COVID-19 Background and Insights: Epidemiology

What do we know about how SARS-CoV-2 has spread globally?

Ashly Pavlovsky, PhD and Zachary Moore, PhD


The Origin of the Pandemic

Throughout December 2019, several patients in Wuhan, Hubei Province, China presented at local hospitals with a persistent cough and pneumonia caused by an unknown source. Many of these patients were directly associated with or in close contact with someone directly associated with the Huanan Seafood Market, a live animal market in that city. Local hospitals were able to identify patients using established pneumonia surveillance techniques which were then modified to identify cases of the novel disease.1,2

On December 31st, a designated hospital was established for patients who were being diagnosed with apparent pneumonia, and a task force was assembled to conduct epidemiologic and etiologic studies of the emergent disease; members included  local and national representatives of the Chinese Center for Disease Control and Prevention (China CDC) as well as the National Health Commission.1,3,4 Samples of the virus were collected from patients’ bronchoalveolar-lavage fluid, and viral genomes were sequenced by the National Institute of Viral Disease Control and Prevention (IVDC).4,5 In early January 2020, this genomic analysis confirmed that patients had been infected with a novel coronavirus of unknown origin, designated as 2019-nCoV (later renamed SARS-CoV-2) with the resultant disease designated COVID-19.4,5

The incidence of COVID-19 continued to rise in China throughout January, and by the end of the month, almost 10,000 cumulative cases had been confirmed in more than 30 provinces.6 Several other countries also reported their first confirmed cases in January including Japan, Singapore, South Korea, United Arab Emirates, and the United States.7,8 By March 11, the World Health Organization (WHO) had declared the outbreak a pandemic with more than 118,000 cases worldwide occurring in 114 countries.9 As of May, there are now outbreaks in more than 185 countries and regions totaling more than 4.4 million confirmed cases and more than 300,000 observed deaths attributed to COVID-19.7,8 The United States has experienced the largest outbreak thus far with more than 1.4 million confirmed cases and more the 80,000 observed deaths. 7,8 Spain and Italy have also experienced large early outbreaks with more than 200,000 confirmed cases each.7,8

The observed mortality rate (percentage of deaths by confirmed cases) for COVID-19 varies amongst countries and overtime but the global average is about 7%.7,8 Some countries have experienced much higher fatality rates than the global average such as Belgium, Italy, the United Kingdom, and France (all >13%)*, as well as the Netherlands and Spain (>10%).7 While other countries have experienced lower death rates such as Germany (4.5%), the United States (6.0%), and China (5.5%).7,8 These differences may be due to variations in testing rates, demographics, and healthcare infrastructure amongst countries. The overall observed mortality rate is lower than the observed mortality rates for SARS-CoV-1 (9.6%) and MERS (35%); however, the cumulative incidence and deaths have been far greater for SARS-CoV-2.10,11 Current forecasts predict that the cumulative COVID-19-related deaths in the United States will continue to rise for several weeks, but the rate of new deaths will stabilize or even reduce if existing social distancing measures continue.12

How the Virus Infects People

Although SARS-CoV-2 likely originated through zoonotic transfer from a live animal at the Huanan Seafood Market, it is clear that human-to-human infection is its main mode of transmission. Infected individuals can transmit SARS-CoV-2 by releasing viral-containing respiratory droplets into the air when they cough or sneeze, which can then be inhaled by individuals who are in close proximity or land on nearby surfaces. Some virus-containing droplets can travel through the air to be inhaled by others potentially meters away.13-16 If the inhaled viral load is large enough, a person may become infected. In a laboratory setting, SARS-CoV-2 viral particles were found to remain viable in aerosol droplets for at least three hours after dispersal.17

In addition to respiratory transmission, contaminated surfaces can be a source of infection if an individual were to touch their eyes, nose, or mouth after contact with the contaminated surface.14 Viable SARS-CoV-2 particles have been detected up to 72 hours after application to plastic as well as stainless steel surfaces and up to 24 hours after application to cardboard surfaces, though the number of virus particles was reduced at these time points.17

Human-to-human transmission of SARS-CoV-2 can occur through contact with family members, spending time in a hospital setting with infected patients, or close contact with infected members of the community.13,18-21 Individuals can be contagious both during the asymptomatic incubation period (2–14 days after exposure) as well as when they begin exhibiting symptoms of the disease.19-22 In addition, some infected individuals never display symptoms, or only minor symptoms, making it challenging to determine who and when an individual is contagious.18,19,22

The reproduction number (R0) is a calculated value that represents the average number of new infections that a contagious individual will cause in a naïve population; thus, the R0 of a virus corresponds to its transmissibility. If the value of R0 is less than 1, the disease will likely decline and die out; if the value is greater than 1, the disease will infect an increasing number of people, potentially leading to a pandemic. For context, the seasonal influenza ranges between 0.9 – 2.1, with an average value of 1.3, and the 1918 – 1919 H1N1 “Spanish Flu” ranged between 1.4 – 2.8.23,24 Preliminary studies from China have estimated the R0 of SARS-CoV-2 to range from 1.4 – 6.5.25 The broad range of estimates is likely due to the different methodologies used to calculate these values as well as to the relatively short time since the onset of the novel virus outbreak. More recent studies based on the SARS-CoV-2 outbreak on the Diamond Princess Cruise ship and the outbreak in South Korea have narrowed the estimate to 2.0 – 4.0, which is similar to SARS-CoV-1.26,27

Currently, the incidence of COVID-19 is slowing or has been effectively contained in some of the earliest-affected countries, such as China, Switzerland, and Germany. Other countries, such as the United States, the United Kingdom, and Canada, are beginning to show indications of reduced transmission of SARS-CoV-2. Still other countries, such as Russia, Brazil, and India, may yet see more rapid transmission of the virus. It will likely take many months for this phase of the pandemic to resolve globally, but as more epidemiological data can be collected, the scientific community will be able to make better recommendations for future management and mititgation of this disease.


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Zachary Moore

Author Zachary Moore

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