At least one-third of the world is under some type of lockdown because of the coronavirus pandemic, as governments urge social distancing to stymie the virus' spread.
That's because the COVID-19 virus is insidious.
"There's significant transmission by people not showing symptoms," Stephen Morse, an epidemiologist at Columbia University, told Business Insider.
According to Robert Redfield, the director of the Centres for Disease Control and Prevention, 25 percent of people infected with the new coronavirus don't present any symptoms or fall ill but can still transmit the illness to others.
Redfield on Tuesday told NPR that "we have pretty much confirmed" that "a significant number of individuals that are infected actually remain asymptomatic."
These asymptomatic carriers, Redfield added, are most likely contributing to the rapid spread of the coronavirus worldwide - the number of confirmed cases passed 1 million this week - and making it challenging for experts to assess the true extent of the pandemic.
"We don't know all the unidentified cases out there," Morse said. "It's mostly sicker people in hospitals who are being tabulated."
The prevalence of asymptomatic transmission doesn't bode well for global containment efforts, as Bill Gates recently wrote in an article published in the New England Journal of Medicine.
"That means COVID-19 will be much harder to contain than the Middle East respiratory syndrome or severe acute respiratory syndrome (SARS), which were spread much less efficiently and only by symptomatic people," Gates said.
The first confirmation that the novel coronavirus could be transmitted by asymptomatic people came in February, when a case study described a 20-year-old woman from Wuhan, China, who passed the coronavirus to five family members but never got physically sick herself.
A World Health Organisation report about the coronavirus outbreak in China, published in February, found few instances in which a person who tested positive never showed any symptoms. Instead, most people who were asymptomatic on the date of their diagnosis (a relatively small group anyway) went on to develop symptoms later.
"The proportion of truly asymptomatic infections is unclear but appears to be relatively rare," the report authors wrote.
In the WHO study, 75 percent of people in China who were first classified as asymptomatic later developed symptoms, ProPublica reported. That means, technically, "presymptomatic transmission" is what's probably common.
Other research has reaffirmed these findings. A CDC study of coronavirus patients in a nursing home in Washington state's King County found that of 23 people who tested positive, only 10 showed symptoms on the day of their diagnosis. Ten people in the other group developed symptoms a week later.
"These findings have important implications for infection control," the authors wrote, adding that many public-health approaches "rely on presence of signs and symptoms to identify and isolate residents or patients who might have COVID-19."
The CDC also evaluated coronavirus patients on the Diamond Princess cruise ship, which was quarantined in Japan in February. Of the 3,711 people onboard, 712 tested positive, but almost 50 percent of them had no symptoms at the time.
Redfield told NPR that "it appears that we're shedding significant virus" about 48 hours before symptoms appear.
"This helps explain how rapidly this virus continues to spread across the country, because we have asymptomatic transmitters and we have individuals who are transmitting 48 hours before they become symptomatic," he added.
A handful of recent studies and reports suggest that presymptomatic and asymptomatic transmission are not unusual.
A small study among Japanese ex-pats evacuated from Wuhan in February revealed that 30.8 percent of people who tested positive showed no symptoms.
Research that examined coronavirus cases in Singapore found that of 157 cases acquired locally, 10 involved presymptomatic transmission. The scientists concluded that most presymptomatic transmission exposure occurred one to three days before a person developed symptoms.
Research from China in February revealed that 13 percent of the 468 confirmed cases studied involved presymptomatic transmission.
The LA Times recently reported that three-quarters of a group of singers who attended a 60-person choir practice got the COVID-19 virus, even though none showed symptoms at the practice.
Last month, 14 NBA players, coaches, and staff tested positive for the coronavirus. Half of them didn't have symptoms when they were diagnosed, according to the Wall Street Journal.
A biotech company in Iceland that has tested more than 9,000 people found that around 50 percent of those who tested positive said they were asymptomatic, the researchers told CNN.
An especially troubling aspect of presymptomatic transmission is that people seem to shed more coronavirus in the earlier stages of their infection. But the average symptom onset takes five days.
Research that examined 23 coronavirus patients in two Hong Kong hospitals found that individuals' viral load - how many viral particles they were carrying and shedding into their environment - peaked during the first week of symptom onset, then gradually declined.
A SARS patient, by contrast, sheds the most virus around seven to 10 days after getting visibly sick.
A study from Guangzhou found similar results: Among 94 patients, people were most contagious right when symptoms started to show, or just before.
One potential group of asymptomatic carriers could be children. Thus far, children are among those least sickened by the coronavirus - but some could be getting very mild infections then spreading the virus.
Research published Wednesday in the journal The Lancet looked at 36 children who tested positive for coronavirus between January 17 and March 1 in three Chinese hospitals. Half of those children had "mild disease with no presenting symptoms," the authors wrote.
Another study, which has yet to be peer-reviewed, found that 56 percent of 700 children infected with COVID-19 in China had mild, if any, symptoms.
John Williams, an expert in pediatric infectious disease at University of Pittsburgh Medical Centre, told ABC that "asymptomatic infection is common in children, occurring in 10-30 percent" of cases.
The WHO and CDC have not yet recommended that healthy members of the general public wear masks while going out in public; only healthcare workers are required to use face protection.
But the White House has just announced a new policy, based on CDC guidance, urging Americans to wear cloth masks when out and about.
The prevalence of presymptomatic transmission is likely a primary reason for the change.
Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN on Wednesday that he would "lean toward" asking everyone to wear masks "if we do not have the problem of taking away masks from the healthcare workers who need them."
He added the shift might be important "particularly now that we're getting some inklings that there's transmission of infection from an asymptomatic person who is not coughing, who is not sneezing, who just appears well."
Face protection for the most part doesn't benefit the wearer; instead, masks primarily protect others from the wearer's germs, since they could be infected but not know.
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