The Epidemiology of SARS-CoV-2

By April 17, 2020Uncategorized

The recent outbreak of coronavirus (COVID-19) has turned into a public health emergency of international concern. With no antiviral drugs or vaccines, and the presence of carriers without obvious symptoms, traditional public health intervention measures are now significantly less effective (1). The pandemic began in late December 2019, when a cluster of patients was admitted to hospitals with an initial diagnosis of pneumonia of an unknown etiology. These patients were epidemiologically linked to a seafood and wet animal wholesale market in Wuhan, Hubei Province, China (2). The timeline of the initial outbreak can be divided into three phases. The local outbreak by exposure in the aforementioned food wholesale market marks the first phase. The second phase started on January 13 and was marked by rapid expansion and spread of the virus within hospitals (nosocomial infection) and by family transmission (close-contact transmission). In this phase, the epidemic spread from Wuhan to other areas. The first case outside of China was reported in Thailand on January 13, caused by a Wuhan resident travelling to this country. Already by January 23, 29 provinces within China, plus six foreign countries, had reported a total of 846 confirmed cases, an approximately 20-fold increase from the first phase. Meanwhile, Wuhan implemented a ‘lock-down’ (i.e., shutting down all movement within and out of the city). Unfortunately, this period coincided with the traditional mass movement of people, a form of ‘home-coming’, before Chinese New Year and thus more than 5 million people had already left Wuhan (1). The third phase started on January 26, and four days later the outbreak was designated a Public Health Emergency of International Concern by WHO. By February 12, newly confirmed cases in China jumped to 14,840. By that time, 25 countries had reported over 60,000 infections (1). Of note, February 3 seems to be a tipping point of the epidemic in China, from which time the daily number of confirmed cases outside Hubei began to decline. Whether it reflects the success of the ‘Wuhan lock-down’ and other public health measures, or virus transmission reduced for other reasons, remains unclear (1). As of March 31, more than 800,000 confirmed cases of COVID-19 have been reported. Although the outbreak began in China, the US, Italy, and Spain all now have more confirmed cases. In particular, the US has nearly 180,000 cases and over 3,400 deaths (compared to ~80,000 cases and ~3,300 deaths in China) (3).

COVID-19 has been found to have higher levels of transmissibility and pandemic risk than the SARS-CoV, as the effective reproductive number (R) of COVID-19 (2.9) is estimated to be higher than the reported R of SARS (1.77) at this early stage (4). However, it is worth noting that R estimates may vary upon numerous biological, socio-behavioral, and environmental factors, and must be interpreted with caution (1). Person-to-person spread of COVID-19 is thought to occur mainly via respiratory droplets, resembling the spread of influenza. With droplet transmission, virus released in the respiratory secretions when a person with infection coughs, sneezes, or talks can infect another person if it makes direct contact with the mucous membranes; infection can also occur if a person touches an infected surface and then touches his or her eyes, nose, or mouth. Droplets typically do not travel more than six feet (about two meters) and do not linger in the air. However, given the current uncertainty regarding transmission mechanisms, airborne precautions are recommended routinely in some countries and in the setting of certain high-risk procedures in others (4).

The interval during which an individual with COVID-19 is infectious is also uncertain. Most data informing this issue are from studies evaluating viral RNA detection from respiratory and other specimens. However, detection of viral RNA does not necessarily indicate the presence of infectious virus. Viral RNA levels appear to be higher soon after symptom onset compared with later in the illness, which raises the possibility that transmission might be more likely in the earlier stage of infection, but additional data is needed to confirm this hypothesis. The duration of viral shedding is also variable; there appears to be a wide range, which may depend on severity of illness. In one study of 137 patients who survived COVID-19, the median duration of viral RNA shedding from oropharyngeal specimens was 20 days (range of 8 to 37 days) (4). Transmission of SARS-CoV-2 from asymptomatic individuals (or individuals within the incubation period) has also been described. However, the extent to which this occurs remains unknown. Large-scale serologic screening may be able to provide a better sense of the scope of asymptomatic infections and inform epidemiologic analysis; several serologic tests for SARS-CoV-2 are under development to help provide a better sense of the scope of asymptomatic infections and inform epidemiologic analysis (4).

Extensive measures to reduce person-to-person transmission of COVID-19 are required to control the current outbreak. Special attention and efforts to protect or reduce transmission should be applied in susceptible populations including children, health care providers, and elderly people (2). The early death cases of COVID-19 outbreak occurred primarily in elderly people, possibly due to a weak immune system that permits faster progression of viral infection. Public services and facilities should provide decontaminating reagents for cleaning hands on a routine basis. Physical contact with wet and contaminated objects should be considered in dealing with the virus, especially agents such as fecal and urine samples that can potentially serve as an alternative route of transmission. China and other countries including the US have implemented major prevention and control measures including travel screenings to control further spread of the virus. Epidemiological changes in COVID-19 infection should be monitored, taking into account potential routes of transmission and subclinical infections, in addition to the adaptation, evolution, and virus spread among humans and possible intermediate animals and reservoirs (2)


  1. Sun J, He W, Wang L, et al. COVID-19: epidemiology, evolution, and cross-disciplinary perspectives. Trends Mol Med. 2020. Epub ahead of print.
  2. Rothan HA, Byrareddy SN: The epidemiology and pathogenesis of coronavirus disease (COVID-19) outbreak. J Autoimmun. 2020:102433-102433. 10.1016/j.jaut.2020.102433
  3. COVID-19 CORONAVIRUS PANDEMIC. (n.d.). Retrieved March 31, 2020, from
  4. McIntosh, K. (2020, March 25). Coronavirus disease 2019 (COVID-19). Retrieved March 31, 2020, from