Introduction: The objective of this study of urban emergency medical services (EMS) and ED healthcare personnel (HCP), was to measure over time new incident cases of both symptomatic and asymptomatic COVID-19.

Hypothesis: The prevalence of symptomatic and asymptomatic SARS-CoV-2 among subjects working on the frontlines of healthcare are higher than expected. The incidence will increase at three months.

Methods: Participants were recruited to participate in the study and enrolled upon completion of the consent process. Subjects were asked to submit a COVID-19 symptom log at initial visit and at the three-month follow-up. Blood and saliva samples were collected from each participant by trained research personnel. Collection sites included fire stations and a centralized testing location in the city of Dallas. The samples underwent reverse transcriptase-polymerase chain reaction (RT-PCR) testing and IgG antibody testing using the Abbot IgG antibody assay. Repeat PCR and serology testing were performed after three months. The primary outcome was COVID infection as identified by IgG and PCR. Descriptive statistics were used to report incidence of cases over the follow-up period.

Results: The cohort comprised of 713 (552 EMS and 161 HCP). Follow-up testing was complete in 549 (77%) individuals. The overall prevalence of diagnosed COVID-19 at enrollment occurred in 3.7% (95% CI 2.5-5.3%) of the subjects, eight by PCR and 18 seropositive for IgG with 50% of these (95% CI 31.6-68.4%) reporting no prior symptoms. Of those who were asymptomatic in the initial test, four had previously tested positive. The incidence of new COVID-19 infection at three months was 56/549 (10.2%, 95% CI 7.9-13.0%). Of these 33/56 (60%, 95% CI 47-72%) were asymptomatic, 44/56 (79%) had antibodies, and 12/56 (21%) were diagnosed by PCR alone. In those asymptomatic with COVID-19, 16/26 (61%), and 26/56 (46%) reported known exposure to COVID-19.

Conclusion: The estimated prevalence at baseline of SARS-CoV-2 amongst frontline personnel was 3.7% with an increase in infected individuals to over 10% after three months. A substantial proportion were asymptomatic infections.

SARS-CoV-2 is a highly infectious virus with significant morbidity and responsible for the COVID-19 pandemic. The World Health Organization has reported that the pandemic is a public health emergency of international concern. It spreads by respiratory droplets and aerosolized particles. In the initial stages of the COVID-19 pandemic there was uncertainty among frontline emergency care professionals, their leaders, and other healthcare personnel as to risk of acquiring COVID-19. This risk was thought to be high by many despite basic mitigation measures such as the use of personal protective equipment (PPE). The rate of COVID-19 has been reported to be as high as 26.7% among healthcare workers in Iran.7 Similarly, EMTs and paramedics have been described as a group with a high rate of exposure to pathogens. This has been attributed to frequent and close contact with patients.

Additionally, asymptomatic infection and transmission are a hazard to frontline personnel battling COVID-19. Numerous case series and observational studies have reported that asymptomatic disease occurs.2-5 One observational study of one cruise ship estimated that 17% of patients infected with COVID-19 are asymptomatic.6 Another modeling study using Bayesian theory estimated the asymptomatic ratio to be 31%.3 A report from Italy suggests a much higher rate.5

There are four Level 1 trauma centers in the city of Dallas, Texas, and most fire departments are staffed with emergency medical technicians (EMT) and paramedics. The high number of frontline personnel in the area and their high rate of exposure to critically ill patients with COVID-19 make them ideal subjects to evaluate the prevalence of SARS-CoV-2. The objective of this study of urban emergency medical services (EMS) and emergency department healthcare professionals (HCP) was to assess this risk by measuring new incident cases of both symptomatic and asymptomatic COVID-19 over a period of three months. 

The EMS agencies in this region participate in a centralized organization that facilitates sharing of information related to clinical guidelines, operational processes, and updates on delivery of care. The organization consists of urban and suburban public agencies ranging from less than 100 members to over 1,000 members. The member EMS agencies and leaders were informed of this study and the personnel from interested agencies were invited to participate. Subjects provided consent prior to participating in the study.

Healthcare professionals working in four of the largest emergency departments in the city of Dallas were invited to participate through their respective leadership channels. No direct solicitation of healthcare professionals outside of local and regional organizations occurred. In addition, a flier that allowed participants to indicate interest was electronically distributed, and consent obtained on the day of initial testing.

In this prospective cohort study of EMS and HCP, volunteer subjects from local EMS agencies and four local hospitals with emergency departments were enrolled from June to August of 2020. Frontline personnel consisted of physicians, nurses, EMTs and paramedics. Data on self-reported exposures, symptoms, and record of previous COVID-19 testing were collected from each participant. Blood and saliva samples were collected from each participant by trained research personnel. Collection sites included fire stations and a centralized location in Dallas. The samples underwent reverse transcriptase-polymerase chain reaction (RT-PCR) testing and IgG antibody testing using the Abbot IgG assay. Repeat PCR and serology testing was performed after three months.

The primary outcome was COVID infection as identified by IgG and PCR. Descriptive statistics were used to report incidence of cases over the follow-up period. Ninety-five percent confidence intervals (95% CIs) were calculated. Stratified analysis was performed and divided into symptomatic and asymptomatic individuals. SARS-CoV-2 infection was defined by the presence of IgG. Sample size was calculated based on the confidence interval width of participants that turned IgG positive and had no symptoms. With a sample size of 1,000 and 800 participants with no symptoms, if 2.5% are IgG positive there will be a CI width of 1.5%-3.8%. Statistics were performed using STATA SE 16 (Statacorp, College Station, TX).

The cohort comprised of 713 (552 EMS and 161 HCP) subjects. Median age was 35 (IQR 31, 42) and 82% were male. Follow-up testing was complete in 549 (77%) individuals. The overall prevalence of diagnosed COVID-19 at enrollment occurred in 3.7% (95% CI 2.5-5.3%) eight by PCR and 18 seropositive for IgG with 50% of these (95% CI 31.6-68.4%) reporting no prior symptoms. Of those who were asymptomatic in the initial test, four had previously tested positive.

At three months, the incidence of new COVID-19 infection was 56/549 (10.2%, 95% CI 7.9-13.0%). Of these 33/56 (60%, 95% CI 47-72%) were asymptomatic; 44/56 (79%) had antibodies; and 12/56 (21%) were diagnosed by PCR alone. In those asymptomatic with COVID-19, 16/26 (61%) and 26/56 (46%) reported known exposure to COVID-19. There was no difference in the prevalence or incidence of infection diagnosed through a positive PCR between EMS and HCP. There was no significant difference between asymptomatic infection between prehospital and hospital personnel. Of all the subjects tested, 12 were IgG positive on the first and second tests (See Table 1 and Table 2). 

In June of 2020, there was approximately a four-percent prevalence of COVID-19 among the frontline personnel in our study. Most of these infections were asymptomatic. These results are similar to a study by Mohr et al., who looked at over 6,000 emergency personnel from around the United States and found that 4.6% of personnel studied had acquired COVID-19 by July 8, 2020.21 EMS and HCP have been at the forefront of implementing strategies to mitigate the risk of COVID-19 transmission in their ranks. Murphy et al.8 demonstrated that programmatic changes targeted toward EMS response to patients with COVID-19 resulted in reduced exposure and promoted responsible PPE utilization, while maintaining a less than 1% incidence throughout their study. Structured efforts aimed at minimizing exposure and promoting PPE use were similarly rolled out in our study area. For example, EMS agencies for which many of the study subjects work were actively involved in efforts such as altered endotracheal intubation clinical guidelines to decrease exposure to aerosolized particles. Others developed aggressive PPE utilization policies such as full-body protective suits when responding to 911 calls on patients suspected of having COVID-19. Other EMS agencies reconfigured their responding personnel structure to minimize the number of individuals entering suspected COVID-19 positive patients’ homes.

Yet, the incidence of new COVID-19 cases rose to 10.2% in a span of three months. Many factors may explain this rise in cases among EMS and HCP, including the inherent risk of treating patients with active COVID-19 infection, often in close quarters like an ambulance or in a hospital room, often with individuals at their most contagious stage.

It is also possible that study participants underprepared for managing patients with COVID-19, a theory supported by Fernandez, et al.,1 who demonstrated that suspicion alone was insufficient to guide EMS personnel in PPE usage in the care of patients suspected of having COVID-19. Additionally, the incidence of community COVID-19 in Dallas County rose dramatically at one point during the study period, with a peak of over 1,600 cases per day reported by county health authorities in late June and early July, 2020.32 This rise in community cases could have also contributed to the increased incidence among participants in the study.

Two additional factors were possible contributors. The first was PPE shortage, rationing and supply chain deficiency, which may have led to inadequate availability and increased reuse of essential PPE such as N95 particulate respirators. The second is community transmission, or non-patient care-related transmission. The importance of these two factors is likely significant and hotly debated. Davido et al.19 found a relationship between attending in-hospital administrative meetings without using a face covering (such as a surgical mask) and a significantly increased likelihood of acquiring COVID-19. The same finding was noted for HCP who reported attending private social gatherings outside work.

The number of asymptomatic COVID-19 positive subjects was a sobering statistic. The finding was not surprising given the known SARS-CoV-2 transmission variabilities and from the experiences of other research groups. Tarabichi et al.16 reported that most of the study EMS personnel who had positive serologic testing were asymptomatic or mildly symptomatic. The same pattern is noted on the HCP side. A study by Lombardi et al.18 studying HCP during Italy’s COVID-19 case surge early in the pandemic demonstrated that a proportion were asymptomatic at the time of the positive test.

Asymptomatic SARS-CoV-2 infection and the risk of asymptomatic transmission heighten the need for adequate PPE use during patient care, disease surveillance through testing, and increased utilization of basic public health measures during non-patient-care-related activities such as proper face coverings and avoidance of poorly-ventilated congregation areas.  

This study was one of the first of its kind during the COVID-19 pandemic. The ability to simultaneously quantify EMS and HCP prevalence and new COVID-19 infection incidence over time was a remarkable accomplishment. However, the follow-up for all EMS and HCP was challenging. Seventy-seven percent is acceptable to our study team, but we would have preferred improved adherence to the follow-up plan. The incidence of infections at three months would likely be higher if a greater number of subjects participated in the follow-up. Our group will seek to mitigate this challenge by facilitating in-station and in-hospital collection of samples by available trained personnel belonging to the subjects’ organizations instead of relying on our personnel’s availability to travel to the sites. The number of asymptomatic personnel infected with COVID-19 was surprising, but the subjectivity of, and reliance on, self-reported symptoms leave the possibility that subjects could have had other symptoms or did not wish to report them.  

Another limitation is that participation in the study relied on memory recall of symptoms and patient exposures. Participants reported perceived symptoms of COVID-19 dating back weeks and months before testing, making exposure-symptomatology-testing timeline associations extremely challenging. Future studies will aim for improved, more accurate reporting mechanisms that allow us to study asymptomatic infected subject timelines more closely.

The estimated prevalence at baseline of SARS-CoV-2 amongst frontline personnel (EMS and HCP) was 3.7% with an increase in infected individuals to over 10% after three months. A substantial proportion were asymptomatic infections. 

The study design and implementation adhered to currently accepted standards for human research. The study design and protocol was submitted to the institutional review board at the University of Texas Southwestern, under study ID STU 2020-0357, submitted as “Frontline Personnel IgG and IgM surveillance in a COVID pandemic. All subjects were volunteers and provided with both verbal and written materials informing them of the study. Written consent was obtained from all subjects.

1. COVID-19 Preliminary Case Series: Characteristics of EMS Encounters with Linked Hospital Diagnoses. Fernandez AR, Crowe RP, Bourn S, Matt SE, Brown AL, Hawthorn AB, Brent Myers J. Prehosp Emerg Care. 2021 Jan-Feb;25(1):16-27. doi: 10.1080/10903127.2020.1792016. Epub 2020 Jul 31.PMID: 32677858 

2. COVID-19 Pandemic: The Role of EMS Physicians in a Community Response Effort. Cabañas JG, Williams JG, Gallagher JM, Brice JH. Prehosp Emerg Care. 2021 Jan-Feb;25(1):8-15. doi: 10.1080/10903127.2020.1838676. Epub 2020 Nov 11.PMID: 33074060 

3. EMS dispatch center activity during the COVID-19 containment. Penverne Y, Jenvrin J, Montassier E. Am J Emerg Med. 2020 Aug 5:S0735-6757(20)30678-1. doi: 10.1016/j.ajem.2020.07.083. Online ahead of print.PMID: 32826107 Free PMC article. No abstract available. 

4. Feedback to Prepare EMS Teams to Manage Infected Patients with COVID-19: A Case Series. Ghazali DA, Ouersighni A, Gay M, Audebault V, Pavlovsky T, Casalino E. Prehosp Disaster Med. 2020 Aug;35(4):451-453. doi: 10.1017/S1049023X20000783. Epub 2020 Jun 8.PMID: 32507122 Free PMC article. 

5. How the COVID-19 Epidemic Affected Prehospital Emergency Medical Services in Tehran, Iran. Saberian P, Conovaloff JL, Vahidi E, Hasani-Sharamin P, Kolivand PH.West J Emerg Med. 2020 Sep 25;21(6):110-116. doi: 10.5811/westjem.2020.8.48679.PMID: 33052824 Free PMC article. 

6. Emergency Medical Services resource capacity and competency amid COVID-19 in the United States: preliminary findings from a national survey. Ventura C, Gibson C, Collier GD.Heliyon. 2020 May 3;6(5):e03900. doi: 10.1016/j.heliyon.2020.e03900. eCollection 2020 May.PMID: 32368629 Free PMC article. 

7. Healthcare Personnel experience of working during the COVID-19 pandemic: A qualitative study. Eftekhar Ardebili M, Naserbakht M, Bernstein C, Alazmani-Noodeh F, Hakimi H, Ranjbar H. Am J Infect Control. 2020 Oct 6:S0196-6553(20)30896-8. doi: 10.1016/j.ajic.2020.10.001. Online ahead of print.PMID: 33031864 Free PMC article. 

8. Occupational exposures and programmatic response to COVID-19 pandemic: an emergency medical services experience. Murphy DL, Barnard LM, Drucker CJ, Yang BY, Emert JM, Schwarcz L, Counts CR, Jacinto TY, McCoy AM, Morgan TA, Whitney JE, Bodenman JV, Duchin JS, Sayre MR, Rea TD.Emerg Med J. 2020 Nov;37(11):707-713. doi: 10.1136/emermed-2020-210095. Epub 2020 Sep 21.PMID: 32958477 Free PMC article. 

9. Effects of COVID-19 Pandemic on Emergency Medical Services. Şan İ, Usul E, Bekgöz B, Korkut S.Int J Clin Pract. 2020 Dec 5:e13885. doi: 10.1111/ijcp.13885. Online ahead of print.PMID: 33280198 

10. The Effect of the COVID-19 Pandemic on the Anxiety Level of Emergency Medical Services Professionals. Usul E, Şan I, Bekgöz B.Psychiatr Danub. 2020 Autumn-Winter;32(3-4):563-569. doi: 10.24869/psyd.2020.563.PMID: 33370767 

11. Effect of the Coronavirus Disease 2019 (COVID-19) Pandemic on the U.S. Emergency Medical Services System: A Preliminary Report. Lerner EB, Newgard CD, Mann NC. Acad Emerg Med. 2020 Aug;27(8):693-699. doi: 10.1111/acem.14051. Epub 2020 Jul 7.PMID: 32557999 Free PMC article. 

12. Antibody Rapid Test Results in Emergency Medical Services Personnel during COVID-19 Pandemic; a Cross Sectional study. Saberian P, Mireskandari SM, Baratloo A, Hasani-Sharamin P, Babaniamansour S, Aliniagerdroudbari E, Jamshididana M. Arch Acad Emerg Med. 2020 Nov 10;9(1):e2. eCollection 2021.PMID: 33313569 Free PMC article. 

13. Medical Leave Associated With COVID-19 Among Emergency Medical System Responders and Firefighters in New York City. Prezant DJ, Zeig-Owens R, Schwartz T, Liu Y, Hurwitz K, Beecher S, Weiden MD. JAMA Netw Open. 2020 Jul 1;3(7):e2016094. doi: 10.1001/jamanetworkopen.2020.16094.PMID: 32706380 Free PMC article. 

14. Home-based Testing for SARS-CoV-2: Leveraging Prehospital Resources for Vulnerable Populations. Goldberg SA, Bonacci RA, Carlson LC, Pu CT, Ritchie CS. West J Emerg Med. 2020 Jun 15;21(4):813-816. doi: 10.5811/westjem.2020.5.47769.PMID: 32726248 Free PMC article. 

15. Emergency Medical Services Personnel Awareness and Training about Personal Protective Equipment during the COVID-19 Pandemic. Cash RE, Rivard MK, Camargo CA Jr, Powell JR, Panchal AR. Prehosp Emerg Care. 2021 Jan 12:1-8. doi: 10.1080/10903127.2020.1853858. Online ahead of print.PMID: 33211613 

16. SARS-CoV-2 Infection among Serially Tested Emergency Medical Services Workers. Tarabichi Y, Watts B, Collins T, Margolius D, Avery A, Gunzler D, Perzynski A. Prehosp Emerg Care. 2021 Jan-Feb;25(1):39-45. doi: 10.1080/10903127.2020.1831668. Epub 2020 Oct 30.PMID: 33026286 

17. COVID-19 Vaccine Acceptability among U.S. Firefighters and Emergency Medical Services Workers: A Cross-Sectional Study. Caban-Martinez AJ, Silvera CA, Santiago KM, Feliciano PL, Burgess JL, Smith DL, Jahnke S, Horn GP, Graber JM. J Occup Environ Med. 2021 Jan 28. doi: 10.1097/JOM.0000000000002152. Online ahead of print.PMID: 33560073 

18. Characteristics of 1573 healthcare workers who underwent nasopharyngeal swab testing for SARS-CoV-2 in Milan, Lombardy, Italy. Lombardi A, Consonni D, Carugno M, Bozzi G, Mangioni D, Muscatello A, Castelli V, Palomba E, Cantù AP, Ceriotti F, Tiso B, Pesatori AC, Riboldi L, Bandera A, Lunghi G, Gori A. Clin Microbiol Infect. 2020 Oct;26(10):1413.e9-1413.e13. doi: 10.1016/j.cmi.2020.06.013. Epub 2020 Jun 20.PMID: 32569835 

19. First wave of COVID-19 in hospital staff members of a tertiary care hospital in the greater Paris area: surveillance and risk factors study. Davido B, Gautier S, Riom I, Landowski S, Lawrence C, Thiebaut A, Bessis S, Perronne V, Mascitti H, Noussair L, Rancon MD, Touraine B, Rouveix E, Herrmann JL, Annane D, de Truchis P, Delarocque-Astagneau E; Garches COVID-19 Collaborative Group. Int J Infect Dis. 2021 Feb 16;105:172-9. doi: 10.1016/j.ijid.2021.02.055. Online ahead of print.PMID: 33607301 

20. Trends in US Emergency Department Visits for Mental Health, Overdose, and Violence Outcomes Before and During the COVID-19 Pandemic. Holland KM, Jones C, Vivolo-Kantor AM, Idaikkadar N, Zwald M, Hoots B, Yard E, D’Inverno A, Swedo E, Chen MS, Petrosky E, Board A, Martinez P, Stone DM, Law R, Coletta MA, Adjemian J, Thomas C, Puddy RW, Peacock G, Dowling NF, Houry D. JAMA Psychiatry. 2021 Feb 3:e204402. doi: 10.1001/jamapsychiatry.2020.4402. Epub ahead of print. PMID: 33533876  

21. COVID Evaluation of Risk in Emergency Departments. Nicholas M. Mohr, MD, MS,a,∗ Karisa K. Harland, PhD, MPH,a Anusha Krishnadasan, PhD,d Patrick Ten Eyck, PhD,b William R. Mower, MD, PhD,e James Willey, MD,c Makini Chisolm-Straker, MD, MPH,f Stephen C. Lim, MD,g L. Clifford McDonald, MD,h Preeta K. Kutty, MD, MPH,h Elisabeth Hesse, MD, MTM&H,i Scott Santibanez, MD, MPHTM,i David A. Talan, MD,a,d,e and Project COVERED Emergency Department Network. Ann Emerg Med. 2021 Jul; 78(1): 27–34 

22. Wang D, Hu B, Hu C et al. Clinical Characteristics of 138 Hospitalized Patients With 2019 Novel Coronavirus-Infected Pneumonia in Wuhan, China. JAMA 2020. 

23. Lai CC, Liu YH, Wang CY et al. Asymptomatic carrier state, acute respiratory disease, and pneumonia due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2): Facts and myths. J Microbiol Immunol Infect 2020. 

24. Nishiura H, Kobayashi T, Suzuki A et al. Estimation of the asymptomatic ratio of novel coronavirus infections (COVID-19). Int J Infect Dis 2020. 

25. Wang Y, Liu Y, Liu L, Wang X, Luo N, Ling L. Clinical outcome of 55 asymptomatic cases at the time of hospital admission infected with SARS-Coronavirus-2 in Shenzhen, China. J Infect Dis 2020. 

26. Day M. COVID-19: identifying and isolating asymptomatic people helped eliminate virus in Italian village. BMJ 2020;368:m1165. 

27. Mizumoto K, Kagaya K, Zarebski A, Chowell G. Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020. Euro Surveill 2020;25. 

28. Ganczak M, Topczewska K. [Risk of occupational exposure to blood-borne pathogens in paramedics]. Med Pr 2018;69:685-694. 

29. Cassaniti I, Novazzi F, Giardina F et al. Performance of VivaDiagTM COVID-19 IgM/IgG Rapid Test is inadequate for diagnosis of COVID-19 in acute patients referring to emergency room department. J Med Virol 2020. 

30. Guo L, Ren L, Yang S et al. Profiling Early Humoral Response to Diagnose Novel Coronavirus Disease (COVID-19). Clin Infect Dis 2020. 

31. Development and Clinical Application of A Rapid IgM-IgG Combined Antibody Test for SARS-CoV-2 Infection Diagnosis. Li Z, Yi Y, Luo X et al. J Med Virol 2020. 

32. “2019 Novel Coronavirus (2019-CoV).” Hhs-Pages, 6 Apr. 2020, www.dallascounty.org/departments/dchhs/2019-novel-coronavirus/summaries.php. Accessed 27 June 2022.