NEW YORK (NYTIMES) - As many as 25 per cent of people infected with the new coronavirus may not show symptoms, the director of the Centres for Disease Control and Prevention (CDC) warns - a startlingly high number that complicates efforts to predict the pandemic's course and strategies to mitigate its spread.
In particular, the high level of symptom-free cases is leading the CDC to consider broadening its guidelines on who should wear masks.
"This helps explain how rapidly this virus continues to spread across the country," the director, Dr Robert Redfield, said in an interview broadcast Tuesday (March 31).
The agency has repeatedly said that ordinary citizens do not need to wear masks unless they are feeling sick. But with the new data on people who may be infected without ever feeling sick, or who are transmitting the virus for a couple of days before feeling ill, Dr Redfield said that such guidance was "being critically rereviewed."
Researchers do not know precisely how many people are infected without feeling ill or if some of them are simply presymptomatic. But since the new coronavirus surfaced in December, they have spotted unsettling anecdotes of apparently healthy people who were unwitting spreaders.
"Patient Z," for example, a 26-year-old man in Guangdong, China, was a close contact of a Wuhan traveller infected with the coronavirus in February. But he felt no signs of anything amiss, not on Day 7 after the contact nor on Day 10 or 11.
Already by Day 7, though, the virus had bloomed in his nose and throat, just as copiously as in those who did become ill. Patient Z might have felt fine, but he was infected just the same.
Researchers now say that people like Patient Z are not merely anecdotes. For example, as many as 18 per cent of people infected with the virus on the Diamond Princess cruise ship never developed symptoms, according to one analysis. A team in Hong Kong suggests that from 20 per cent to 40 per cent of transmissions in China occurred before symptoms appeared.
The high level of covert spread may help explain why the coronavirus is the first virus that is not an influenza virus to set off a pandemic.
The new virus spreads about as easily as flu, "and when's the last time anyone thought anything about stopping influenza transmission, short of the vaccine?" said Dr Michael Osterholm, an infectious disease expert at the University of Minnesota.
With any vaccine still in early development, the best way to mitigate the pandemic is social distancing, he and other experts said. Because people may be passing the virus on to others even when they feel fine, asking only unwell people to stay home is unlikely to be enough.
This is why many experts, going against recommendations by the CDC and the World Health Organisation, are now urging everyone to wear masks - to prevent those who are unaware they have the virus from spreading it.
Like influenza, some experts now say, this virus appears to spread both through large droplets and droplets smaller than 5 micrometers - termed aerosols - containing the virus that infected people might release especially while coughing but also while merely exhaling. They emphasised that the level of virus in both types of particles is low, so simply jogging or walking by an infected person does not put people at risk.
"If you have a passing contact with an infectious person, you would have a very, very low chance of transmission occurring," said Dr Benjamin Cowling, an epidemiologist at the University of Hong Kong.
The risk goes up with sustained contact - during face-to-face conversation, for example, or by sharing the same air space for a prolonged time.
In addition to its confusing stance on masks, "the WHO has been saying aerosol transmission doesn't occur, which is also perplexing,"Dr Cowling said, adding, "I think both are actually wrong."
Experts agreed that infections were being passed along by people who do not report symptoms - what they call asymptomatic transmissions - but they also noted some confusion around the term.
"There's no standard definition for it, and you could say to yourself, well, that's kind of ridiculous: You either have symptoms or you don't," said Dr Jeffrey Shaman, an infectious diseases expert at Columbia University.
But studies by his team have shown, he said, that some people never notice their symptoms, others are unable to distinguish the infection from their smoker's cough or allergies or other conditions, and still others may feel every pain acutely.
Where the definitions may matter is in being able to understand the true scope of the pandemic.
Dr Cowling's team has analysed data from China at various stages in the pandemic. The WHO's mission to China concluded that most people who were infected with the virus had significant symptoms. But in the early weeks of the epidemic, his analysis shows, China set a high bar for what constituted a confirmed case of infection - requiring respiratory symptoms, fever and a chest X-ray for pneumonia.
Their definition left out mild and asymptomatic cases and, as a result, the team vastly underestimated the scale and nature of the outbreak there.
"We've estimated in China that between 20 per cent and 40 per cent of transmission events occurred before symptoms appeared," Dr Cowling said.
A separate analysis of the hundreds of people cloistered aboard the Diamond Princess cruise ship bears out this scale. Once the ship docked in Japan on Feb 5, researchers tested all of the passengers and reviewed those who tested positive for the virus on multiple occasions over a two-week period. They found that 18 per cent of the infected passengers remained symptom-free throughout.
"The substantial asymptomatic proportion for Covid-19 is quite alarming," said Dr Gerardo Chowell, an epidemiologist at Georgia State University who worked on the analysis.
Dr Chowell noted that the passengers on the ship tended to be older and therefore more likely to develop symptoms. He estimated that about 40 per cent in the general population might be able to be infected without showing signs of it.
Several studies have shown now that people infected with the new coronavirus are most contagious about one to three days before they begin to show symptoms. This presymptomatic transmission was not true of the coronaviruses that caused severe acute respiratory syndrome (Sars) and Middle East respiratory syndrome (Mers).
"This is where we got very lucky with Sars, was that it really didn't transmit until after people were showing symptoms, and that made it much easier to detect it and shut it down with aggressive public health measures," said Dr Carl Bergstrom, an expert in emerging infectious diseases at the University of Washington in Seattle.
With the new coronavirus, there is transmission by healthy-seeming people, and often severe symptoms and a high fatality rate. "That whole combination makes it very, very tough to fight using standard public health measures," he said.