Can people who are asymptomatic spread coronavirus? What we know right now

The World Health Organization incited backlash from public health officials during a news conference on June 8 when Maria Van Kerkhove, the WHO's technical lead for the COVID-19 response, said it's "very rare" for asymptomatic carriers of COVID-19 to spread the virus. The statement came after Kerhove said that in countries tracking asymptomatic cases, they are "following contacts and they're not finding secondary transmission onward."

A day later, the WHO backtracked and clarified the statement, with Kerkhove noting that the WHO "actually doesn't have that answer yet," regarding if -- and how often -- asymptomatic carriers of COVID-19 spread the virus.

Now, many people are confused about whether or not asymptomatic carriers can spread COVID-19 -- but the question isn't really about that. It's safe to assume that asymptomatic carriers can spread the disease, based on early and ongoing studies about the nature of COVID-19. 

The bigger question is about how contagious these cases really are -- especially now that the US is seeing a massive surge in cases after lockdown restrictions loosened.

Remember that information about the novel coronavirus changes rapidly: We are presented with new facts every day. This article discusses the concept of asymptomatic COVID-19 cases at the time of writing, and we'll update this story as more information becomes available. 

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What it means to be an asymptomatic carrier 

If you're an asymptomatic carrier of a disease, it means you have the disease but don't show any symptoms. 

In the case of COVID-19, this can be a serious threat because of the way this virus spreads: People who are unaware that they have the virus may not stay home or take precautions, such as wearing a mask or staying six feet apart from others, when they leave their homes. 

There's a lot of confusion about what "asymptomatic" really means -- part of that comes from a lack of data about asymptomatic COVID-19 carriers, but a large part of that stems from the many different uses of the word "asymptomatic." 

People can contract the novel coronavirus and be truly asymptomatic -- meaning the virus infects them and runs its course without ever producing symptoms. 

Then there are people who are "presymptomatic," which refers to the time period between infection and appearance of symptoms. With many viruses, people are contagious during presymptomatic phases, and we know this to be true about COVID-19. 

Someone may be presymptomatic for several days, and if that person wasn't self-isolating during the presymptomatic phase (which, naturally, isn't top-of-mind), they could have passed the virus onto everyone they came into close contact with. 

Finally, there are mild cases of COVID-19, in which an infected person might show acute symptoms, such as a light cough, some mild body aches or other symptoms typical of the common cold. These people may never know they have COVID-19 because their symptoms aren't severe enough to warrant a test, thus they never receive a diagnosis. 

People with mild cases may not feel sick enough to stay home from work or avoid running errands. After all, life doesn't stop for a common cold -- pre-pandemic, it was pretty common to go about your daily obligations despite a mild cold, and many people still operate with that mindset. 

"Asymptomatic" has been used to describe all of the above scenarios, which doesn't help the case for figuring out whether asymptomatic transmission of COVID-19 is significant or not.

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Can asymptomatic people spread COVID-19? 

Generally, asymptomatic people can spread the disease they have. This is thought to be true for the novel coronavirus, too, although officials don't yet know how common the spread of COVID-19 by asymptomatic carriers is. 

Studies have shown that people can be contagious in the first several days of having the virus, before they show symptoms. One study actually estimated that more than 40% of novel coronavirus cases were transmitted in the presymptomatic phase. And, in a study that analyzed samples from sailors on the USS Theodore Roosevelt, about 20% of infected people reported that they never developed symptoms. 

A survey from the CDC reports that 54% of people who tested positive for COVID-19 couldn't recall how they got it. The remaining 46% of respondents who tested positive reported that they had close contact with someone who was sick, such as a family member, before they developed the disease. 

This indicates that the majority of people could have contracted COVID-19 from an asymptomatic person they'd been in close contact with, or from a symptomatic passer-by, such as someone who coughed near you at the grocery store. 

The report concludes by saying this ambiguity "underscores the need for isolation of infected persons, contact tracing and testing during ongoing community transmission, and prevention measures including social distancing and use of cloth face coverings."

So, it's clear that people can contract the disease but remain symptomless. It's also clear that people who do develop symptoms could potentially spread the virus before they knew they had it.

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How many people with coronavirus are asymptomatic?

Estimates as to the percentage of asymptomatic coronavirus cases vary widely. Experts have estimated that anywhere from 25% to 80% of people with COVID-19 never develop symptoms. Here's a look at some of the research surrounding asymptomatic COVID-19:

• As early as January 2020, researchers noted that presymptomatic transmission can likely explain secondary cases of COVID-19.
• In April 2020, researchers suggested that the virus is most contagious before or at the onset of symptoms.
• A May 27 study in the Journal of American Medicine reported that more than 40% of study participants who tested positive for COVID-19 were asymptomatic. 
• Another study found that 104 of 128 (81%) positive cases on a cruise ship showed no symptoms. 
• In New Orleans, Louisiana, a study conducted by a hospital system in the city found that 75% of infectious people were asymptomatic.

Why are these findings important? 

If people have COVID-19 and don't know it, it's unlikely they'll take maximum precautions to prevent themselves from spreading the virus. 

Some officials have voiced concern that the WHO's initial statement that asymptomatic spread is "very rare" discouraged mask-wearing and social distancing, which could significantly worsen the spread of COVID-19 if asymptomatic people can readily transmit the virus. This is why many experts argue that it's sensible to assume asymptomatic spread is a threat. 

How does COVID-19 spread? 

As a quick reminder, SARS-CoV-2, the virus that causes the illness COVID-19, spreads through respiratory droplets, such as saliva and mucus. This means it primarily spreads when people who have the virus cough, sneeze and talk in the presence of other people. Direct contact with people who are sick also spreads the virus.

It's also possible the COVID-19 can spread when people touch surfaces that have the SARS-CoV-2 virus on them, although the CDC says this isn't thought to be the main mode of transmission. 

According to the CDC, RNA from the virus has been detected in other bodily fluids and byproducts, such as blood and stool, but experts do not yet know if COVID-19 can spread through these substances. 

The best way to prevent the spread of COVID-19 is to practice good personal hygiene (e.g. wash your hands often), stay home when you can (and definitely when you're sick), wear a face covering when you go out, and avoid contact with others who are sick.