At least one third of the world's population is under some type of isolation due to the coronavirus pandemic as governments urge people to distance themselves to stop the virus from spreading.
This is because the COVID-19 virus is insidious.
“People often show no symptoms of the disease,” Stephen Morse, an epidemiologist at Columbia University, told Business Insider.
According to Robert Redfield, director of the Centers for Disease Control and Prevention, 25 percent of people infected with the coronavirus do not show any symptoms, but may still transmit the disease to others.
Redfield said Tuesday that “we have confirmed” that “a significant number of infected people are in fact asymptomatic.”
These asymptomatic carriers, Redfield added, are likely contributing to the rapid spread of the coronavirus globally – the number of confirmed cases topped 1 million this week – and makes it difficult for experts to assess the true extent of the pandemic.
The prevalence of asymptomatic transmission does not bode well for global containment efforts, as Bill Gates recently wrote in an article published in the New England Journal of Medicine.
“This means that COVID-19 will be much more difficult to contain than respiratory syndrome in the Middle East or Severe Acute Respiratory Syndrome (SARS), which spread much less efficiently and all infected people had symptoms,” Gates said.
What do we know about asymptomatic transmission.
The first confirmation that the new coronavirus could be spread by people without symptoms came in February, when a case study described a 20-year-old woman from Wuhan, China, who transmitted the coronavirus to five family members but was not sick herself.
A World Health Organization report on the coronavirus outbreak in China, published in February, found several cases in which a person who tested positive had no symptoms. Instead, most people who were asymptomatic at the date of diagnosis (at least in a relatively small group) developed symptoms later.
“The proportion of truly asymptomatic infections is unclear, but appears to be relatively rare,” the report's authors write.
According to ProPublica, in a WHO study, 75% of people in China who were first classified as asymptomatic subsequently developed symptoms. This technically means that 'asymptomatic transmission' is what is common.
Other studies have confirmed these findings. A CDC study of coronavirus patients in a nursing home in King County, Washington, found that out of 23 people who tested positive, only 10 had symptoms on the day of their diagnosis. Ten people in the other group developed symptoms after a week.
“These findings are essential for infection control,” the authors write, adding that many public health approaches “rely on the presence of signs and symptoms to identify and isolate residents or patients who may have COVID-19.”
The CDC has also evaluated coronavirus patients on the Diamond Princess cruise ship, which was quarantined in Japan in February. Of the 3,711 people on board, 712 tested positive, but almost 50 percent of them showed no symptoms of the disease.
There are many other examples of asymptomatic transmission.
Redfield said it 'looks like we are shedding the virus' about 48 hours before symptoms appear.
“This helps explain how quickly this virus continues to spread around the world, because we have asymptomatic transmitters and we have people who transmit the virus 48 hours before they become symptomatic,” he added.
Several recent studies and reports indicate that asymptomatic transmission is not uncommon.
A small study of Japanese residents evacuated from Wuhan in February found that 30.8% of people who tested positive had no symptoms.
Studies that looked at coronavirus cases in Singapore found that out of 157 locally acquired cases, 10 had pre-symptomatic transmission. Scientists concluded that the greatest pre-symptomatic infection occurred one to three days before a person developed symptoms.
Research in China in February found that 13 percent of the 468 confirmed cases studied were associated with asymptomatic transmission.
14 players, coaches and NBA staff tested positive for the coronavirus last month. According to the Wall Street Journal, half of them were asymptomatic at diagnosis.
Researchers told CNN that a biotech company in Iceland that tested over 9,000 people found that about 50 percent of those who tested positive said they had no symptoms.
A particularly troubling aspect of pre-symptomatic transmission is that people seem to shed more coronavirus in the earlier stages of their infection. But on average, symptoms start after five days.
Studies that examined 23 coronavirus patients in two hospitals in Hong Kong showed that people's viral load – how many viral particles they carried and shed into the environment – peaked during the first week of symptom onset and then gradually declined.
The SARS patient, on the other hand, sheds the greatest amount of the virus about 7-10 days after becoming noticeably sick.
A study in Guangzhou found similar results: among 94 patients, people were most infectious when or just before symptoms began.
Children may be asymptomatic carriers.
Children may be one potential group of asymptomatic carriers. Until now, children are among those least affected by coronavirus, but some may have a very mild infection and then spread the virus.
The study, published Wednesday in The Lancet, looked at 36 children who tested positive for the coronavirus between January 17 and March 1 at three Chinese hospitals. The authors wrote that half of these children had 'mild illness without symptoms'.
Another study, which has not yet been peer-reviewed, found that 56 percent of the 700 children infected with COVID-19 in China had mild symptoms, if any.
John Williams, an expert on infectious diseases in children at the University of Pittsburgh Medical Center, said that “asymptomatic infection is common in children and occurs in 10-30 percent of cases.”
Wearing masks can help reduce asymptomatic transmission.
The WHO and CDC have not yet recommended that healthy members of the general public wear masks when going out in public; only healthcare professionals should use face protection.
But the White House has just announced a new policy, based on the CDC leadership, urging Americans to wear cloth masks when they leave their homes.
Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN on Wednesday that he would be “leaning towards asking everyone to wear masks.”
He added that the change could be important “especially now that we get some suspicion that there is transmission from an asymptomatic person who does not cough, who does not sneeze, who looks good.”
Most of the face protection does not benefit the wearer; instead, masks primarily protect others from the wearer's germs, as they can be contaminated without being aware of it.
This article was published by Business Insider.
Sources: Photo: Indonesian Foreign Ministry via AP
