I just read an article that makes the case that Infected people without symptoms might be driving the spread of coronavirus more than we realized. It's a longish article so I'll simply provide two excerpts that give a flavor for what's being learned. The studies cited are obviously from mainstream medical and scientific sources:
On Tuesday, Dr. Sandra Ciesek, director of the Institute of Medical Virology in Frankfurt, Germany, tested 24 passengers who had just flown in from Israel.
Seven of the 24 passengers tested positive for coronavirus. Four of those had no symptoms, and Ciesek was surprised to find that the viral load of the specimens from the asymptomatic patients was higher than the viral load of the specimens from the three patients who did have symptoms.
Early, large-scale studies using mathematical modelling of outbreaks in Tianjin, China, and Singapore in January and February have also found significant amounts of spread by people who had not yet developed symptoms.
A study posted Sunday by Belgian and Dutch researchers shows that between 48% and 66% of the 91 people in the Singapore cluster contracted the infection from someone who was pre-symptomatic. Of the 135 people in the Tianjin cluster, between 62% and 77% caught it from someone was pre-symptomatic.
One of the study's lead authors, Tapiwa Ganyani at the Data Science Institute at Hasselt University in Belgium, noted in an email to CNN that these are estimates with uncertainties.
Canadian, Dutch and Singaporean researchers looked at the same outbreaks in Tianjin and Singapore and found that infection was transmitted on average 2.55 days and 2.89 days before symptom onset respectively in each location.
"Our analysis would suggest that presymptomatic transmission is pretty commonplace," said the study's lead author, Caroline Colijn, who leads the mathematics, genomics and prediction in infection and evolution research group at Simon Fraser University in British Columbia.
CDC officials have said that while it's clear asymptomatic spread does happen, it does not appear to be the driver of the outbreak -- or as the CDC says on its website, asymptomatic transmission "is not thought to be the main way the virus spreads."
"If I were writing that CDC webpage today, I would phrase that a little more towards transmission before symptoms show up," Colijn said.
Ciesek, the German virologist, agrees that the CDC should tweak its statement.
"I read it before, and I wondered to myself -- why are they so sure of this?" she said.