COVID-19: Is Asymptomatic Transmission Real?
We're going to start a new series today, one that aims to apply rigorous scientific analysis to the 2019 Coronavirus originating from Wuhan, China. The idea is to answer questions that have a real, day-to-day impact on the members of the Code 1 Family and beyond.
If anything has proven to be true with respect to the Coronavirus pandemic, it is that information seems flip 180 degrees from one day to the next. Up is down. Right is Left.
Question: Can COVID-19 be spread when someone is asymptomatic?
Answer: Yes. This does appear to be the case.
Background: One of the most frightening aspects of the COVID-19 pandemic is its penchant for asymptomatic transmission (AT). When reports first began emerging from China, we learned that this infection had an unusual penchant for asymptomatic spread and that it had an estimated 14 day incubation* period.
* An incubation period is defined as the time between infection and the first appearance of symptoms.
Although not common in our everyday experience, the idea of diseases spreading without the ill having symptoms is well known to the medical community. One famous American example was the case of Typhoid Mary, who spread typhoid fever on and off for several years despite repeated claims that she herself never had symptoms.
More famously, one of the most widespread and consequential pandemics in history was the Bubonic Plague (also known as "The Black Death"), which appears to have had an ongoing role in world history going back centuries.
Bubonic plague has a known incubation period from 2-10 days, which meant that it was usually able to spread far and wide from just a small locus (often traders).
COVID-19 (aka SARS-COV-2, aka Wuhan Virus, aka "Coronavirus" and its derivatives) presented something of an uncertainty.
There was an alarming initial report published in the New England Journal of Medicine on 30 January making the case for asymptomatic transmission (AT). The report was based on a case report of several patients in Germany that had reportedly been infected by an asymptomatic woman in early January.
This certainly added to the widespread concern about the ease of COVID-19's transmissibility. However, this study had a fatal flaw: the authors never contacted the so-called "Patient Zero" and assumed asymptomatic status based on the reports of those around her. When later contacted by the German equivalent to the CDC, the woman reported that she had in fact been quite symptomatic and strove to hide it from her colleagues. This forced the authors to retract the paper and sowed worldwide confusion about whether and individual had to be sick in order to spread the disease.
Furthermore, COVID-19 (SARS-COV-2) should have acted like it's close cousin, SARS-CoV-1 (aka the "SARS" outbreak of 2003). That virus has been well-studied and is known to have a shorter (5-day) incubation period and is most transmissible during viral replication in the lower respiratory tract (such as the lungs), which generally means a more vigorous symptomatic response. COVID-19 (SARS-CoV-2), by contrast, has a longer (14-day) incubation period and replicates in the upper respiratory tract, often without any symptoms at all.
Which brings us to the latest and likeliest best evidence for asymptomatic transmission: In a study published just a few days ago in the New England Journal of Medicine, the authors describe AT in a skilled nursing facility (SNF) in Washington State from a symptomatic worker.
In a nutshell, the worker became symptomatically positive for COVID-19 on 1 Mar 2020. Approximately 12 and 18 days later the residents of the SNF were tested for virus using nasal swabs:
76 SNF residents were tested.
Each resident was asked to recall whether they had experienced any of the signs or symptoms associated with COVID-19, including: Fever, cough, shortness of breath.
48/76 (63%) had positive tests.
27/48 (56% of positives) reported being completely asymptomatic at the time of testing (though many later became symptomatic).
There are some caveats which apply to this study:
It took place in one enclosed and high contact facility
More than half of the residents tested had some sort of cognitive impairment (though I will note that as a clinician, in the acute phase of an illness patients are still able to somewhat articulate discomfort, pain, or other symptoms).
What does this mean for the future? Perhaps improved and more widespread testing in asymptomatic populations most at risk of infection, such as nursing homes, hospitalized patients, prisons, etc.
Thanks for reading and I welcome your feedback.