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First published Feb. 13, 2020.
Last updated October 11, 2021, @ 5 pm PT.


The novel coronavirus, SARS-CoV-2, is the cause of COVID-19, a severe acute respiratory disease first detected in Wuhan, China. The spread of COVID-19 is a global health risk that has had a significant impact on the economy and the daily lives of people across the globe.

Signs & Symptoms

The Centers for Disease Control & Prevention (CDC) believes at this time that symptoms of COVID-19 may appear in as few as two days or as long as 14 after exposure. Individuals with symptoms including, fever, coughing, and shortness of breath should self-isolate and seek COIVD-19 diagnostic and medical advice.1,7 The clinical spectrum of COVID-19 ranges from mild, cold-like symptoms to severe pneumonia, respiratory failure, and in some cases, death.7

Anyone can be infected by the virus that causes COVID-19, but certain populations appear to be more vulnerable to severe infection: recent evidence suggests that older individuals and those with underlying medical conditions are more likely to experience complications or hospitalization.7
Spread of Infection

The WHO has designated the outbreak a global health emergency.3 As of October 11, there have been 238,012,352 confirmed COVID-19 cases (up 5,905,313 from September 27). Of the total cases, 144,111,887 individuals have recovered, and 4,854,787 have died (up 102,380 from September 27), of which 714,053 are from the U.S. (up 23,665 from September 27).

The case fatality ratio (CFR), measures the proportion of deaths among identified confirmed cases. The relative risk of mortality for different groups of people and countries helps guide resource allocation and policy decisions. The level of transmission in a population may be underestimated if individuals with mild or no symptoms are missing from the data or if under-served groups have decreased access to health care or testing.4

the spread of Infection: U.S. Status Report

COVID-19 cases have been reported in all 50 states, with more than 50,000 cases reported in almost every state. Despite these guidelines, COVID-19 cases are surging throughout the southern and western United States. Despite social distancing guidelines and orders to wear masks in public places to prevent community spread, COVID-19 cases are surging across the United States. Many states, particularly in the Midwest, are reporting record numbers of daily cases.

On Mar. 11, 2020, the WHO Director-General announced that COVID-19 can be characterized as a pandemic.   On Feb. 28, 2020, the WHO raised the Global Risk Assessment level for COVID-19 from high to very high.2   Person-to-person spread (or community spread) has occurred in China and now also appears to be happening in countries all over the world.7
Anatomy of SARS-CoV-2

From the beginning of the COVID-19 outbreak, the combination of many expedited studies has revealed an unprecedented amount of information for a new disease, including the causative virus (SARS-CoV-2), its genetic sequence, PCR assays, and serological analyses. Efforts are rapidly ongoing with the aim of quickly designing effective treatments, such as a vaccine or antiviral drug.

SARS-CoV-2 Mechanism of Action

Once in the body, SARS-CoV-2 recognizes and attaches to a transmembrane protein called ACE2 found mainly on cells that line our airways. The effectiveness of SARS-CoV-2 in triggering respiratory disease may be due to the evolved structure of the Spike (S) protein, which contains the receptor-binding domain for ACE2 and creates the virus’s recognizable ‘crown’ shape. After binding to an ACE2 homodimer, the virus enters the cell through a process called receptor-mediated endocytosis. Inside the cell, the virus releases its RNA and takes advantage of the host cell’s machinery, allowing it to reproduce, escape, and continue infecting human tissue.

The situation in Numbers:
How COVID-19 Compares to Other Outbreaks

As more cases emerge, medical professionals and researchers have been able to identify distinguishing traits of COVID-19. Here is a look at other notable viral diseases to show how COVID-19 compares.

Covid-19 Vaccines & variants:
What We Know & What We’re Still Learning

As more and more people are getting vaccinated, public health recommendations are being updated to reflect the new situation. As of May 13, 2021, the CDC’s official guidelines for the U.S. stated that fully vaccinated people no longer need to wear a mask or physically distance in most settings. These updates are a result of the high efficiency of the EUA COVID-19 vaccines and a growing body of evidence that fully vaccinated people are less likely to have an asymptomatic infection or transmit SARS-CoV-2 to others.23

What We Know

All EUA COVID-19 vaccines currently being distributed are highly effective in preventing COVID-19. In international, randomized, placebo-controlled studies, the Pfizer-BioNTech and Moderna COVID-19 Vaccines were 95% and 94.1% effective in preventing COVID-19 disease.24,25 The Janssen vaccine was 77% effective in preventing severe/critical COVID-19 occurring at least 14 days after vaccination and 85% effective in preventing severe/critical COVID-19 occurring at least 28 days after vaccination.26

What we’re still learning

A growing body of evidence suggests that fully vaccinated people are less likely to have an asymptomatic infection and potentially less likely to transmit SARS-CoV-2. In addition, preliminary studies show some protection against emerging variants of concern (VOC), including Alpha (originally identified in the United Kingdom) and Beta (originally identified in South Africa).27

Preventative measures such as mask use and social distancing continue to be important and are still recommended in healthcare and other high-risk settings, as well as during travel via plane, bus, train, and other public transportation. Individuals who are not yet fully vaccinated are still recommended to follow preventative measures.23

  1. Coronavirus Disease 2019 (COVID-19) – Symptoms. Centers for Disease Control and Prevention. Published April 27, 2020. Accessed April 28, 2020.
  2. World Health Organization (2020). Novel Coronavirus (2019-NCoV) Situation Reports.
  3. Chen N, Zhou M, Dong X, et al. Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study. The Lancet. 2020.
  4. “The Urgent Questions Scientists Are Asking About Coronavirus.” The New York Times, 10 Feb. 2020,
  5. Branswell H. The Coronavirus Questions That Scientists Are Racing to Answer. Scientific American. swer/. Published January 28, 2020.
  6. Zhao Y, Zhao Z, Wang Y, et al. Single-cell RNA expression profiling of ACE2, the putative receptor of Wuhan 2019-nCov. American Journal of Respiratory and Critical Care Medicine. 2020. doi:10.1164/rccm.202001-0179LE
  7. Centers for Disease Control and Prevention. Published January 10, 2020. Accessed March 13, 2020.
  8. Zhao S, Lin Q, Ran J, et al. Preliminary estimation of the basic reproduction number of novel coronavirus (2019-nCoV) in China, from 2019 to 2020: A data-driven analysis in the early phase of the International Journal of Infectious Diseases. 2020. doi:10.1016/j.ijid.2020.01.050
  9. “Cumulative Number of Reported Probable Cases of Severe Acute Respiratory Syndrome (SARS).” World Health Organization, World Health Organization, 23 July 2015,
  10. “Middle East respiratory syndrome.” World Health Organization, World Health Organization,
  11. Factsheet about Middle East respiratory syndrome coronavirus (MERS-CoV). European Centre for Disease Prevention and Control.  Published February 10, 2020. Accessed February 11, 2020.
  12. Types of Influenza Viruses. Centers for Disease Control and Prevention. Published November 18, Accessed February 11, 2020.
  13. “Weekly U.S. Influenza Surveillance Report.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 7 Feb. 2020,
  14. Biggerstaff M, Cauchemez S, Reed C, Gambhir M, Finelli L. Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: a systematic review of the literature. BMC Infectious Diseases. 2014;14(1). doi:10.1186/1471-2334-14-480
  15. Transmission of Avian Influenza A Viruses Between Animals and People. Centers for Disease Control and Prevention. Published February 10, 2015. Accessed February 11, 2020.
  16. Biggerstaff M, Cauchemez S, Reed C, Gambhir M, Finelli L. Estimates of the reproduction number for seasonal, pandemic, and zoonotic influenza: a systematic review of the literature. BMC Infect Dis. 2014;14(1):480. doi:10.1186/1471-2334-14-480
  17. Fraser C, Cummings DAT, Klinkenberg D, Burke DS, Ferguson NM. Influenza Transmission in Households During the 1918 Pandemic. Am J Epidemiol. 2011;174(5):505-514. doi:10.1093/aje/kwr122
  18. Morens DM, Fauci AS. The 1918 influenza pandemic: insights for the 21st century. J Infect Dis. 2007;195(7):1018-1028. doi:10.1086/511989
  19. Lessler J, Reich NG, Brookmeyer R, Perl TM, Nelson KE, Cummings DAT. Incubation periods of acute respiratory viral infections: a systematic review. Lancet Infect Dis. 2009;9(5):291-300. doi:10.1016/S1473-3099(09)70069-6
  20. Taubenberger JK, Baltimore D, Doherty PC, et al. Reconstruction of the 1918 Influenza Virus: Unexpected Rewards from the Past. mBio. 2012;3(5). doi:10.1128/mBio.00201-12
  21. 1918 Pandemic (H1N1 virus). Published March 20, 2019. Accessed March 16, 2020.
  22. History of 1918 flu pandemic. Published March 21, 2018. Accessed March 16, 2020.
  23. Centers for Disease Control and Prevention. Updated May 13, 2021. Accessed May 21, 2021.
  24. Centers for Disease Control and Prevention. Published December 11, 2020. Accessed May 21, 2021.
  25. Centers for Disease Control and Prevention. Published December 18, 2020. Accessed May 21, 2021.
  26. Centers for Disease Control and Prevention. Published May 13, 2021. Accessed February 27, 2021.
  27. Centers for Disease Control and Prevention. Updated April 2, 2021. Accessed May 21, 2021.

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