A lot of people infected with the coronavirus have very mild or even no symptoms, or ones that don’t match the usual markers of fever, dry cough or difficulty breathing. The discovery of larger numbers of so-called asymptomatic cases, initially thought to be rare, underscores a key challenge in stopping the pandemic: If people don’t know they’re infected, they’re probably not taking steps to prevent transmitting it.

1. Why is this virus different?

Peak concentration of the coronavirus in the upper airway seems to occur around the time an infected person first develops symptoms. That’s usually when a person is most contagious -- coughing, sneezing or otherwise shedding virus. With this virus, however, a carrier is also capable of transmitting it even before getting the telltale signs of infection, as some early studies from China, Germany and Singapore indicated. And some people with the virus never feel ill, yet have been implicated in its spread. The extent to which these “silent carriers” may also be “silent spreaders” isn’t clear. It’s difficult to pinpoint the source of an individual infection, especially as the virus becomes entrenched in communities. Researchers at the University of Oxford estimated in March that a third to a half of transmissions occur from pre-symptomatic people.

2. How many asymptomatic cases are there?

The number is “significant,” U.K. doctors wrote in a letter to the Lancet medical journal in April. A lack of resources for testing in many places, at least in the initial weeks of the pandemic, meant screening has focused on sick people displaying classic symptoms of Covid-19, as the disease is called. When health authorities have widened their testing, they typically find many more positive results than they had assumed. A study of cases aboard the Diamond Princess cruise ship, which docked in Japan, found that 328 of the 634, or 52%, of people who tested positive had no obvious symptoms at the time of testing. A retrospective analysis found that 10 of 43 of those asymptomatic cases did develop symptoms over a 15-day period. Testing in China, the Netherlands and Iceland, on board a U.S. aircraft carrier and among obstetric patients in New York City, found anywhere from 43% to 88% of people testing positive were asymptomatic at the time.

3. Are such cases truly asymptomatic?

No. Data from China indicate that at least a fifth of patients who have no signs of Covid-19 at the time of testing positive are incubating the disease and do develop symptoms days later. Some people with the coronavirus develop atypical symptoms -- ones that don’t meet the definitions health authorities use to identify likely cases. These signs include diarrhea and the abrupt loss of smell and taste. That’s a big change from back in February when a joint World Health Organization-China mission concluded that asymptomatic infection “appears to be relatively rare.” The WHO said in April that the risk of catching Covid-19 from someone with no symptoms at all is “very low.”

4. Why is this a problem?

Some researchers have suggested that unrecognized carriers may be a reason why this coronavirus has spread extremely quickly relative to others, such as the one responsible for severe acute respiratory syndrome, or SARS, almost 20 years ago. Containment measures are predicated on early detection and isolation, but patients with mild, non-specific, or no symptoms are more difficult to identify. Such cases can also increase the risk to health-care workers if a patient undergoes certain high-risk medical or surgical procedures, such as a colonoscopy. Fourteen hospital staff were infected after a patient in China tested positive days after surgery, and they had failed to take necessary infection-control precautions.

5. What can we do about it?

Researchers at the Yale School of Public Health showed in July that solely isolating symptomatic patients wouldn’t be enough to prevent a post-lockdown resurgence. They emphasized the need for rapid contact tracing and testing to identify asymptomatic carriers. Where tests aren’t widely available, X-rays may help. Italian researchers found chest images can be used to identify probable cases among people displaying mild or no symptoms. Secondly, health officials recommend wearing face masks or cloth barriers covering the nose and mouth to minimize both the projection of virus-carrying respiratory secretions from an infected person and the risk of a non-infected person inhaling virus particles. Thirdly, practice physical distancing and avoid congregating in large groups.

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