Overview of COVID-19
Last Updated: October 19, 2021
The COVID-19 pandemic has exploded since cases were first reported in China in December 2019. As of October 18, 2021, more than 240 million cases of COVID-19—caused by SARS-CoV-2 infection—have been reported globally, including more than 4.9 million deaths.1
Individuals of all ages are at risk for SARS-CoV-2 infection and severe disease. However, the probability of serious COVID-19 disease is higher in people aged ≥60 years, those living in a nursing home or long-term care facility, and those with chronic medical conditions. In an analysis of more than 1.3 million laboratory-confirmed cases of COVID-19 that were reported in the United States between January and May 2020, 14% of patients required hospitalization, 2% were admitted to the intensive care unit, and 5% died.2 The percentage of patients who died was 12 times higher among those with reported medical conditions (19.5%) than among those without medical conditions (1.6%), and the percentage of those who were hospitalized was six times higher among those with reported medical conditions (45.4%) than among those without medical conditions (7.6%). The mortality rate was highest in those aged >70 years, regardless of the presence of chronic medical conditions. Among those with available data on health conditions, 32% had cardiovascular disease, 30% had diabetes, and 18% had chronic lung disease. Other conditions that may lead to a high risk for severe COVID-19 include cancer, kidney disease, liver disease (especially in patients with cirrhosis), obesity, sickle cell disease, and other immunocompromising conditions. Transplant recipients and pregnant people are also at a higher risk of severe COVID-19.3-10
Data from the United States suggest that racial and ethnic minorities experience higher rates of COVID-19 and subsequent hospitalization and death.11-15 However, surveillance data that include race and ethnicity are not available for most reported cases of COVID-19 in the United States.4,16 Factors that contribute to the increased burden of COVID-19 in these populations may include over-representation in work environments that confer higher risks of exposure to COVID-19, economic inequality (which limits people’s ability to protect themselves against COVID-19 exposure), neighborhood disadvantage,17 and a lack of access to health care.16 Structural inequalities in society contribute to health disparities for racial and ethnic minority groups, including higher rates of comorbid conditions (e.g., cardiac disease, diabetes, hypertension, obesity, pulmonary diseases), which further increases the risk of developing severe COVID-19.15
Like other RNA viruses, SARS-CoV-2 is constantly evolving through random mutations. New mutations can potentially increase or decrease infectiousness and virulence. In addition, mutations can increase the virus’ ability to evade adaptive immune responses from past SARS-CoV-2 infection or vaccination. This may increase the risk of reinfection or decrease the efficacy of vaccines.18 There is already evidence that some SARS-CoV-2 variants have reduced susceptibility to plasma from people who were previously infected or immunized, as well as to certain monoclonal antibodies (mAbs) that are being considered for prevention and treatment.19
Since December 2020, several variants have been identified that have now been assigned Greek letter designations by the World Health Organization (WHO). SARS-CoV-2 variants are designated as variants of concern (VOC) if they display certain characteristics, such as increased transmissibility or virulence. In addition, vaccines and/or therapeutics may have decreased effectiveness against VOC, and the mutations found in these variants may interfere with diagnostic test targets. The designation variant of interest (VOI) is used for important variants that have not yet been fully characterized; however, the designations and definitions for these variants differ between organizations.20,21 In September 2021, the Centers for Disease Control and Prevention (CDC) added a new designation for variants: variants being monitored (VBM). This refers to variants for which the data indicate a potential or clear impact on approved or authorized medical countermeasures, or variants that have been associated with more severe disease or increased transmission rates; however, these variants are either no longer detected or are circulating at very low levels in the United States. As such, these variants do not pose a significant and imminent risk to public health in the United States.
The B.1.617.2 (Delta) variant, which was first identified in India and has been designated a VOC, is the dominant variant in the United States since the summer of 2021. The Delta variant is more infectious than other variants, leading to increased transmissibility.22 The B.1.1.7 (Alpha) variant that was first seen in the United Kingdom is more infectious and may be more virulent than earlier variants.23-25 The B.1.351 (Beta) variant that was originally identified in South Africa has spread to many other countries, including the United States. The P.1 (Gamma) variant was originally identified in Manaus, Brazil, and has also emerged in the United States. These variants, which were previously designated as VOC, are now classified as VBM. Other VBM in the United States include the B.1.427/B.1.429 (Epsilon) variants that were originally identified in California, the B.1.526 (Iota) variant that was originally identified in New York, and the B.1.617.1 (Kappa) variant that was first identified in India. For a detailed discussion on the susceptibility of certain VOC, VOI , and VBM to available anti-SARS-CoV-2 mAbs, please see Anti-SARS-CoV-2 Monoclonal Antibodies.
The data on the emergence, spread, and clinical relevance of these new variants is rapidly evolving; this is especially true for research on how variants might affect transmission rates, disease progression, vaccine development, and the efficacy of current therapeutics. Because the research on variants is moving quickly and the classification of the different variants may change over time, websites such as the CDC COVID Data Tracker, CoVariants.org, and WHO’s Tracking SARS-CoV-2 Variants provide regular updates on the data for SARS-CoV-2 variants. The COVID-19 Treatment Guidelines Panel reviews the emerging data on these variants, paying particular attention to research on the impacts of these variants on testing, prevention, and treatment.
The estimated incubation period for COVID-19 is up to 14 days from the time of exposure, with a median incubation period of 4 to 5 days.6,26,27 The spectrum of illness can range from asymptomatic infection to severe pneumonia with acute respiratory distress syndrome and death. Among 72,314 people with COVID-19 in China, 81% of cases were reported to be mild (defined in this study as no pneumonia or mild pneumonia), 14% were severe (defined as dyspnea, respiratory frequency ≥30 breaths/min, oxygen saturation [SpO2] ≤93%, a ratio of arterial partial pressure of oxygen to fraction of inspired oxygen [PaO2/FiO2] <300 mm Hg, and/or lung infiltrates >50% within 24 to 48 hours), and 5% were critical (defined as respiratory failure, septic shock, and/or multiorgan dysfunction or failure).28 In a report on more than 370,000 confirmed COVID-19 cases with reported symptoms in the United States, 70% of patients experienced fever, cough, or shortness of breath, 36% had muscle aches, and 34% reported headaches.2 Other reported symptoms have included, but are not limited to, diarrhea, dizziness, rhinorrhea, anosmia, dysgeusia, sore throat, abdominal pain, anorexia, and vomiting.
The abnormalities seen in chest X-rays of patients with COVID-19 vary, but bilateral multifocal opacities are the most common. The abnormalities seen in computed tomography of the chest also vary, but the most common are bilateral peripheral ground-glass opacities, with areas of consolidation developing later in the clinical course of COVID-19.29 Imaging may be normal early in infection and can be abnormal in the absence of symptoms.29
Common laboratory findings in patients with COVID-19 include leukopenia and lymphopenia. Other laboratory abnormalities have included elevated levels of aminotransferase, C-reactive protein, D-dimer, ferritin, and lactate dehydrogenase.
Although COVID-19 is primarily a pulmonary disease, emerging data suggest that it also leads to cardiac,30,31 dermatologic,32 hematologic,33 hepatic,34 neurologic,35,36 renal,37,38 and other complications. Thromboembolic events also occur in patients with COVID-19, with the highest risk occurring in critically ill patients.39
The long-term sequelae of COVID-19 survivors are currently unknown. Persistent symptoms after recovery from acute COVID-19 have been described (see Clinical Spectrum of SARS-CoV-2 Infection). Lastly, SARS-CoV-2 infection has been associated with a potentially severe inflammatory syndrome in children (multisystem inflammatory syndrome in children, or MIS-C).40,41 Please see Special Considerations in Children for more information.
- Johns Hopkins. COVID-19 Dashboard by the Center for Science and Engineering. 2021. Available at: https://coronavirus.jhu.edu/map.html. Accessed October 18, 2021.
- 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. 2020;69. Available at: https://www.cdc.gov/mmwr/volumes/69/wr/pdfs/mm6924e2-H.pdf.
- Cai Q, Chen F, Wang T, et al. Obesity and COVID-19 severity in a designated hospital in Shenzhen, China. Diabetes Care. 2020;43(7):1392-1398. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32409502.
- Centers for Disease Control and Prevention. Coronavirus disease 2019 (COVID-19): cases in U.S. 2020. Available at: https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/cases-in-us.html. Accessed November 25, 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;69(15):458-464. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32298251.
- Guan WJ, Ni ZY, Hu Y, et al. Clinical characteristics of coronavirus disease 2019 in China. N Engl J Med. 2020;382(18):1708-1720. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32109013.
- 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. JAMA Intern Med. 2020;180(7):934-943. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32167524.
- Palaiodimos L, Kokkinidis DG, Li W, et al. Severe obesity, increasing age and male sex are independently associated with worse in-hospital outcomes, and higher in-hospital mortality, in a cohort of patients with COVID-19 in the Bronx, New York. Metabolism. 2020;108:154262. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32422233.
- Zambrano LD, Ellington S, Strid P, et al. Update: characteristics of symptomatic women of reproductive age with laboratory-confirmed SARS-CoV-2 infection by pregnancy status - United States, January 22–October 3, 2020. MMWR Morb Mortal Wkly Rep. 2020;69(44):1641-1647. Available at: https://www.ncbi.nlm.nih.gov/pubmed/33151921.
- Centers for Disease Control and Prevention. COVID-19 (coronavirus disease): people with certain medical conditions. 2021. Available at: https://www.cdc.gov/coronavirus/2019-ncov/need-extra-precautions/people-with-medical-conditions.html. Accessed September 16, 2021.
- Azar KMJ, Shen Z, Romanelli RJ, et al. Disparities In outcomes among COVID-19 patients in a large health care system in California. Health Aff (Millwood). 2020;39(7):1253-1262. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32437224.
- Gold JAW, Wong KK, Szablewski CM, et al. Characteristics and clinical outcomes of adult patients hospitalized with COVID-19—Georgia, March 2020. MMWR Morb Mortal Wkly Rep. 2020;69(18):545-550. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32379729.
- Gross CP, Essien UR, Pasha S, Gross JR, Wang SY, Nunez-Smith M. Racial and ethnic disparities in population-level COVID-19 mortality. J Gen Intern Med. 2020;35(10):3097-3099. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32754782.
- Nayak A, Islam SJ, Mehta A, et al. Impact of social vulnerability on COVID-19 incidence and outcomes in the United States. medRxiv. 2020;Preprint. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32511437.
- Price-Haywood EG, Burton J, Fort D, Seoane L. Hospitalization and mortality among black patients and white patients with COVID-19. N Engl J Med. 2020;382(26):2534-2543. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32459916.
- Centers for Disease Control and Prevention. Health equity considerations and racial and ethnic minority groups. 2020. Available at: https://www.cdc.gov/coronavirus/2019-ncov/community/health-equity/race-ethnicity.html. Accessed November 24, 2020.
- Kind AJH, Buckingham WR. Making neighborhood-disadvantage metrics accessible—the neighborhood atlas. N Engl J Med. 2018;378(26):2456-2458. Available at: https://www.ncbi.nlm.nih.gov/pubmed/29949490.
- Walensky RP, Walke HT, Fauci AS. SARS-CoV-2 variants of concern in the United States-challenges and opportunities. JAMA. 2021;325(11):1037-1038. Available at: https://www.ncbi.nlm.nih.gov/pubmed/33595644.
- Wang P, Nair MS, Liu L, et al. Antibody resistance of SARS-CoV-2 variants B.1.351 and B.1.1.7. Nature. 2021;593(7857):130-135. Available at: https://www.ncbi.nlm.nih.gov/pubmed/33684923.
- World Health Organization. Tracking SARS-CoV-2 variants. 2021; https://www.who.int/en/activities/tracking-SARS-CoV-2-variants/. Accessed October 18, 2021.
- Centers for Disease Control and Prevention. SARS-CoV-2 variant classifications and definitions. 2021. Available at: https://www.cdc.gov/coronavirus/2019-ncov/cases-updates/variant-surveillance/variant-info.html. Accessed April 5, 2021.
- Centers for Disease Control and Prevention. Delta variant: what we know about the science. 2021. Available at: https://www.cdc.gov/coronavirus/2019-ncov/variants/delta-variant.html. Accessed September 16, 2021.
- Leung K, Shum MH, Leung GM, Lam TT, Wu JT. Early transmissibility assessment of the N501Y mutant strains of SARS-CoV-2 in the United Kingdom, October to November 2020. Euro Surveill. 2021;26(1). Available at: https://www.ncbi.nlm.nih.gov/pubmed/33413740.
- Davies NG, Barnard RC, Jarvis CI, et al. Report: continued spread of VOC 202012/01 in England. 2020. Available at: https://cmmid.github.io/topics/covid19/reports/uk-novel-variant/2020_12_31_Transmissibility_and_severity_of_VOC_202012_01_in_England_update_1.pdf.
- Murugan NA, Javali PS, Pandian CJ, Ali MA, Srivastava N, Jeyaraman J. Computational investigation of increased virulence and pathogenesis of SARS-CoV-2 lineage B.1.1.7. bioRxiv. 2021;Preprint. Available at: https://www.biorxiv.org/content/10.1101/2021.01.25.428190v1.
- Li Q, Guan X, Wu P, et al. Early transmission dynamics in Wuhan, China, of novel coronavirus-infected pneumonia. N Engl J Med. 2020;382(13):1199-1207. Available at: https://www.ncbi.nlm.nih.gov/pubmed/31995857.
- Lauer SA, Grantz KH, Bi Q, et al. The incubation period of coronavirus disease 2019 (COVID-19) from publicly reported confirmed cases: estimation and application. Ann Intern Med. 2020;172(9):577-582. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32150748.
- Wu Z, McGoogan JM. Characteristics of and important lessons from the coronavirus disease 2019 (COVID-19) outbreak in China: summary of a report of 72,314 cases from the Chinese Center for Disease Control and Prevention. JAMA. 2020;323(13):1239-1242. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32091533.
- Shi H, Han X, Jiang N, et al. Radiological findings from 81 patients with COVID-19 pneumonia in Wuhan, China: a descriptive study. Lancet Infect Dis. 2020;20(4):425-434. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32105637.
- Liu PP, Blet A, Smyth D, Li H. The science underlying COVID-19: implications for the cardiovascular system. Circulation. 2020;142(1):68-78. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32293910.
- Madjid M, Safavi-Naeini P, Solomon SD, Vardeny O. Potential effects of coronaviruses on the cardiovascular system: a review. JAMA Cardiol. 2020;5(7):831-840. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32219363.
- Sachdeva M, Gianotti R, Shah M, et al. Cutaneous manifestations of COVID-19: report of three cases and a review of literature. J Dermatol Sci. 2020;98(2):75-81. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32381430.
- Henry BM, de Oliveira MHS, Benoit S, Plebani M, Lippi G. Hematologic, biochemical and immune biomarker abnormalities associated with severe illness and mortality in coronavirus disease 2019 (COVID-19): a meta-analysis. Clin Chem Lab Med. 2020;58(7):1021-1028. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32286245.
- Agarwal A, Chen A, Ravindran N, To C, Thuluvath PJ. Gastrointestinal and liver manifestations of COVID-19. J Clin Exp Hepatol. 2020;10(3):263-265. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32405183.
- Whittaker A, Anson M, Harky A. Neurological manifestations of COVID-19: a systematic review and current update. Acta Neurol Scand. 2020;142(1):14-22. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32412088.
- Paniz-Mondolfi A, Bryce C, Grimes Z, et al. Central nervous system involvement by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). J Med Virol. 2020;92(7):699-702. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32314810.
- Pei G, Zhang Z, Peng J, et al. Renal involvement and early prognosis in patients with COVID-19 pneumonia. J Am Soc Nephrol. 2020;31(6):1157-1165. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32345702.
- Su H, Yang M, Wan C, et al. Renal histopathological analysis of 26 postmortem findings of patients with COVID-19 in China. Kidney Int. 2020;98(1):219-227. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32327202.
- Bikdeli B, Madhavan MV, Jimenez D, et al. COVID-19 and thrombotic or thromboembolic disease: implications for prevention, antithrombotic therapy, and follow-up: JACC state-of-the-art review. J Am Coll Cardiol. 2020;75(23):2950-2973. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32311448.
- Chiotos K, Bassiri H, Behrens EM, et al. Multisystem inflammatory syndrome in children during the coronavirus 2019 pandemic: a case series. J Pediatric Infect Dis Soc. 2020;9(3):393-398. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32463092.
- Belhadjer Z, Meot M, Bajolle F, et al. Acute heart failure in multisystem inflammatory syndrome in children in the context of global SARS-CoV-2 pandemic. Circulation. 2020;142(5):429-436. Available at: https://www.ncbi.nlm.nih.gov/pubmed/32418446.