A new study, which hasn’t appeared yet, estimates that people who do not display any symptoms yet, transmit around 10% of infections. Until now, the coronavirus, SARS-CoV-2, has infected more than 200,000 people worldwide.
Because the virus is new, scientists are still trying to understand how it behaves. Their findings will help officials to create new ways to intervene and stem the flow, while also finding responses to the pandemic.
Scientists from the University of Texas at Austin are part of this activity and are trying to figure out how quickly the virus can spread. Their recent findings aren’t made public yet, but you’ll soon find them in the journal Emerging Infectious Diseases.
The serial interval
The research was conducted by scientists from the United States, France, China, and Hong Kong. They were very interested in finding out how quickly SARS-CoV-2 can spread.
When estimating how quickly a virus will spread, scientists pay close attention to two primary factors:
- The reproduction number — how many people each individual with the virus is possible to infect.
- The serial interval — the time between one person developing the symptoms of a condition and a second person becoming infected and developing symptoms.
The researchers found in this recent study that the serial interval for SARS-CoV-2 was approximately 4 days. They also 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 passes the virus to someone else, even if they do not show any symptoms. The new study is one of the first to analyze this phenomenon for SARS-CoV-2.
In their investigation, researchers studied 450 case reports from 93 Chinese cities. They were the ones who found the most convincing evidence to date that people can pass the virus to someone else before the symptoms appear, a scenario known as presymptomatic transmission.
The scientists measured that people who had not developed any symptoms transmitted around 10% of the cases studied by them. It isn’t clear yet what role does asymptomatic or presymptomatic transmission might play in the coronavirus pandemic. Their findings will help in making better predictions about how the virus will develop.
Some people who have contracted SARS-CoV-2 display no symptoms, and it is likely that only 1–3% of people are asymptomatic carriers. In addition, in all COVID-19 cases, there is a specific period of time between infection and the apparition of the 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 examined incubation time found that the median for developing symptoms is 5.1 days, and 97.5% of those developing symptoms do so within 11.5 days.
Moreover, understanding how easily these presymptomatic individuals can 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 new study also explained that, with more and more people contracting the virus, their latest estimates might change. Scientists are still working around the clock in order to understand how this virus works, but we have to wait for more data before we can make solid conclusions about the silent transmission rate.