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The following is a curated review of key information and literature about this topic. It is not comprehensive of all data related to this subject.
Health care personnel wear personal protective equipment (PPE) to minimize their exposure to infectious microorganisms or hazardous materials in the clinical environment. When caring for patients with communicable infections, transmission-based equipment including gloves, gowns and eye and mouth protection is used to interrupt the transmission of a pathogen in addition to standard precautions such as hand hygiene. In addition to placing a patient in appropriate isolation, PPE is the first line of protection for health care personnel, supplemented by robust administrative, environmental and engineering controls (CDC, July 2015).
Epidemiologic evidence suggests SARS-CoV-2 is transmitted primarily through droplet inhalation containing infectious virus produced when coughing, sneezing, talking or singing (WHO-China Report, February 2020; Zhu, February 2020; WHO, 2020; Azimi, July 2020; Cheng, May 2020; Liu, June 2020; Chan, January 2020; Huang, January 2020; Pung, March 2020; Luo, March 2020). Self-inoculation following virus exposure on contaminated surfaces may also play a role in transmission. SARS-CoV-2 small droplet nuclei can become aerosolized and stay in the air for several hours during aerosol-generating procedures (AGPs); in these instances airborne transmission of SARS-CoV-2 appears possible (FGI, 2018; Klompas, July 2020; Hamner, May 2020, Heinzerling, April 2020). AGPs include endotracheal intubation, bronchoscopy, non-invasive ventilation, tracheotomy, manual ventilation before intubation, cardiopulmonary resuscitation, sputum induction and autopsy.
The extent to which airborne spread occurs not associated with AGPs is an area of controversy. Epidemiologic reports of transmission while on buses, during choir practice, via fecal aerosols and in nursing homes with poor ventilation (Hamner, May 2020; Shen, September 2020; de Man, August 2020; Kang, September 2020) suggest that in some scenarios aerosol-based (e.g. airborne) transmission is possible, but several of these studies are confounded by persons being in close proximity for an extended time and/or being unmasked and do not account for contacts outside of these particular moments. It is likely that SARS-CoV-2 aerosol-based transmission may happen, but based on current epidemiologic data, it does not appear to be a primary mode of transmission. The reproductive number of SARS-CoV-2 is approximately 2.5, which is similar to influenza; in comparison, the reproductive number of measles, which does transmit via aerosols, is 18 (Klompas, July 2020; Petersen, July 2020). It is likely that the viral inoculum needed to establish SARS-CoV-2 infection is much higher than that of measles. The symptomatic secondary attack rate of SARS-CoV-2 in households in published studies has ranged from 4.6% to 17.1% (Jing, June 2020; Cheng, May 2020; Zhang, May 2020; Burke, March 2020); this is less consistent with overt airborne transmission. Ultimately infectious risk likely depends on multiple factors including the duration of exposure, patient comorbidities and the size of the viral inoculum. Global experience with SARS-CoV-2 to date suggests droplet transmission is the primary mechanism of disease spread.
SARS-CoV-2 may also be transmitted indirectly through contact with contaminated surfaces via fomites (van Doremalen, April 2020; Verbeek, May 2020). Infectious viral particles contaminating surfaces can remain viable from a period of hours to days depending on numerous factors (including temperature, humidity, surface type and sunlight exposure). In addition to either droplet or airborne precautions (see below), the CDC recommends contact precautions when caring for patients with suspected or confirmed SARS-CoV-2 infection.
When health care personnel in an area with at least moderate SARS-CoV-2 community spread are participating in direct care of patients with confirmed or suspected SARS-CoV-2, CDC recommends using the following PPE to prevent transmission in addition to standard precautions:
- A National Institute for Occupational Safety and Health (NIOSH)-approved N95 or equivalent or higher-level respirator (or face mask if a respirator is not available) instead of a facemask when AGPs are being utilized or during surgical procedures that may generate aerosols
- Eye protection
If a SARS-CoV-2 positive patient is not undergoing AGPs the IDSA Guidelines on Infection Prevention in Patients with Known COVID-19 note that a surgical mask can be used (or a N95, N99 or PAPR). During periods of PPE shortage the IDSA guidelines panel recommend using a surgical mask or reprocessed respirator (as opposed to using no mask).
During critical PPE shortages CDC notes that extended use of facemasks can be utilized; a facemask should be removed if it becomes soiled or damaged. Face shields may also help in extending the use of facemasks (by reducing the risk for facemask contamination during patient care). The optimal use of face shields remains to be determined.
Careful donning and doffing of PPE using the correct sequence is paramount to avoid contamination and exposure to infection; see this video example created by the Society of Healthcare Epidemiologists of America (Ortega, June 2020).
In an area with at least moderate community SARS-CoV-2 spread, when health care personnel are in the clinical setting but not caring for a person who is suspected or confirmed to have SARS-CoV-2 infection, given that asymptomatic and presymptomatic transmission does occur, the most effective measure to prevent transmission is to use face masks (see Masks and Face Coverings for the Public section; Moriarty, March 2020; Kimball, March 2020; Wei, April 2020; He, April 2020). Studies have shown symptom screening of health care personnel would have missed an important number of health care personnel with SARS-CoV-2 infection who were working in the clinical setting, and that in the absence of transmission-based precautions, outbreaks of COVID-19 can occur in health care settings (Chow, April 2020; Heinzerling, April 2020). Given such evidence, universal masking in health care settings is recommended, in addition to daily symptom screening prior to entering the facility (CDC, July 2020). The CDC recommends that facemasks be used preferentially over cloth face coverings, when available. As noted above, medical-grade masks should be used when treating patients with potential or documented COVID-19.
In areas with moderate to elevated community transmission, CDC recommends that health care personnel wear eye protection during all patient care encounters (CDC, July 2020) given concerns about mucosal surfaces as a route of transmission (see eye protection section). During any AGPs (as outlined above), a fit-tested N95 respirator or a powered air purifying respirator (PAPR) should be worn instead of a surgical mask.
In summary: The primary mode of SARS-CoV-2 transmission appears to be inhalation of respiratory droplets. In specific scenarios, particularly indoor settings with poor ventilation, aerosol transmission may also occur; however, epidemiologic data is not consistent with aerosols being the primary mode of transmission. Fomite and contact transmission have been proposed, but are likely to be an uncommon mode of transmission. Based on this information, when caring for patients with SARS-CoV-2 infection in medical settings, health care personnel should wear a gown, gloves, eye protection and an N95 or equivalent respirator; if aerosol-generating procedures are being undertaken, a higher-level respirator is needed.
COVID-19 seropositivity and asymptomatic rates in healthcare workers are associated with job function and masking (Sims, November 2020).
Overall, in this prospective cohort study in Michigan, among health care employees who were exposed to persons with COVID-19, those who wore a mask were less likely to be seropositive than those who did not. Additionally, a large proportion of seropositive employees did not have symptoms of COVID-19 in the month prior to testing. The results are limited by several methodologic concerns.
- 20,614 out of approximately 43,000 employees of a health system with eight hospitals in Detroit, Michigan who provided an online assessment of demographic, clinical and exposure information and a blood sample.
- The mean age of all participants was 43.1 (+/- 13.0 years).
- SARS-CoV-2 IgG positivity rates associated with different job functions.
- The antibody test used was the EUROIMMUN platform.
- An internal validation demonstrated a specificity and sensitivity of 99.35% (95% CI: 97.93-99.86%) and 98.14% (95% CI: 97.75-99.22%), respectively.
- 1,818 (8.8%) participants were seropositive and 1.7% had equivocal results.
- In those who had equivocal results and were tested again 7 days later, 10.8% became seropositive.
- Among seropositive individuals, 44% reported that they were asymptomatic during the month prior to blood collection.
- Among all participants, 42.5% (95% CI: 41.8-43.2%) reported a direct exposure to COVID-19, defined by an interaction within 6 feet of a COVID-19 positive individual for more than 10 minutes.
- Among the COVID-19 exposed participants, 12.5% (95% CI: 11.8-13.2%) were seropositive.
- Participants in job categories involving direct patient care had a higher seropositive rate (9.5%; 95% CI: 9.1-10.0%) than those who did not (7.0%; 95% CI: 6.3-7.6%; p<0.0001).
- Those with frequent patient contact (phlebotomy, respiratory therapy and nursing) had a significantly higher seropositive rate (11.0%; 95% CI: 10.4-11.7%) than those with intermittent patient contact (physicians or clinical support with patient contact [7.4%; 95% CI: 6.7-8%]).
- Of participants who reported an exposure to COVID-19 at work or elsewhere, 76% reported wearing a mask at any time.
- Wearing an N95/PAPR mask had a significantly lower seropositivity rate (10.2%) compared to surgical/other masks (13.1%) or no mask (17.5%).
- At the time of the study, the community prevalence of SARS-CoV-2 infection in Michigan was high. This study did not examine/attempt to differentiate infections acquired in the health care setting vs. community-based acquisition.
- The authors note some persons were exposed outside of the health care setting, but do not report how many exposures this was. Presumably when people were exposed outside of the health care setting, they were not wearing PPE. An additional analysis without these exposures would have been helpful.
- The data on differences in serological status by mask type examines N95s vs. all other masks. A comparison of surgical masks and N95 masks directly would be useful.
- Whether other PPE was in use in the health care setting (face shields, gowns, gloves, etc.) in addition to masks was not assessed; this may have a significant impact on the results.
- All data were self-reported and may be subject to recall bias.
- Since IgG usually takes at least 2 weeks to develop, recently infected persons may have been missed.
Seroprevalence of SARS-CoV-2 among frontline health care personnel in a multistate hospital network - 13 academic medical centers, April-June 2020 (CDC, September 2020).
Overall, in this study assessing antibodies in health care personnel across 13 geographically diverse academic medical centers, 6% had antibodies to SARS-CoV-2. Seroprevalence was lower among providers who used face coverings.
- The study was conducted by the Influenza Vaccine Effectiveness in the Critically Ill (IVY) Network, a collaboration of academic medical centers in the United States conducting epidemiologic studies on influenza and COVID-19.
- 13 IVY medical centers participated; each enrolled a convenience sample of health care personnel between April and June 2020. The total number of participants was 3,248.
- To evaluate the prevalence and features of SARS-CoV-2 infection among frontline U.S. health care personnel.
- 194 (6.0%) of the sample had positive test results for SARS-CoV-2 antibodies.
- Seroprevalence by hospital ranged from 0.8% to 31.2% (median=3.6%).
- Among the 194 seropositive participants, 56 (29%) reported no symptoms since February, 86 (44%) did not believe they previously had COVID-19 and 133 (69%) did not report a prior diagnosis of COVID-19.
- Seroprevalence was lower among health care personnel who reported always wearing a face covering (defined as a surgical mask, N95 respirator or PAPR) while caring for patients (5.6%), compared with those who did not (9.0%) (p=0.012).
- The likelihood of health care personnel volunteering for studies may have been affected by their perceived risk of acquiring SARS-CoV-2, which may have skewed the results.
- Information on infection prevention strategies at the individual sites was not assessed.
- The study did not assess the likelihood of health care personnel being infected in the health care setting versus in the community.
- The study did not assess differences in the types of face coverings used at each site.
Risk of COVID-19 among frontline health care workers and the general community: a prospective cohort study (Nguyen, September 2020).
Overall, in this prospective study using self-reported data from a smartphone app, there was an increased risk for SARS-CoV-2 infection among frontline health care personnel compared with the general community. This risk was higher among individuals in direct contact with patients with COVID-19 who either reported inadequate PPE availability or reusing PPE.
- 2,810,103 users of the COVID Symptom Study app in the U.S. and U.K.
- 134,885 (4.8%) participants reported being a frontline health care worker.
- To investigate the risk of testing positive for COVID-19, the risk of developing symptoms associated with SARS-CoV-2 infection or both.
- The prevalence of COVID-19 was 2,747 cases per 100,000 among frontline health care personnel compared with 242 cases per 100,000 in the general community.
- Frontline workers were about 3.4 times more likely to test positive for COVID-19 than the general population.
- Compared with health care personnel who reported adequate PPE, frontline health care workers reporting PPE reuse had an increased risk of a positive COVID-19 test (adjusted HR 1.46, 95% CI 1.21-1.76); inadequate PPE was associated with a comparable increase in risk after multivariable adjustment (1.31, 1.10-1.56).
- Frontline health care personnel with inadequate PPE caring for patients with COVID-19 had an increased risk for COVID-19 after multivariable adjustment (adjusted HR 5.91, 95% CI 4.53-7.71) compared with those with adequate PPE not caring for patients with suspected or confirmed COVID-19.
- Health care personnel in nursing homes most frequently (16.9%) reported inadequate PPE, while inpatient providers reported reuse of PPE most often (23.7%).
- Among frontline health care personnel reporting adequate PPE, the risk for COVID-19 was increased for those caring for patients with suspected COVID-19 (adjusted HR 2.39, 95% CI 1.90-3.00) and for those caring for patients with documented COVID-19 (4.83, 3.99-5.85), compared with health care workers who did not care for either group.
- Findings were based on self-reported data on COVID-19 testing positivity, which could have led to recall bias.
- The survey did not enquire about specific occupations, experience level, type of PPE used, receipt of PPE training or frequency of exposure to patients with SARS-CoV-2 infection or aerosolizing procedures.
- Data collection was through a smartphone survey, which may skew respondents’ age group and limits respondents to those with access to a smartphone, which may result in selection bias.
- The study took place between March and April 2020, when universal mask use in health care settings was not common; the findings may not be generalizable to the present.
Specific risk factors for SARS-CoV-2 transmission among health care workers in a university hospital (Celebi, August 2020)
In this case-control study, health care personnel had a higher risk of acquiring SARS-CoV-2 when caring for patients with COVID-19 than health care workers who did not. Inappropriate use of PPE was associated with an increased risk of infection.
- 703 health care personnel at an academic medical center in Turkey who were screened for SARS-CoV-2.
- The case control portion of the study consisted of 47 cases and 134 controls.
- Risk factors for SARS-CoV-2 transmission among health care personnel.
- The positivity rates for SARS-CoV-2 among physicians, nurses, cleaning personnel and other occupations were 6.3%, 8.0%, 9.1% and 2.6%, respectively.
- Among the health care personnel who were screened for SARS-CoV-2 by RT-PCR, 8.3% worked in COVID-19 units and 3.4% did not work in COVID-19 units (RR=2.449, p=0.027).
- Inappropriate use of PPE during the care of suspected or confirmed cases of COVID-19 (OR=11.295, CI=2.183-59.429, P=0.04) and staying in the same break room as other health care personnel without wearing a medical mask for more than 15 minutes (OR=7.422, CI=1.898-29.020, P=0.04) were associated with SARS-CoV-2 transmission.
- The study did not assess the frequency, intensity and duration of exposure for each health care worker. The data obtained in the case-control study relied on self-report, which could have led to recall bias.
- The sample size was relatively small.
Epidemiology of and risk factors for coronavirus infection in health care workers: a living rapid review (Chou, July 2020).
Overall, in this review, health care personnel experienced a significant burden of SARS-CoV-2 infection, and PPE use appeared to be associated with a reduction in risk of infection.
- 64 studies that reported the incidence of or outcomes associated with coronavirus infections in health care personnel and risk factors for health care worker infection.
- 18 studies examined SARS-CoV-2.
- To examine risk factors for health care worker SARS-CoV-2, SARS-CoV-1 and MERS-CoV infection.
- 10 studies and 5 case series on incidence of infection were evaluated.
- The incidence of SARS-CoV-2 infection ranged from 3.8%-38.9%.
- 3 retrospective studies addressed risk factors associated with SARS-CoV-2 infection in health care personnel.
- Risk factors included working in a high-risk versus general department (relative risk [RR], 2.13 [CI, 1.45-3.95]); suboptimal handwashing before or after patient contact (RR, 3.10 [CI, 1.43-6.73] and 2.82 [CI, 1.11-7.18], respectively); longer work hours (log-rank p=0.02); and improper PPE use (RR, 2.82 [CI, 1.11-7.18]).
- In another study, 41 health care personnel were exposed to a patient with SARS-CoV-2 infection during an aerosol-generating procedure; 85% used face masks and 15% used respirators. No cases of infection occurred.
- The number of studies found was limited and heterogenous; no statistics could be applied.
- The quality of the studies was variable.
Physical distancing, face masks and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis (Chu, June 2020).
Overall, in this meta-analysis, there was a protective effect with the use of physical distancing between patients, and the use of masks and eye protection when caring for patients infected with SARS-CoV-2. None of the studies compared the efficacy of different kinds of masks.
- 172 observational studies from 16 countries across six continents.
- 44 comparative studies in health care and non-health-care settings (n=25,697 patients).
- To systematically review the effect of physical distance, face masks and eye protection on transmission of SARS-CoV-2, SARS-CoV and MERS-CoV.
- Some studies included direct evidence on SARS-CoV-2 (64 studies, of which 7 were comparative in design); 55 of the studies reported on SARS-CoV and 25 on MERS-CoV.
- Transmission of virus was lower with physical distancing of 1 meter or more, compared with a distance of less than 1 meter (n=10,736, pooled adjusted odds ratio 0.18, 95% CI 0.09-0.38).
- Protection was increased as distance was lengthened (change in RR 2.02 per m; pinteraction=0.041).
- Face mask use resulted in a large reduction in risk of infection (n=2647; aOR 0.15, 95% CI 0.07-0.34), with stronger associations using N95 or similar respirators compared with disposable surgical masks or similar (pinteraction=0.090).
- The addition of a mask or respirator decreased the transmission risk to 3.1%.
- Eye protection was associated with lower rates of infection (n=3713; aOR 0.22, 95% CI 0.12-0.39).
- Risk of bias was generally low to moderate after considering the observational design.
- None of the studies compared the efficacy of medical masks to that of N95 respirators in preventing transmissions, rather the efficacy of masks versus no masks.
- The primary studies on face masks in this meta-analysis had low-certainty evidence, and no studies were randomized controlled trials.
Personal protective equipment and intensive care unit health care worker safety in the COVID-19 era (PPE-SAFE): an international survey (Tabah, June 2020).
Overall, in this survey of reported PPE practices and availability in health care personnel, there were widespread shortages and reuse of single-use PPE items, and half of the respondents were never fit-tested for masks.
- International web-based survey from 6 continents of 2,711 health care personnel on PPE use and availability when caring for intensive care COVID-19 patients.
- Survey respondents included physicians (67%), nurses (27%) and allied health care personnel (6%).
- To survey health care personnel on availability and use of PPE while caring for COVID-19 patients in the intensive care unit.
- Most health care personnel used N95 respirators (58%), face shields (62%) and waterproof gowns (67%) when caring for SARS-CoV-2 patients in the ICU.
- A third of health care personnel reported that N95/FFP2/FFP3 masks or PAPR were missing and 33% that PPE was either washed or reused.
- Adverse effects of PPE were associated with longer shift durations and included heat (1,266, 51%), thirst (1,174, 47%), pressure areas (1,088, 44%), headaches (696, 28%), inability to use the bathroom (661, 27%) and extreme exhaustion (492, 20%).
- Voluntary survey and responses reflect opinions and perceptions alone.
- The study has an over-representation of physicians, which may underestimate the burden of adverse effects caused by PPE.
Survival of SARS-CoV-2 and influenza virus on the human skin: Importance of hand hygiene in COVID-19 (Hirose, October 2020). A model evaluated the stability of SARS-CoV-2 and influenza A virus (IAV) on human skin surfaces versus other surfaces (stainless steel/glass/plastic) and the disinfection effectiveness of 80% (w/w) ethanol. SARS-CoV-2 and IAV were inactivated more rapidly on skin surfaces than on other surfaces and the survival time was significantly longer for SARS-CoV-2 than for IAV (9.04 h [95% confidence interval, 7.96–10.2 h] vs. 1.82 h [1.65–2.00 h]), indicating that SARS-CoV-2 may have a higher risk of contact transmission than IAV.
What do we know about SARS-CoV-2 transmission? A systematic review and meta-analysis of the secondary attack rate and associated risk factors (Koh, October 2020). A review compared 43 studies of household secondary attack rate, 18 studies of secondary attack rate in health care settings and 17 studies of secondary attack rate in other settings. The pooled household secondary attack rate was 18.1% (95% CI, ), with significant heterogeneity ranging from 3.9% to 54.9% across studies. Secondary attack rate of symptomatic index cases was higher than asymptomatic cases (RR: 3.23; 95% CI, 1.46-7.14). Adults showed higher susceptibility to infection than children (RR: 1.71; 95% CI, 1.35-2.17). In health care settings, secondary attack rate was estimated at 0.7% (95% CI, 0.4%-1.0%).
Characteristics and transmission dynamics of COVID-19 in health care personnel at a London teaching hospital (Zheng, July 2020). In this retrospective review, authors sought to examine the transmission dynamics of SARS-CoV-2 infection in health care personnel at a teaching hospital. They found the rates of COVID-19 paralleled the community cases. They attributed the lack of increased transmission among health care workers to the use of PPE.
Contact tracing assessment of COVID-19 transmission dynamics in Taiwan and risk at different exposure periods before and after symptom onset (Cheng, May 2020). In this prospective case-ascertained study of 100 patients with confirmed COVID-19 and 2,761 close contacts in Taiwan, household and non-household family contacts had the highest secondary clinical attack rate, disproportionally higher than health care exposures (4.6% vs. 5.3% vs. 0.9%, respectively). The authors attribute the lower health care secondary attack rate to several factors, including PPE use and admission to the hospital at later stages of infection when the amount of viable virus is considerably lower.
Risk factors of health care personnel with coronavirus disease 2019: a retrospective cohort study in a designated hospital of Wuhan in China (Ran, March 2020). In this retrospective review of 72 health care workers with COVID-19, incorrect handwashing, suboptimal hand hygiene before and after contact with patients and improper PPE were associated with increased risk of SARS-CoV-2 infection (2.64, p<.05; 3.10, p<.01; 2.43, p<.01; 2.82, p<.05).