Nearly Half Of All COVID-19 Coronavirus Infections May Come From Asymptomatic People

Is it possible to have COVID-19 coronavirus and not even know it? Yes. Of greater concern to scientists is what may be happening to those asymptomatic carriers and how many people they may be unwittingly infecting. The numbers may be higher than we previously imagined, according to a new study.

With virtually any pathogenic outbreak, some people will become infected and never experience any symptoms. They’re known as asymptomatic carriers. With some pathogens, this is fairly benign, with the asymptomatic person getting over the disease and without them spreading it widely.

Apparently, that’s not the case with SARS-CoV-2, the virus underlying COVID-19. It’s been suspected for months that some people may be asymptomatic carriers, and the fear was that some of them would end up infecting other people without even knowing it.

An historical example of this phenomenon is Typhoid Mary. Mary Mallon was an Irish-born cook who was believed to have infected 53 or more people with Typhus, without ever being sick with it herself.

Is it possible that there are COVID-19 Charlies or Colleens out there? While it’s unlikely that a significant number of asymptomatic carriers of SARS-CoV-2 could match Typhoid Mary’s infection count, there are a lot more of them then there was of her. Both medically and mathematically, that may pose a real risk.

The research folks over at Scripps Research Translational Institute have crunched the numbers from 16 different COVID-19 patient cohorts from around the world. They wanted to get a real estimate of just how many COVID-19 cases could be connected to those who spread the virus without any knowledge of having it. They published their findings in the June 3 edition of Annals of Internal Medicine, and the numbers are surprising.

They believe that at least 30% of all COVID-19 cases were acquired from asymptomatic carriers. More likely, they say, that number is between 40 and 45%.

To be clear, it’s highly unusual for a respiratory virus to be spread when the symptoms of that respiratory virus are not present. Most of those viruses cause symptoms in their hosts shortly after infection. Additionally, it’s those symptoms – coughing, sneezing, mucus production and touching of mucous membranes – that aid the spread of the virus.

SARS-CoV-2 seems like the “black ops” version of respiratory viruses. It sneaks in unseen, doesn’t make any noise and gets its unwitting host to help it replicate and spread. Pretty slick, and pretty scary.

Dr. Eric Topol, director and founder of the Scripps RT Institute, had this to say to Time magazine, “The range we found is extraordinarily high. That means the range of what can happen with SARS-CoV-2 is from no symptoms to [death]. That’s not at all similar to any virus or pathogen we’ve experienced that has killing potential in the past. What we have here is an extraordinary spectrum, including this quiet, stealth mode of infecting somebody.”

Along with his co-author, Daniel Oran, Topol did a search for studies which included asymptomatic people. They keyed in on 16 different cohorts from around the world. Included among those cohorts were the crews of the USS Theodore Roosevelt and Charles DeGaulle, aircraft carriers which each experienced outbreaks.

They also reviewed studies including Japanese citizens evacuated from Wuhan, China, residents of a King County, WA nursing facility, residents of Vo, Italy, the passengers on the Diamond Princess cruise ship, prison inmates, homeless shelters, college students and a group of more than 13,000 Icelanders who had volunteered to be tested.


Of the 16 study cohorts reviewed, only 5 included secondary or follow-up testing of the participants. These studies revealed that a small percentage of the people who were asymptomatic at the time of their first positive test went on to actually develop symptoms. Those people were considered pre-symptomatic, meaning they tested positive and then later developed symptoms.

These people were separated from the truly asymptomatic for purposes of the Scripps study. In the 2,300 person cohort in Vo, Italy, an astonishing 41 percent of people tested positive without developing symptoms within a 14 day period. Previous studies have shown that a preponderance of people infected who get symptoms will do so within 5 to 14 days after their exposure and infection.

It was once believed that asymptomatic people were so because they had low levels of the virus and/or weren’t shedding the virus the way those with symptoms might. Recent research shows that asymptomatic people often have virus levels similar to those of symptomatic people.

That means that, even without visible or identifiable symptoms of the virus, viral load and shedding levels in asymptomatic people may be dangerously high. It also means they are equally capable of spreading it.

Topol also suggests that, in spite of a lack of symptoms, COVID-19 may be damaging the bodies of asymptomatic carriers in other ways. It was noted that among the Diamond Princess cruise ship cohort, 331 people tested positive but had no symptoms. Of these, 76 showed signs of lung damage normally associated with COVID-19 infection when they were given lung CT scans later.

“People who are getting infection without symptoms are actually doing a lot of damage to their bodies and they don’t know it,” says Topol. In the South Korean cohort, this finding was supported, too, when the 10 asymptomatic patients also showed similar issues.

Topol believes his team’s findings are “basically the reason why we have to all wear masks—because nobody knows who is an asymptomatic carrier. The person doesn’t know it and the person’s contacts don’t know it. That has enormous implications and it’s an area we need to study more, on how to test people without symptoms on a very large scale to understand these people better and follow them to determine precisely their ability to transmit [the virus].”

Without testing the entire population, it is impossible to absolutely know what impact asymptomatic disease has and what it means. Is it possible that asymptomatic people simply have a better immune defense against a broad range of coronaviruses? Are their immune systems just generally more hardy and responsive? Do they just have a weaker, less potent version of the virus?

Perhaps a more important question, at least from a public health policy perspective, is how long are asymptomatic people infectious? We’ve assumed a 14 day window, based on the accepted symptom incubation period. Topol’s analysis of data from the USS Theodore Roosevelt, however, suggests a longer infectious period. This has implications for reopening cities and states, how long to quarantine people exposed while traveling upon their return to the US and how long masks will be needed in public.

Testing everyone, or nearly everyone in a community population would be ideal for identifying asymptomatic cases. That would allow public health officials to educate these folks on the value and need for physical distancing, masks and other measures to prevent the spread of the virus.

Short of that, Topol’s team thinks technology can help. They’re studying heart rate changes by capturing them on fitness watches, smartphone apps and wristbands. They believe that changes in heart rate may signal infection, particularly if it happens across a community. Heart rate variability in individuals can be a result of any number of non-virus causes. They say variations across a community, however, may indicate an infection cluster, signalling a need for more testing in that community to identify infected individuals and undertake measures to mitigate the further spread of COVID-19.

“If even a portion of the 100 million Americans who have a smartwatch or fitness band are involved, then we could go in and do studies for information we are missing now—antigen testing, antibody testing and we can look for transmissibility,” says Topol. “The priorities during a pandemic are absolutely to look after the sick. But we also shouldn’t miss how important this area of asymptomatic spread is to understand. For every one person who is sick, there are a whole lot of people who have the virus and don’t know it.”

While infection rates, hospitalizations and death counts continue to move in the right direction, every new detail we learn about COVID-19 coronavirus gives us another tool in beating it. The more we know about this coronavirus, the better prepared we’ll be when the next one comes along, too. Considering how many mistakes were made in the response to this outbreak, we need all the help we can get.

Keep the faith and keep after it!

Related Content –
Childhood Obesity Crisis Worsened By COVID-19 Coronavirus Lockdowns
Study Says Wearing Masks May Reduce COVID-19 Coronavirus Transmission
COVID-19 Coronavirus Causing Unprecedented Mental And Emotional Trauma Worldwide

Success! You're on the list.

Journal Reference –

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.