A new study— which has yet to appear — estimates that people who do not yet display any symptoms transmit around 10% of infections.
The coronavirus, SARS-CoV-2, has infected more than 160,000 people worldwide.
Because the virus is novel, scientists are scrambling to understand how it behaves. Their findings will guide responses to the pandemic and help officials design ways to intervene and stem the flow.
Scientists from the University of Texas at Austin are part of this frenzied activity and are trying to understand how quickly the virus can spread. Their recent findings are awaiting publication in the journal Emerging Infectious Diseases.
The research involved scientists from the United States, France, China, and Hong Kong. They were particularly interested in understanding how quickly SARS-CoV-2 might spread.
When scientists are estimating how quickly a virus will spread, they pay attention to two primary factors:
1. The reproduction number — or how many people each individual with the virus is likely to infect.
2. The serial interval — or time between one person developing the symptoms of a condition and a second person becoming infected and developing symptoms.
The researchers in the latest study found that the serial interval for SARS-CoV-2 was approximately 4 days.
They explain that a shorter serial interval makes an epidemic harder to contain and more likely to spread quickly.
“Ebola, with a serial interval of several weeks, is much easier to contain than influenza, with a serial interval of only a few days. Public health responders to Ebola outbreaks have much more time to identify and isolate cases before they infect others,” explains Prof. Lauren Ancel Meyers from the University of Texas at Austin. “The data suggest that this coronavirus may spread like the flu. That means we need to move quickly and aggressively to curb the emerging threat.”
A “silent transmission” occurs when someone who has contracted the virus shows no symptoms but passes the virus to someone else. The current study is one of the first to examine this phenomenon for SARS-CoV-2.
To investigate, the researchers studied 450 case reports from 93 Chinese cities.
They found the most convincing evidence to date that people can transmit the virus before symptoms begin, a scenario that is also known as presymptomatic transmission.
The authors estimate that people who had not yet developed symptoms transmitted around 10% of the cases they studied.
It is not clear how much of a role asymptomatic or presymptomatic transmission might play in the coronavirus pandemic. These findings might help make better predictions about how the virus will behave.
Some people who have contracted SARS-CoV-2 display no symptoms, and it is likely that only 1–3% of people are so-called asymptomatic carriers.
However, in all cases of COVID-19, there is a lag between infection and the start of symptoms, which is known as the incubation period.
The time between transmission and symptoms could be anything from 2–14 days.
A recent study that investigated incubation time concludes that the median for developing symptoms is 5.1 days, and 97.5% of those developing symptoms do so within 11.5 days.
Understanding how easily these presymptomatic individuals transmit the virus is important.
“This provides evidence that extensive control measures, including isolation, quarantine, school closures, travel restrictions, and cancellation of mass gatherings may be warranted. Asymptomatic transmission definitely makes containment more difficult.”
– Prof. Lauren Ancel Meyers
The authors of the current study also make it clear that, as the numbers of people who contract the virus rises, their latest estimates might change.
With scientists working around the clock to understand how the virus works, we will need to wait for more data before we can make solid conclusions about silent transmission rates.