk setting.

We performed a multicenter cross-sectional study across four CACs in Toronto, Canada. Each CAC prospectively identified HCWs with COVID-19 between March 15, 2020, and March 14, 2021. All asymptomatic HCWs working in the CACs were required to undergo SARS-CoV-2 testing if they had unprotected close contact with anyone with COVID-19, if they traveled, or if they developed any symptoms whether minimal or atypical. ⁵ Each CAC implemented a standard hierarchy of controls ⁶ that focussed on potential points of transmission risk (Table 1). Those performing nasopharyngeal testing performed hand hygiene and wore surgical masks, eye protection, gown, and gloves according to Canadian recommendations. There were no changes in PPE recommendations during the study period. The primary outcome was the rate of CAC HCW positivity for SARS-CoV-2 compared to the rest of the Ontario population according to publicly reported rates. The secondary outcome was the number of CAC HCWs positive for SARS-CoV-2 who worked in a patient-facing role and regularly performed nasopharyngeal swabs or examine patients, compared to CAC HCWs (eg. administrative staff) who had no patient contact. The primary outcome was evaluated using the χ² test. Based on our fixed sample size, using a 2-sided α of 0.05, we had a power of 80% to detect a difference of 2%. A Poisson regression model with a generalized estimating equation was created for the secondary analysis that accounted for clustering among HCWs at the same CAC. As a sensitivity analysis, the primary outcome was compared again based on a study period ending December 31, 2020, prior to the start of COVID-19 vaccination of HCWs. Research ethics review was not required because the study met criteria for exemption; the project was deemed improvement in quality and not human-subject research.

Table 1.

Hierarchy of Controls Implemented in COVID-19 Assessment Centers where 21,951 Patients Tested Positive for SARS-CoV-2 between March 15, 2020, and March 14, 2021

Note. HVAC, heating, ventilation and air conditioning; HCW, healthcare worker.

^a High-risk exposure defined as any close contact (within 2 m) with unmasked individual for 10-minutes or longer where HCW was either not wearing a mask, or eye protection, or both.

During the study period, 354,027 patients were tested across the 4 CACs, and 21,951 (6.2%) were confirmed positive for SARS-CoV-2, including 4,097 (4.3%), 2,830 (3.8%), 4,887 (5.8%) and 10,137 (10.1%) at the 4 CACs. Table 2 summarizes the outcomes of 470 HCWs working in the CACs. Overall HCW positivity rate for SARS CoV-2 was 2.3% (11 of 470) compared to 2.2% in the Ontario population (P = .82). We detected no significant difference in the rate of HCW infections between patient-facing and non–patient-facing roles, with 2.3% and 2.2% of HCWs positive, respectively (relative risk, 0.89; 95% confidence interval [CI], 0.49–1.65; P = .72). In the sensitivity analysis, the overall HCW positivity rate for SARS CoV-2 was 1.7% (8 of 470) compared to 1.2% in the rest of Ontario (P = .34).

Table 2.

Number of Total and SARS-CoV-2–Positive Individuals Included In Study

Note. CAC, COVID-19 assessment center; HCW, healthcare worker.

Our results show that when embedded within a comprehensive bundle of measures designed to minimize COVID-19 transmission, the use of surgical masks was effective in protecting HCWs given a rate of infection similar to a population average that included nonessential workers. The similar infection rates between clinical and nonclinical staff suggest that most infections that did occur were likely acquired outside the CACs.

Variability in practice exists regarding whether surgical masks or N95 respirators are used for routine care of suspected or confirmed COVID-19 patients, including during testing for SARS-CoV-2. A recent systematic review reported limited to no evidence regarding the risk of aerosol transmission related to nasopharyngeal or oropharyngeal swabs in the detection of SARS-CoV-2. ^{Reference Agarwal, Fernando and Honarmand9} Our study helps to address this important gap in the literature and supports existing international guidelines recommending droplet and contact precautions for this specimen collection.

This study has several limitations. It was limited by the observational design and small sample size. There were differences in the number of HCWs and the relative time working at each CAC, which may have affected the exposure risk between sites. We attempted to account for clustering within sites using generalized estimating equation model in the secondary analysis. The patient population had a test positivity rate of 6% and generally exposures during testing were brief. However, a detectable difference in SARS-CoV-2 infection risk would be expected if these practices were inadequate, given that these HCWs were within close contact to nearly 22,000 patients with COVID-19, have similar or higher expected nonoccupational risks for COVID-19 compared to the general population, and are more likely to be tested. ^{Reference Schwartz, Achonu and Buchan10}

Our findings provide supporting evidence for the effectiveness and safety of this combination of infection prevention and control measures, which includes PPE of a surgical mask, eye protection, gown, and gloves in the collection of nasopharyngeal and oropharyngeal swabs for SARS-CoV-2.