Abstract
Background
Emerging research has examined the prevalence of severe acute respiratory syndrome virus 2 (SARS-CoV-2) infections in numerous settings, but a critical gap in knowledge is an understanding of the rate of infection among first responders.
Methods
We conducted a prospective serial serologic survey by recruiting public first responders from Cleveland area emergency medical services agencies and fire departments. Volunteers submitted a nasopharyngeal swab for SARS-CoV-2 PCR testing and serum samples to detect the presence of antibodies to SARS-CoV-2 on two visits scheduled approximately 3 weeks apart.
Results
296 respondents completed a first visit and 260 completed the second. While 71% of respondents reported exposure to SARS-CoV-2, only 5.4% (95% CI 3.1-8.6) had positive serologic testing. No subjects had a positive PCR. On the first visit, eight (50%) of the test-positive subjects had no symptoms and only one (6.2%) sought healthcare or missed school or work. None of the subjects that tested negative on the first visit were positive on their second.
Conclusions
While our results show a relatively low rate of test positivity for SARS-CoV-2 amongst first responders, most were either asymptomatic or mildly symptomatic. The potential risk of asymptomatic transmission both between first responders and from first responders to vulnerable patients requires more study.
Key words:
Introduction
Emergency first responders are likely uniquely at risk for contracting the severe acute respiratory syndrome virus 2 (SARS-CoV-2) due to the nature of their work environment (Citation1). While many individuals that contract the illness may manifest symptoms that prompt testing and quarantine, there is a growing body of evidence around the potential for and risk of asymptomatic spread, which in turn may lead to under-estimates of disease prevalence (Citation2). To date, the extent of asymptomatic SARS-CoV-2 infections has been reasonably well-characterized in general community samples, cruise ship passengers, women in childbirth, hospital workers and long-term care residents (Citation3–6). Evidence for asymptomatic infection in emergency workers during the 2002 SARS outbreak has been documented, but less is known about the risk for infection, asymptomatic or otherwise, in first responders during the current SARS-CoV-2 pandemic (Citation7). In order to understand (Citation1) the risk of COVID-19 infection and (Citation2) the rate of asymptomatic infection in this population, we conducted a prospective PCR and serologic survey of first responders in the Cleveland, Ohio area during the first few months of the 2019 novel coronavirus pandemic.
Methods
Setting
Cleveland is located in Cuyahoga county, which has a population of approximately 1.2 million individuals. On the last date of our study (June 2, 2020), there were a cumulative of 4,821 reported cases and 1,151 hospitalizations reported in the county (Citation8). Cleveland EMS, consisting of approximately 300 first-responders, responds to over 100,000 9-1-1 calls per year. Cleveland Fire, an organization consisting of approximately 800 first-responders, provides support to Cleveland EMS. In the context of the pandemic, all personnel were fit tested, trained in proper PPE donning and doffing, and had N95 masks available for patient care. There were no periods where N95 masks were not available. Except for the initial stages of the pandemic when surgical masks were used, most personnel wore N95 masks for patients with COVID-19 symptoms and either an N95 or surgical masks for all other patient care. Surgical masks, where tolerated, were also placed on all patients. Each ambulance received a surface wipe down after each call involving a patient with COVID-19 symptoms and a more intensive aerosol disinfectant application once per day. When not on calls, EMS crews were mandated to wear surgical or cloth masks at the stations or anytime they could not appropriately apply social distancing. Personnel self-monitored for symptoms daily. Agencies were notified of COVID-19 positive patients by the hospitals’ infection control staff.
Study Design
The protocol and sample size were designed in accordance with the Population-based age-stratified seroepidemiological investigation protocol for COVID-19 produced and maintained by the World Health Organization (WHO) (Version 1.1, published March 17, 2020) (Citation9). Participants were volunteer adults aged 18 and over, currently working as emergency services personnel from Cleveland Emergency Medical Services (EMS) and Cleveland Fire. Subjects were recruited to the study by the distribution of study pamphlets to their worksites. Study subjects indicated interest by submitting contact information through a secure online form or leaving voicemail on a dedicated study phone line. Study staff used this information to contact subjects by their preferred means (phone or email), and scheduled a videoconferencing intake visit where subject authentication, informed consent and enrollment took place. Once this was completed, dedicated support staff contacted subjects to schedule appointments for 2 study visits to take place 3 weeks apart (in accordance with the WHO COVID-19 seroepidemiological protocol). Appointments were offered during regular work hours (7 AM to 6 PM on weekdays) at a single, centrally located drive-through testing facility with phlebotomy capabilities. Testing at the facility was dedicated to subjects of this study only. If subjects missed their initially scheduled appointment, they were given an opportunity to reschedule within study parameters. All study surveys were submitted electronically by email, with the second survey automated to be sent out the day before their second visit. The electronic consent process and survey administration were completed using REDCap (Citation10). There was no financial incentive for participation in the study.
The intake survey asked about general demographic information, occupation and title of the respondent. Both the intake and follow-up surveys assessed for any known exposure to individuals with the novel 2019 coronavirus (COVID-19), symptoms (if any) and whether symptoms led volunteers to seek healthcare, miss work or school or be hospitalized. The follow-up survey also included questions on subjects’ field activity, personal protective equipment (PPE) usage and training, and suspected exposures to COVID-19. The majority of questions were drawn from the WHO seroepidemiological protocol, while questions regarding PPE availability and usage were introduced to provide context for the study results.
A nasopharyngeal swab and blood sample were collected from each participant during both study visits. Testing of swabs was conducted using a COVID-19 RT-PCR test that qualitatively detects nucleic acid from SARS-CoV-2. Initial PCR results were based on a relatively low threshold of positivity for target nucleic acid sequences. After completion of the study, the SARS-CoV-2 PCR results were re-interpreted using a higher threshold for test positivity, in accordance with updated manufacturer guidelines. The change in threshold occurred after our study was completed and was done at a system level, independent of our study activity. Serum samples were screened for the presence of COVID-19 virus specific IgG and IgM using ELISA testing by Epitope Diagnostics, Inc (http://www.epitopediagnostics.com/covid-19-elisa). Reported estimates of the sensitivities and specificities of the serologic assays vary depending on methodology and timing. For the IgG serology, sensitivity has been reported as 100%, (95% CI 84.6–100) and specificity 88.7% (95% CI, 77.0–95.7). For the IgM serology, sensitivity is reported at 81.8% (95% CI, 48.2–97.7) after 20 days of infection onset, and specificity as 97.2% (95% CI, 92.1–99.4).
The primary outcome was the prevalence of COVID-19 infection (as defined by positive serology or PCR testing) in the population. The confidence interval of the prevalence of infection was calculated using the Clopper-Pearson interval method. Presentation of the remaining measures and survey responses was descriptive, with stratification by initial testing status. Statistical analyses were conducted using R (version 3.5.1; R Core Team). This study was approved by the MetroHealth Institutional Review Board.
Results
296 participants completed the initial sampling visit between April 20 and May 19, 2020, while 260 completed the second sampling visit between May 18 and June 2, 2020 (). The intake survey was completed by 296 subjects (100% of those completing the first visit), while the second survey was only completed by 208 (70.3% of those that completed the first visit).
Figure 1. Study subject flow diagram.
First visit survey responses and testing results are shown in . 71% of the first responders that participated in our study reported exposure to individuals with COVID-19. However, only 16 (5.4%, 95% CI 3.1–8.6) had positive results of either serology or PCR assays. 8 (50%) of the test positive volunteers reported no symptoms. There were no discernable patterns in the number of reported symptoms when comparing test positive and test negative individuals (). 15 individuals had either a positive IgG or IgM, while 1 participant had both positive IgG and IgM results. That participant was the only one who reported a positive PCR prior to the study, and the only one who reported missing work and seeking healthcare in the positive result group. On the follow-up visit, none of the subjects who were negative on the initial screen tested positive (). Of the subjects that tested positive on the first visit, 6 subjects (40%) remained positive while 9 (60%) became negative (1 did not complete the follow-up sample visit). None of the subjects in our sample had positive PCR samples (after test threshold revision) or serologic evidence of contracting the illness between the first and second visits, even though the number subjects reporting exposure rose from 168 to 197.
Figure 2. The number of symptoms reported by testing result.
Table 1. Demographics, reported symptoms and SARS-CoV-2 screening results in a cohort of first responders from the Cleveland area (N = 296)
Table 2. Follow-up survey responses and SARS-CoV-2 screening results among subjects completing a second visit (N = 260)
179 of 205 (87.3%) subjects who completed the follow-up survey reported responding to emergencies in the field. Within these field active respondents, the majority reported adequate PPE training (97.1% of those testing negative on the first visit and 88.9% of those testing positive on the first visit) and supplies (75.9% of those testing negative on the first visit and 88.9% of those testing positive on the second visit). Field active respondents reported proper PPE usage an average of 94.9% of the time (median 100%, range 25–100).
Discussion
While more than two thirds of emergency medical workers in our cohort reported exposure to individuals with COVID-19, SARS-CoV-2 infection remained infrequent among these essential workers in Cleveland, Ohio. Half of those that tested positive reported no symptoms, raising concern for the potential of asymptomatic infection in this cohort.
None of the subjects in our sample had evidence of current infection by PCR, or serologic evidence of contracting the illness between the first and second visits. On the contrary, the number of positive serologies declined in-between visits. This latter finding is not unsurprising, considering recent work suggests that when compared to symptomatic individuals, asymptomatic individuals have less robust serologic responses and a faster decline as well as greater frequency of resolution in IgG levels (Citation11).
8 of 16 (50%) test-positive subjects in our study were asymptomatic, and the majority had symptoms that did not prompt them to seek healthcare. The degree to which these positive results may represent community transmission, work-site transmission (within stations or work partners) or patient-to-responder transmission is difficult to elucidate in our study. Results of participants survey data suggest that patient and/or coworker contacts were likely sources of infection. While the majority of the first responders in our study reported compliance with PPE, adequate PPE supplies and comfort with PPE training, anything short of 100% compliance should be regarded as inadequate. Empirically, PPE utilization and standard hygiene appear to be very effective at reducing the nosocomial acquisition of SARS-CoV-2 (Citation12). In the case of the SARS epidemic of 2002, the finding that no paramedics in Toronto, Canada became ill was attributed to similar precautions undertaken by first responders (Citation13). Outside of patient contact, work-site contact between colleagues should be assessed as a potential source – particularly in close quarters within ambulance and within fire-station congregations.
While the true impact of asymptomatic infection remains unclear, some have considered it a major barrier to control of the pandemic (Citation2). Importantly, symptom-based screening that relied on 2 or more symptoms would have missed the majority (10, or 62.5%) of positive subjects in our sample. This issue has been demonstrated in other healthcare worker populations. More research is needed to identify the efficacy of personal protective practices amongst first responders and the actual impact, if any, these asymptomatic first responders might have on the transmission of SARS-CoV-2.
Our study is strengthened by its sample size and the testing of first responders at two different time-points as new COVID-19 cases were peaking in the Cleveland area. Our findings are still likely limited by volunteer bias, although this may have been somewhat mitigated by a robust turnout relative to the estimated number of personnel who staff the EMS and Fire Departments. Unfortunately, we were unable to explore differences in characteristics between respondents and non-respondents as we had no data on the latter. Another limitation in our study is that the sensitivity, specificity and significance of serologic testing amongst asymptomatic or mildly symptomatic individuals remain unclear (Citation14). Asymptomatic patients may mount less robust immune responses so serologic testing results in our study could underestimate the true prevalence. Finally, our results may be limited to the regional and temporal aspects of the COVID-19 pandemic in the greater Cleveland area, but the concerns raised by our findings should be universal.
In summary, despite high rates of exposure, a small proportion of first responders in our study had laboratory evidence of SARS-CoV-2 infection. While uncommon, asymptomatic infection was detected in half of those that tested positive. More research is needed to determine the significance, if any, of asymptomatic infection in this population.
Additional information
Funding
References
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