Why are some people with coronavirus asymptomatic — and what makes them so contagious?

“The potential for transmission from asymptomatic people has been cited as an important factor in controlling the spread of COVID-19, but there is limited information about the clinical course and viral loads of asymptomatic people with SARS-CoV-2 infection,” the researchers wrote.

What they found: “Many individuals with SARS-CoV-2 infection remained asymptomatic for a prolonged period, and viral load was similar to that in symptomatic patients,” the scientists concluded. “Therefore, isolation of infected persons should be performed regardless of symptoms.”

The study was led by Seungjae Lee, an associate professor at Soonchunhyang University College of Medicine in South Korea, was peer reviewed, and carried out earlier in the pandemic; the researchers analyzed swabs taken from the group between March 6 and March 26.

Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said that in his 40 years of dealing with viral outbreaks, he’s never seen anything like COVID-19, particularly in one singular way that helped lead to one of the biggest public health crises in a generation.

“I’ve been dealing with viral outbreaks for the last 40 years. I’ve never seen a single virus — that is, one pathogen — have a range where 20% to 40% of the people have no symptoms,” he told a recent House Committee on Energy and Commerce hearing on the novel coronavirus pandemic.

The World Health Organization currently estimates that 16% of people with COVID-19 are asymptomatic and can transmit the coronavirus, while other data show that 40% of coronavirus transmission is due to carriers not displaying symptoms of the illness.

The infectiousness of asymptomatic individuals relative to those who are symptomatic is 75%, according to the Centers for Disease Control and Prevention, based on studies of “viral shedding” dynamics — that is, how much of the virus they transmit through talking or breathing.

Not everyone responds the same to COVID-19 infection. “The different host immune responses to SARS-CoV-2 infection may partially explain why males and females, young and old persons infected with this virus have markedly distinct disease severity,” the researchers wrote.

So why are some people asymptomatic while others are not? “SARS-CoV-2, which is the virus causing coronavirus disease 2019 (COVID-19), uses the angiotensin-converting enzyme 2 (ACE2) as a cell receptor to invade human cells,” they added.

Those ACE2 receptors or “doorways” appear to be more prevalent in older people and those who are obese than younger people. That may go some way in explaining why so many young people have not been as badly affected by the virus as those older than 60, for instance.

Previous exposure to other coronaviruses, which can give people “T-cell immunity” to similar viruses, receiving a lower viral load of COVID-19 and other lucky genetic variations may also contribute to why some people having less severe or not symptoms to infection.

There is currently no vaccine for COVID-19, although several companies say they’ve made progress with trials. However, unlike with influenza viruses for which there are several vaccines, people have generally not built up an immunity to COVID-19 over multiple generations.

Related: Feeling lax about masks? Think again. Here’s how many lives could be saved if everyone wore a mask — starting today

But a strong immune response to an infection from a virus such as COVID-19 can sometimes cut both ways. Case in point: Doctors and members of the public were spooked by how otherwise strong, healthy people fell victim to the 1918 influenza.

Doctors today attribute that to the “cytokine storm,” a process where the immune system in healthy people reacts so strongly as to hurt the body and ultimately cause damage to the organs and, in the most severe cases, organ failure.

A hallmark of some viruses: A surge of immune cells and their activating compounds (cytokines) effectively turned the body against itself, led to an inflammation of the lungs, severe respiratory distress, leaving the body vulnerable to secondary bacterial pneumonia.

“A major difference between Spanish flu and COVID-19 is the age distribution of fatalities,” according to a recent report by Deutsche Bank DB, . “For COVID-19, the elderly have been overwhelmingly the worst hit, but the young working-age population were severely hit in 1918.

“In fact, the death rate from pneumonia and influenza that year among 25-34-year-olds in the United States was more than 50% higher than that for 65-74-year-olds. A remarkable difference to Covid-19,” Deutsche Bank said.

Public-health officials have advised people to keep a distance of six feet from one another. Face masks are designed to prevent the wearer, who may be infected with COVID-19 but have very mild or no symptoms, from spreading invisible droplets to another person and thereby infecting them too.

It doesn’t matter if someone appears sick or not. Asymptomatic transmission “is the Achilles’ heel of COVID-19 pandemic control through the public-health strategies,” according to a May 28 editorial in the New England Journal of Medicine. It said the SARS-CoV-1 is even more contagious than SARS-CoV-2.

The editorial, by researchers Monica Gandhi, Deborah Yokoe and Diane Havlir at the University of California, San Francisco, referred to recent study of a coronavirus outbreak at a skilled nursing facility that “strongly demonstrate that our current approaches are inadequate.”

They said those symptoms include high genetic relatedness, transmission primarily through respiratory droplets, and the frequency of lower respiratory symptoms (fever, cough, and shortness of breath), with both infections developing a median of five days after exposure.

This University of California, San Francisco study said there’s a high viral load of SARS-CoV-2 shedding in the upper respiratory tract, even among pre-symptomatic patients, “which distinguishes it from SARS-CoV-1, where replication occurs mainly in the lower respiratory tract.”

“Despite the deployment of similar control interventions, the trajectories of the two epidemics have veered in dramatically different directions,” they added. “Within 8 months, SARS was controlled after SARS-CoV-1 had infected approximately 8,100 persons in limited geographic areas.”

Governments around the world are struggling to stop the spread of the pandemic. (An epidemic is a disease that infects regions or a community.) The “Spanish flu” from 1918 to 1919 and Black Death from 1347 to 1351 were two of the most extreme pandemics ever recorded.

As of Saturday, COVID-19, the disease caused by the virus SARS-CoV-2, had infected at least 19.4 million people globally and close to 5 million in the U.S. It had killed over 723,531 people worldwide and at least 162,181 in the U.S., according to data aggregated by Johns Hopkins University.

The stock market has been on a wild ride in recent months. The Dow Jones Industrial Index DJIA, +0.17% and the S&P 500 SPX, +0.06% ended up Friday as investors awaited round two of a fiscal stimulus; the Nasdaq Composite’s COMP, -0.87% 7-day winning streak ended.