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, also called Typhoid Mary, 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 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 tell-tale 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 published April 16. 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 classical 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.