A Novel Coronavirus – Determining Its Spread and Severity
Amesh A. Adalja, MD, FACP, FACEP, FIDSA | January 22, 2020
The last several days of infectious disease headlines have been focused on a mysterious outbreak in Wuhan, China, that has many concerning characteristics. This cluster of pneumonia cases—9 of which have been fatal, with others apparently severe—involves individuals who have contracted a novel coronavirus. The initial cases involved those that reportedly had exposure to a seafood market that housed many types of animals. The initial evaluation of these patients did not reveal any of the likely pathogens, and therefore these patients were determined to have an unknown viral pneumonia, which has since been determined to be caused by a novel coronavirus.
These initial cases, which numbered in the 40s, rapidly increased to several hundred additional cases, uncovered by wider surveillance of pneumonia cases, some of which did not have contact with the market initially viewed as the epicenter. Subsequently, cases were uncovered in Thailand, Japan, South Korea, Taiwan, and the United States, as well as in other cities in China. The cases diagnosed in other countries were in people who had traveled to Wuhan. Healthcare workers also have been infected. These events have led to justified precautions in many countries that are on alert for importation of cases. The US Centers for Disease Control and Prevention (CDC) has issued health alerts and is screening for cases at 3 airports in the US (JFK, San Francisco, and LAX) with 2 more to be added (Chicago O’Hare and Atlanta). The World Health Organization (WHO) will soon convene to determine whether a public health emergency exists.
Comparisons to SARS
Even before the identification of a novel coronavirus as the culprit virus, this outbreak had been compared by journalists and infectious disease experts to the 2003 outbreak of severe acute respiratory syndrome (SARS), a disease caused by a novel coronavirus with animal origins that also occurred in China around this time of the year. In that incident, more than 8,000 cases and almost 800 deaths occurred as the virus swept the globe and created international disruption. Another novel coronavirus that shares a predilection for severe disease with SARS, Middle East respiratory syndrome (MERS), has also been invoked.
It is important to note that the current novel infection is, thus far, unlike SARS (and MERS) in at least 2 very important respects. First, the rate of fatalities currently appears to be much lower. SARS had about a 10% fatality rate, while MERS fatality rates reach around 35%. There is also some question as to the degree of human-to-human transmission that is possible with this virus. It is increasingly becoming clear, however, that this does not appear to be a single source animal exposure outbreak, and some human-to-human transmission has occurred. But to what degree and with what sustainability human-to-human transmissibility is possible remains to be determined.
Coronaviruses Are a Major Cause of the Common Cold but Have Pandemic Potential
Post-SARS, it has been common to associate coronaviruses almost exclusively with severe disease. However, it is crucial to remember that these viruses, which resemble a crown when viewed with an electron microscope, are much more likely to cause uncomplicated upper respiratory infections, such as the common cold. Before SARS, the first human coronaviruses described were thought of as one of the many common cold viruses, without much concern for any more severe infections in people with a normally functioning immune system. SARS drastically changed that misperception and has since placed coronaviruses, for good reason, near the top of pandemic threats.
Those reasons include the fact that coronaviruses are spread via the respiratory route, have shown their ability to cause severe disease, have many strains circulating in animal reservoirs such as bats, and have no effective vaccine or antiviral available.
This reconceptualization of the risk of coronaviruses has since led to the discovery of novel coronaviruses. In addition to MERS, 2 other human coronaviruses have been discovered: NL63 and HKU1. In the case of HKU1, it was found that this virus had been circulating clandestinely even before SARS and was capable of causing critical illness.
Key Questions: Spectrum of Illness, Animal Reservoir, Family Tree
The concern regarding this outbreak will remain justified until some key questions are answered.
- What is the severity of illness?
Most respiratory infections have a spectrum of illness; some have very minor symptoms, while others can have more serious complications, such as pneumonia or respiratory failure. Understanding what the usual course of infection is will be crucial. Is this novel coronavirus syndrome more akin to the usual coronavirus infection, or is it more like MERS and SARS?
- Do those who have died or are critically ill have other medical conditions, such as diabetes, emphysema, or cardiovascular disease, that explain the severity of illness? How is critical illness being defined?
- Which animal in the live market is the likely origin for the virus, and how is the virus acquired?
- Has this virus been circulating in the population for some time? What is the baseline serological positivity?