I got an interesting question about asymptomatic COVID-19 this week that seemed relatively straightforward, but is actually much more complicated. Isn’t it a good thing that a lot of people with COVID-19 have mild illness or no illness at all? Here’s why the answer to that question is both yes and no.

With any disease, fewer symptoms and less suffering is absolutely, unequivocally a good thing. Even with COVID-19, we’d all prefer to have a mild illness over severe acute respiratory distress syndrome (ARDS), the most common serious complication of COVID-19. Patients with ARDS require mechanical ventilation, which we know are in short supply, and are at a significant risk of dying. Additionally, the more asymptomatic COVID-19 cases that haven’t yet been detected, the lower the mortality rate will ultimately turn out to be.

So in that sense, yes of course it’s a good thing that a lot of people have mild illness. That does not, of course, take from the seriousness of this pandemic nor the travesty that more than 60,000 people have lost their lives so far. However, it appears that people with mild disease or no illness at all are contributing to the spread of COVID-19, which complicates the answer to this question considerably. Here’s why:

Respiratory pathogens spread person-to-person when an infected person coughs or sneezes. The physical process of coughing dislodges the virus from the upper respiratory tract and expels droplets containing virus feet into the air. Sneezing is a similar action that loosens and ejects virus from the nasal cavity. If another individual is within the strike zone (estimated to be about six feet), they can breathe in the virus thus causing a new infection. If no people are within the strike zone, droplets can fall on surfaces where viruses can live for hours or days awaiting someone to touch and transport the virus from their hands to their face. These dynamics of pathogen spread underlie the recommendations for physical distancing, respiratory hygiene, disinfecting commonly touched surfaces, and hand washing.

Some pathogens including MERS and SARS original, cousins of the virus that causes COVID-19, are most commonly spread when a person is at their sickest. Sick people are easy to identify and isolate, so it becomes rather straightforward to stop the virus transmission cycle. What is unique about the SARS-CoV-2 virus is that it appears to be able to spread by asymptomatic individuals or during the pre-symptomatic phase before someone knows they have it. This becomes dangerous because healthy people are more likely to go out in public and spread the virus to their community without ever knowing it.

Asymptomatic transmission of COVID-19 was dismissed early on because spread of other respiratory pathogens requires coughing and sneezing. Without symptoms, how could the virus expel itself from the respiratory tract of one person to infect another? Well, we actually expel droplets while talking and even just breathing too, it’s just that most of the time these droplets don’t carry a high volume of viruses when we’re healthy. While we don’t know the amount of SARS-CoV-2 required to initiate a new infection, spread among people without symptoms suggests that enough virus is being transmitted this way.

So how many people with SARS-CoV-2 infection don’t develop symptomatic COVID-19 and thus contribute to asymptomatic spread? This is hard to quantify at this stage in the pandemic because healthy people aren’t generally getting tested for the virus. We have a few hints where screening (testing healthy people) was conducted. For example, everyone on the Diamond Princess cruise ship got a COVID-19 test irrespective of symptoms. A modeling study was carried out that estimated the percentage of asymptomatic cases to be about 18%.

Another subset of healthy-appearing people we have to think about are those that may contribute to spread before they develop symptoms during the pre-symptomatic phase. A study evaluating seven clusters of COVID-19 in Singapore showed that about 6% of cases represented pre-symptomatic transmission.

Asymptomatic and pre-symptomatic transmission of COVID-19 has prompted a shift in the official endorsement for face masks for the general public. The CDC is now recommending that people wear cloth face coverings when in a public setting where physical distancing may be difficult to maintain (such as a grocery store or pharmacy). Importantly they are not recommending general use of medical-grade face masks. N95 and other medical masks should be reserved for front-line healthcare and emergency workers. Cloth masks are still not fully effective for preventing infection and it’s important that they not replace physical distancing, respiratory hygiene, disinfecting commonly touched surfaces, and hand washing. Instead, cloth face masks should be worn when you leave your house in addition to continuing each of these other measures.

The most important impact that cloth face masks will make in this pandemic will be to contain virus that healthy people are unknowingly shedding in their communities. Because you could spread the virus without knowing it, wearing a cloth face mask will help to protect those around you.

Additional resources:
EBSCO COVID-19 Healthcare Resource Center
Should you request a COVID-19 test?
Is the U.S. doing too much or too little to combat COVID-19?