COVID-19 spread is fueled by 'stealth transmission'

News
By Rachael Rettner
published

Cases of COVID-19 that fly under the radar — without being diagnosed — appear to fuel the rapid spread of the disease.

A busy pedestrian street crossing.
(Image credit: Shutterstock)

Cases of the new coronavirus disease (COVID-19) that fly under the radar — without being detected or diagnosed — may have fueled the rapid spread of the disease, according to a new study.

The study found that people with COVID-19 who didn't get diagnosed, likely because they didn't feel very sick, were the source of at least two-thirds of documented COVID-19 cases in China in the early days of the outbreak.

"The explosion of COVID-19 cases in China was largely driven by individuals with mild, limited, or no symptoms who went undetected," study co-author Jeffrey Shaman, a professor of environmental health sciences at Columbia University Mailman School of Public Health, said in a statement.

"Undetected cases can expose a far greater portion of the population to [the] virus than would otherwise occur. … These 'stealth transmissions' will continue to present a major challenge to the containment of this outbreak going forward," Shaman said.

Related: What are the symptoms of the new coronavirus?

For the study, published Monday (March 16) in the journal Science, the researchers developed a computer model to simulate the spread of SARS-CoV-2, the virus that causes COVID-19, among 375 cities in China, including Wuhan, where the outbreak began. For the model, they combined data on reported infections with information on people's movements (obtained from mobile phone data).

They estimated that, prior to the lockdown of Wuhan on Jan. 23, about 86% of all COVID-19 infections in China were undetected. In other words, for every confirmed case of COVID-19, there were six undetected cases, according to The Washington Post. These undetected cases were responsible for the majority of the disease spread prior to the lockdown, the researchers said.

The findings have implications for COVID-19 spread in the rest of the world, as many countries are behind on testing for the disease. The results suggest that the number of cases worldwide could be five to 10 times higher than what has been reported, meaning the true number of cases could be higher than 1.5 million, according to Quartz.

Related: How do the new coronavirus tests work?

"We know this is just the tip of the iceberg," Shaman said at a press conference Monday (March 16), according to Quartz. "The question is how much of the iceberg is submerged. We can argue about the exact number, but if you're not in a place where you're "not proactively looking for people and testing, then the majority of infections will go undetected."

After China implemented the lockdown on Wuhan, officials were able to detect the majority of cases, 65%, and reduce the spread of the disease.

The findings suggest "a radical increase in the identification and isolation of currently undocumented infections would be needed to fully control SARS-CoV-2," the authors concluded.

Originally published on Live Science. 

Rachael Rettner
Rachael Rettner
Contributor

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.

4 Comments Comment from the forums
  • Professor G
    The report is more or less a restatement of the obvious. As will almost all diseases particularly Viral diseases there are going to be many if not a majority of individuals who are asymptomatic or have very minor symptoms, so minor that they did not feel they were sick. One hardly needs a computer program to deduce this just some Medical Common Sense or Logic if you need to say that.

    What is disturbing is the conclusion that more testing is needed. Why? How is this different from any other Flu? If the author is correct and more than 1.5 million individuals have had the disease what is the mortality rate using those numbers? My calculator says about 0.5% so not a lot different than the typical Flu. Of course it could be much lower if more than 1.5 million have had COVID-19.

    Should everyone on Planet Earth by tested annually for Influenza, Corona, HIV etc etc. I would be a Bonanza for the Clinical Testing Industry and a major invasion of Individual Rights.

    It would be more humorous if Life as we know it was not being put on hold.
    Reply
  • N0mad
    Science exists to seek truth. A common, verifiable reality.
    Promotion of ideology and politics, above science, is how we got into this mess. Your individual "liberty", @Professor G, does not rate against the importance of mitigation and herd immunity, to protect those less resilient than you.
    And I'm a geek, so I skew more than a little libertarian, myself. But still, no.

    Just because something is obvious, doesn't make it verifiably true. This highly contagious pathogen spreads most effectively through unaffected carriers, specifically younger humans. It's more lethal, more quickly, than the flu.
    The behavior of Disney visitors the day before close, of revelers in major cities around Saint Patrick's, of the TSA after the announcement of border closings, is unconscionable.
    If even a few people's behaviors can be influenced by these facts, more of the vulnerable and the unlucky (and yes, the reckless, irresponsible, and stupid) will survive.

    Both my partner and I are the most resilient of our families, both sides, all generations.
    But she's a paramedic. And I'm likely high-risk myself, due to previous lung conditions (childhood pneumonia, bouts of bronchitis, long-ago smoking habit). Neither one of us is very young.
    Our parents, on the other hand, are very high-risk. They all have underlying health conditions; make bad life choices; lead stressful, immunocompromised lives...
    We accept that they will die. But it would be nice if they stick around for a bit longer, and avoid the agonizing death that Covid-19 brings.
    "Humorous", maybe for you. Tone-deaf and utterly devoid of empathy and kindness, definitely.
    Reply
  • djjmc
    Professor G said:
    The report is more or less a restatement of the obvious. As will almost all diseases particularly Viral diseases there are going to be many if not a majority of individuals who are asymptomatic or have very minor symptoms, so minor that they did not feel they were sick. One hardly needs a computer program to deduce this just some Medical Common Sense or Logic if you need to say that.

    What is disturbing is the conclusion that more testing is needed. Why? How is this different from any other Flu? If the author is correct and more than 1.5 million individuals have had the disease what is the mortality rate using those numbers? My calculator says about 0.5% so not a lot different than the typical Flu. Of course it could be much lower if more than 1.5 million have had COVID-19.

    Should everyone on Planet Earth by tested annually for Influenza, Corona, HIV etc etc. I would be a Bonanza for the Clinical Testing Industry and a major invasion of Individual Rights.

    It would be more humorous if Life as we know it was not being put on hold.

    My conservitive estimate, as of march the 17th was a bare minimum of 800k cases or a max of 8.3 million. I was basing the high number on the stats that came out of South Korea (which seemed to have completed the most testing and has a good health system). South Korea has a death rate of 0.01%.

    If we change it to a 1% fatality rate then the minimum cases are over 800K. However this is still too little, as it doesnt take into account the people that are about to die, and the corresponding cases that could be related.
    Reply
  • icanactuallydomath
    Professor G said:
    The report is more or less a restatement of the obvious. As will almost all diseases particularly Viral diseases there are going to be many if not a majority of individuals who are asymptomatic or have very minor symptoms, so minor that they did not feel they were sick. One hardly needs a computer program to deduce this just some Medical Common Sense or Logic if you need to say that.

    What is disturbing is the conclusion that more testing is needed. Why? How is this different from any other Flu? If the author is correct and more than 1.5 million individuals have had the disease what is the mortality rate using those numbers? My calculator says about 0.5% so not a lot different than the typical Flu. Of course it could be much lower if more than 1.5 million have had COVID-19.

    Should everyone on Planet Earth by tested annually for Influenza, Corona, HIV etc etc. I would be a Bonanza for the Clinical Testing Industry and a major invasion of Individual Rights.

    It would be more humorous if Life as we know it was not being put on hold.

    If you are a professor, you're a Professor of Things That Don't Involve Math At All. It's mind boggling to me that people can post things that are so obviously stupid. The number of deaths is a function not just of the fatality rate but the number of people who get infected. In an average year about 8% of the US population gets infected with influenza, which with a fatality rate of approximately 0.2% kills about 56,800 people. If this thing infects even in 10% of the population, which is a vast underestimate of the total number if we continue to have half-assed containment measures, then with a fatality rate of 1% it will kill 355,000 people.
    Reply