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ACEP COVID-19 Field Guide

Table of Contents

Severity and Progression of Disease


The median incubation period for COVID-19 is 4 to 5 days; the range is 2 to 14 days (Figure 8.1).

Figure 8.1 Proportion of symptomatic cases per day since infection.


Studies suggest that illness severity can range from mild to critical (Table 8.1):

  • Mild to moderate: 81%;
  • Severe: 14%; and
  • Critical: 5%.

Table 8.1 Illness severity for COVID-19.


According to reports from China:

  • Deaths occurred among patients with critical illness, and the overall case fatality rate was 2.3%.
  • The case fatality rate among patients with critical disease was 49%.
  • Among children, illness severity was lowever, with:
    • 94% having asymptomatic, mild, or moderate disease; 
    • 5% having severe disease; and 
    • <1% having critical disease.

Clinical progression

It is important to note that some patients rapidly deteriorate 1 week after illness onset. 

Among patients who developed severe disease, the medium time to dyspnea ranged from 5 to 8 days; the median time to ARDS ranged from 8 to 12 days; and the median time to ICU admission ranged from 10 to 12 days, with 26% to 32% of patients admitted to the ICU. There have been different reports, but mortality among patients admitted to the ICU ranged from 39% to 72%. The median length of hospitalization among survivors was 10 to 13 days. 

Development of ARDS was reported in:

  • 3% to 17% of all patients; 
  • 20% to 42% of hospitalized patients; and
  • 67% to 85% of patients admitted to the ICU.

Risk factors for severe illness

Age is a strong risk factor for severe illness, complications, and death. According to CDC reports, among the 1,482 patients hospitalized with COVID-19, 74.5% were aged ≥50 years, and 54.4% were male. The hospitalization rate among patients identified through COVID-NET during this 4-week period was 4.6 per 100,000 population. Rates were highest (13.8) among adults aged ≥65 years (Figure 8.2).

Among more than 44,000 confirmed cases of COVID-19 in China, the case fatality rate was highest among older persons: 

  • ≥80 years: 14.8%;
  • 70 to 79 years: 8.0%;
  • 60 to 69 years: 3.6%;
  • 50 to 59 years: 1.3%;
  • 40 to 49 years: 0.4%; and
  • <40 years: 0.2%.

Patients in China with no reported underlying medical conditions had an overall case fatality of 0.9%, but case fatality was higher for patients with comorbidities: 

  • 10.5% for those with cardiovascular disease;
  • 7.3% for diabetes; 
  • Approximately 6% for chronic respiratory disease;
  • Approximately 6% for hypertension; and
  • Approximately 6% for cancer.

Heart disease, hypertension, prior stroke, diabetes, chronic lung/respiratory disease, cancer , and chronic kidney/renal disease and obesity have all been associated with increased illness severity and adverse outcomes.

Figure 8.2 Laboratory-confirmed coronavirus disease 2019 (COVID-19)–associated hospitalization rates,* by age group — COVID-NET, 14 states,a March 1, 2020, to March 30, 2020 (Garg S, Kim L, Whitaker M, et al. Hospitalization rates and characteristics of patients hospitalized with laboratory-confirmed coronavirus disease 2019 — COVID-NET, 14 states, March 1-30, 2020. MMWR Morb Mortal Wkly Rep. 2020 Apr 17;69(15):458-464. doi:10.15585/mmwr.mm6915e3).


COVID-NET = Coronavirus Disease 2019–Associated Hospitalization Surveillance Network

* Number of patients hospitalized with COVID-19 per 100,000 population.

a Counties included in COVID-NET surveillance: California (Alameda, Contra Costa, and San Francisco counties); Colorado (Adams, Arapahoe, Denver, Douglas, and Jefferson counties); Connecticut (New Haven and Middlesex counties); Georgia (Clayton, Cobb, DeKalb, Douglas, Fulton, Gwinnett, Newton, and Rockdale counties); Iowa (one county represented); Maryland (Allegany, Anne Arundel, Baltimore, Baltimore City, Calvert, Caroline, Carroll, Cecil, Charles, Dorchester, Frederick, Garrett, Harford, Howard, Kent, Montgomery, Prince George’s, Queen Anne’s, St. Mary’s, Somerset, Talbot, Washington, Wicomico, and Worcester counties); Michigan (Clinton, Eaton, Genesee, Ingham, and Washtenaw counties); Minnesota (Anoka, Carver, Dakota, Hennepin, Ramsey, Scott, and Washington counties); New Mexico (Bernalillo, Chaves, Dona Ana, Grant, Luna, San Juan, and Santa Fe counties); New York (Albany, Columbia, Genesee, Greene, Livingston, Monroe, Montgomery, Ontario, Orleans, Rensselaer, Saratoga, Schenectady, Schoharie, Wayne, and Yates counties); Ohio (Delaware, Fairfield, Franklin, Hocking, Licking, Madison, Morrow, Perry, Pickaway and Union counties); Oregon (Clackamas, Multnomah, and Washington counties); Tennessee (Cheatham, Davidson, Dickson, Robertson, Rutherford, Sumner, Williamson, and Wilson counties); and Utah (Salt Lake County).

The hospitalization rate among children is shown in Figure 8.3.

Figure 8.3 COVID-19 cases among children* aged <18 years, among those with known hospitalization status (N = 745),a by age group and hospitalization status — United States, February 12, 2020, to April 2, 2020 (CDC COVID-19 Response Team. Coronavirus disease 2019 in children — United States, February 12-April 2, 2020. MMWR Morb Mortal Wkly Rep. 2020 Apr 10;69(14):422-426. doi:10.15585/mmwr.mm6914e4).


* Includes infants, children, and adolescents.

a Number of children missing hospitalization status by age group: <1 year (303 of 398; 76%); 1-4 years (189 of 291; 65%); 5-9 years (275 of 388; 71%); 10-14 years (466 of 682; 68%); and 15-17 years (594 of 813; 73%).


  1. Guan WJ, Ni ZY, Hu Y, et al. Clinical characteristics of coronavirus disease 2019 in China  [published online ahead of print, 2020 Feb 28]. New Eng J Med. 2020;NEJMoa2002032. doi:10.1056/NEJMoa2002032
  2. Wang D, Hu B, Hu C, et al. Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirus-infected pneumonia in Wuhan, China [published online ahead of print, 2020 Feb 7]. JAMA. 2020;e201585. doi:10.1001/jama.2020.1585
  3. Wu C, Chen X, Cai Y, et al. Risk factors associated with acute respiratory distress syndrome and death in patients with coronavirus disease 2019 pneumonia in Wuhan, China [published online ahead of print, 2020 Mar 13]. JAMA Intern Med. 2020;e200994. doi:10.1001/jamainternmed.2020.0994
  4. Yang X, Yu Y, Xu J, et al. Clinical course and outcomes of critically ill patients with SARS-CoV-2 pneumonia in Wuhan, China: a single-centered, retrospective, observational study [published online ahead of print, 2020 Feb 24] [published correction appears in Lancet Respir Med. 2020 Apr;8(4):e26]. Lancet Respir Med. 2020. doi:10.1016/S2213-2600(20)30079-5
  5. Zhou F, Yu T, Du R, et al. Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study [published correction appears in Lancet. 2020 Mar 28;395(10229):1038] Lancet. 2020;395(10229):1054-1062. doi:10.1016/S0140-6736(20)30566-3
  6. Wu Z, McGoogan JM. Characteristics of and important lessons from the coronavirus disease 2019 (COVID-19) outbreak in China: summary of a report of 72314 cases from the Chinese Center for Disease Control and Prevention [published online ahead of print, 2020 Feb 24]. JAMA. 2020;10.1001/jama.2020.2648. doi:10.1001/jama.2020.2648
  7. CDC COVID-19 Response Team. Severe outcomes among patients with coronavirus disease 2019 (COVID-19) — United States, February 12-March 16, 2020. MMWR Morb Mortal Wkly Rep. 2020 Mar 27;69(12);343-346. doi:10.15585/mmwr.mm6912e2
  8. Arentz M, Yim E, Klaff L, et al. Characteristics and outcomes of 21 critically ill patients with COVID-19 in Washington State [published online ahead of print, 2020 Mar 19]. JAMA. 2020;e204326. doi:10.1001/jama.2020.4326
  9. Livingston E, Bucher K. Coronavirus disease 2019 (COVID-19) in Italy [published online ahead of print, 2020 Mar 17]. JAMA. 2020;10.1001/jama.2020.4344. doi:10.1001/jama.2020.4344
  10. Novel Coronavirus Pneumonia Emergency Response Epidemiology Team. [The epidemiological characteristics of an outbreak of 2019 novel coronavirus diseases (COVID-19) in China]. Zhonghua Liu Xing Bing Xue Za Zhi. 2020 Feb 17;41(2):145-151. doi:10.3760/cma.j.issn.0254-6450.2020.02.003
  11. CDC COVID-19 Response Team. Preliminary estimates of the prevalence of selected underlying health conditions among patients with coronavirus disease 2019 — United States, February 12-March 28, 2020. MMWR Morb Mortal Wkly Rep. 2020 Apr 3;69(13);382–386. doi:10.15585/mmwr.mm6913e2
  12. Dong Y, Mo X, Hu Y, et al. Epidemiology of COVID-19 among children in China. Pediatrics. 2020 Apr;e20200702.
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