Early on in the COVID-19 pandemic, there were a lot of big questions about the basic properties of SARS-CoV-2: how quickly did it spread, could it spread from asymptomatic people, what was the typical mortality rate, and so on. We quickly started getting answers on some of these, but they were all imperfect in various ways. We could trace all the cases in controlled environments, like a cruise ship or aircraft carrier, but these probably wouldn’t reflect the virus’s spread in more typical communities. Or, we could trace things in real-world communities, but that approach would be far less certain to capture all the cases.
Over time, we’ve gotten lots of imperfect records, but we’ve started to build a consensus out of them. The latest example of this—a paper that describes contact tracing all cases that originated in Hunan, China—provides yet another set of measures of the virus’s behavior and our attempts to control infection. Papers like this have helped build the consensus on some of the key features of things like asymptomatic spread and the impact of contact tracing, so we thought it was a good chance to step back and look at this latest release.
Trace all the cases
The new work, done by an international team of researchers, focuses on the spread of SARS-CoV-2 in Hunan Province during the first outbreak after its origins in nearby Hubei. During the period of study, health authorities started by identifying cases largely by symptoms, and they then switched to a massive contact tracing effort and aggressive isolation policies. These efforts shut the outbreak down by early March. And, thanks to them, we have very detailed information on viral cases: 1,178 infected individuals, another 15,648 people they came in contact with, and a total of nearly 20,000 potential exposure events.