At a glance
Clinicians and public health professionals can find information and evidence on this page to evaluate patients infected with SARS-CoV-2, the virus that causes COVID-19.
Key points
For evidence-based treatment recommendations, visit the Infectious Diseases Society of America (IDSA) COVID-19 Treatment and Management Guidelines and the American College of Physician (ACP) Clinical Guidelines and Recommendations on COVID-19. Also, get information on Variants of the Virus and COVID-19 Vaccines.
About COVID-19
- COVID-19 can vary from asymptomatic infection to critical illness. Symptoms and severity can change during illness.
- Infected people can transmit SARS-CoV-2, the virus that causes COVID-19, before symptom onset.
Testing
- When testing for SARS-CoV-2 infection, CDC recommends a viral test, either molecular (nucleic acid amplification test [NAAT], such as polymerase chain reaction [PCR]) or antigen tests that detect SARS-CoV-2 or its components.
- Do not use a serologic test that detects antibodies.
- A chest radiograph or computed tomography (CT) images alone are not recommended to diagnose COVID-19, but might be helpful in assessing and managing patients.
- Do not use a serologic test that detects antibodies.
- Early testing is recommended for all people who experience COVID-19 symptoms, particularly those at risk for severe illness:
- Age is the strongest risk factor for severe COVID-19.
- The number of underlying medical conditions also increases risk for severe COVID-19
- Age is the strongest risk factor for severe COVID-19.
Prevention
- Staying up to date on COVID-19 vaccines is the best way to prevent future SARS-CoV-2 infections, hospitalizations, long-term sequelae, and death.
Incubation period
Incubation periods may differ by SARS-CoV-2 variant. Meta-analyses of studies published in 2020 identified a pooled mean incubation period of 6.5 days from exposure to symptom onset. 1A study conducted during high levels of Delta variant transmission reported a period of 4.3 days, 2and during high levels of Omicron variant transmission, a median incubation period of 3–4 days. 34
Presentation
Asymptomatic and Pre-symptomatic Presentation
SARS-CoV-2 infection may not elicit symptoms in some people (asymptomatic) and may elicit symptoms after a positive test (pre-symptomatic presentation). 56It is unclear what percentage of people who initially appear asymptomatic progress to clinical disease. Children are more likely to remain asymptomatic than adults. People may have abnormalities on chest imaging consistent with COVID-19 before symptom onset or a positive COVID-19 test. 7
Symptomatic Presentation
Symptoms can be difficult to differentiate from, and can overlap with, other viral respiratory illnesses such as influenza (flu) and respiratory syncytial virus (RSV). COVID-19 can vary from asymptomatic infection to critical illness; symptoms and severity can change during the illness. Because symptoms may progress quickly, close follow-up is needed, especially for:
- Older adults
- People with disabilities
- People with immunocompromising conditions
- People with certain underlying medical conditions
The Infectious Diseases Society of America (IDSA) COVID-19 Treatment guidelines define the following groups to facilitate appropriate treatment:
- Mild to Moderate Illness: Individuals who have any of the various signs and symptoms of COVID-19 and do not need supplemental oxygen who have an oxygen saturation (SpO2) ≥94% on room air at sea level.
- Severe Illness: Individuals who have SpO2 <94% on room air at sea level or needing supplemental oxygen.
- Critical Illness: Individuals who have respiratory failure who are subcategorized as:
- Needing high-flow oxygen or non-invasive ventilation
- Needing mechanical ventilation and extracorporeal membrane oxygenation (ECMO)
- Needing high-flow oxygen or non-invasive ventilation
To learn more about COVID-19 treatment, please refer to the ISDA Guidelines on the Treatment and Management of Patients with COVID-19 and the ACP Clinical Guidelines and Recommendations on COVID-19.
Radiographic Considerations and Findings
Patients with severe COVID-19 may demonstrate bilateral air-space consolidation on chest radiographs.8 Chest computed tomography (CT) images may demonstrate bilateral, peripheral ground glass opacities and consolidation. 910Less common CT findings can include intra- or interlobular septal thickening with ground glass opacities (hazy opacity) or focal and rounded areas of ground glass opacity surrounded by a ring or arc of denser consolidation (reverse halo sign). 9
Multiple studies have reported abnormalities on CT or chest radiograph in people who are asymptomatic, pre-symptomatic, or before RT-PCR detects SARS-CoV-2 RNA in nasopharyngeal specimens.10
Symptoms
Common COVID-19 symptoms 11include:
- Fever
- Cough
- Shortness of breath
- Fatigue
- Headache
- Myalgia
Some people have gastrointestinal symptoms such as:
- Nausea
- Vomiting
- Diarrhea
Gastrointestinal symptoms can occur prior to having fever or lower respiratory tract signs and symptoms. 12Loss of smell and taste can occur, although these symptoms are less common since Omicron began circulating. 13 People can experience SARS-CoV-2 infection (asymptomatic or symptomatic), even if they are up to date with COVID-19 vaccines or were previously infected. 14
Several studies report ocular symptoms 15associated with SARS-CoV-2 infection, including:
- Redness
- Tearing
- Dry eye or foreign body sensation
- Discharge or increased secreations
- Itching or pain
Dermatologic manifestations have also been associated with COVID-19 and may be associated with increased severity. 16The timing of skin manifestations in relation to other COVID-19 symptoms and signs is variable. 17Images of cutaneous findings in COVID-19 are available from the American Academy of Dermatology
Transmission
People infected with SARS-CoV-2 can transmit the virus if they are vaccinated or unvaccinated, 1819asymptomatic, pre-symptomatic, 20or symptomatic. Peak transmissibility occurs from prior to symptom onset to a few days after, but most people can shed virus up to 10 days following infection. 21
Clinicians should consider encouraging all patients to follow CDC's Respiratory Virus Guidance to help lower health risks caused by SARS-CoV-2 and other respiratory viruses. Clinicians should also recommend that patients who are infected with SARS-CoV-2 follow CDC guidance for preventing spread of respiratory viruses when they are infected.
Co-circulation of influenza, RSV, and SARS-CoV-2
Clinicians can find useful resources to help in their decision making and actions when influenza, respiratory syncytial virus (RSV), and SARS-CoV-2 are co-circulating.
- Information for Clinicians on Influenza Virus Testing: Assists clinicians with influenza testing and treatment of patients with acute respiratory illness symptoms.
- Influenza Antiviral Medications: Information on antiviral treatment of influenza.
- IDSA COVID-19 Guidelines: Information on the treatment and management of patients with COVID-19
- RSV: For Healthcare Providers: Guidance on testing, vaccines for adults 60 years and older, and prophylaxis for infants and young children.
- Alene M, Yismaw L, Assemie MA, Ketema DB, Gietaneh W, Birhan TY. Serial interval and incubation period of COVID-19: a systematic review and meta-analysis. BMC Infect Dis. Mar 11 2021;21(1):257. doi:10.1186/s12879-021-05950-x
- Grant R, Charmet T, Schaeffer L, et al. Impact of SARS-CoV-2 Delta variant on incubation, transmission settings and vaccine effectiveness: Results from a nationwide case-control study in France. The Lancet Regional Health - Europe. 2022/02/01/ 2022;13:100278. doi:10.1016/j.lanepe.2021.100278
- Song JS, Lee J, Kim M, et al. Serial Intervals and Household Transmission of SARS-CoV-2 Omicron Variant, South Korea, 2021. Emerg Infect Dis. Mar 2022;28(3):756-759. doi:10.3201/eid2803.212607
- Jansen L, Tegomoh B, Lange K, et al. Investigation of a SARS-CoV-2 B.1.1.529 (Omicron) Variant Cluster - Nebraska, November-December 2021. MMWR Morb Mortal Wkly Rep. Dec 31 2021;70(5152):1782-1784. doi:10.15585/mmwr.mm705152e3
- Kimball A, Hatfield KM, Arons M, et al. Asymptomatic and Presymptomatic SARS-CoV-2 Infections in Residents of a Long-Term Care Skilled Nursing Facility - King County, Washington, March 2020. MMWR Morb Mortal Wkly Rep. Apr 3 2020;69(13):377-381. doi:10.15585/mmwr.mm6913e1
- Roxby AC, Greninger AL, Hatfield KM, et al. Detection of SARS-CoV-2 Among Residents and Staff Members of an Independent and Assisted Living Community for Older Adults - Seattle, Washington, 2020. MMWR Morb Mortal Wkly Rep. Apr 10 2020;69(14):416-418. doi:10.15585/mmwr.mm6914e2
- Kronbichler A, Kresse D, Yoon S, Lee KH, Effenberger M, Shin JI. Asymptomatic patients as a source of COVID-19 infections: A systematic review and meta-analysis. International Journal of Infectious Diseases. 2020/09/01/ 2020;98:180-186. doi:10.1016/j.ijid.2020.06.052
- Sadiq Z, Rana S, Mahfoud Z, Raoof A. Systematic review and meta-analysis of chest radiograph (CXR) findings in COVID-19. Clinical Imaging. 2021/12/01/ 2021;80:229-238. doi:10.1016/j.clinimag.2021.06.039
- Kanne JP, Bai H, Bernheim A, et al. COVID-19 Imaging: What We Know Now and What Remains Unknown. Radiology. Jun 2021;299(3):E262-e279. doi:10.1148/radiol.2021204522
- Doerschug KC, Schmidt GA. Pulmonary Aspects of COVID-19. Annu Rev Med. Jan 27 2022;73:81-93. doi:10.1146/annurev-med-042220-014817
- Stokes EK, Zambrano LD, Anderson KN, et al. Coronavirus Disease 2019 Case Surveillance - United States, January 22-May 30, 2020. MMWR Morb Mortal Wkly Rep. Jun 19 2020;69(24):759-765. doi:10.15585/mmwr.mm6924e2
- Yang L, Tu L. Implications of gastrointestinal manifestations of COVID-19. The Lancet Gastroenterology & Hepatology. July2020, 629-630; doi:10.1016/S2468-1253(20)30132-1
- Tenforde MW, Rose EB. Characteristics of Adult Outpatients and Inpatients with COVID-19 — 11 Academic Medical Centers, United States, March–May 2020. MMWR July 3, 2020.
- COVID-19 Vaccine Breakthrough Infections Reported to CDC — United States, January 1–April 30, 2021. MMWR Morb Mortal Wkly Rep 2021;70:792–793. DOI:10.15585/mmwr.mm7021e3Nasiri N, Sharifi H, Bazrafshan A, Noo
- Nasiri N, Sharifi H, Bazrafshan A, Noori A, Karamouzian M, Sharifi A. Ocular Manifestations of COVID-19: A Systematic Review and Meta-analysis. J Ophthalmic Vis Res. Jan-Mar 2021;16(1):103-112. doi:10.18502/jovr.v16i1.8256
- Tan SW, Tam YC, Oh CC. Skin manifestations of COVID-19: A worldwide review. JAAD International. March 2021, 119-133. doi:10.1016/j.jdin.2020.12.003[GW((1]
- Genovese G, Moltrasio C, Berti E, Giovanni Genovese, Marzano, AV. Skin Manifestations Associated with COVID-19: Current Knowledge and Future Perspectives. Dermatology. 2021, 237, 1-12. doi.org/10.1159/000512932
- Singanayagam A, Hakki S, et al. Community transmission and viral load kinetics of the SARS-CoV-2 delta (B.1.617.2) variant in vaccinated and unvaccinated individuals in the UK: a prospective, longitudinal, cohort study. The Lancet Infectious Diseases. Vol 22, Issue 2. February 2022, 183-195. doi.org/10.1016/S1473-3099(21)00648-4
- Eyre DW, Taylor D, Purver M, et al. Effect of Covid-19 Vaccination on Transmission of Alpha and Delta Variants. New England Journal of Medicine. 2022;386(8):744-756. doi:10.1056/NEJMoa2116597
- Thompson HA, Mousa A, Dighe A, et al. SARS-CoV-2 setting-specific transmission rates: a systematic review and meta-analysis. Clin Infect Dis. Feb 9 2021; doi:10.1093/cid/ciab100z
- Singanayagam A, Patel M, et al. Duration of infectiousness and correlation with RT-PCR cycle threshold values in cases of COVID-19, England, January to May 2020. Eurosurveillance. Vol. 25, Issue 32. August 2020.