An early warning for the United States
Around twelve days ago the US, as a whole, went above Reff=1.0.
On 1 September in our blogpost "Late summer upsurge on two continents" the US Reff was below 1.0 but we listed 22 US States with Reff above the redline of 1.0. here. Today, we report that COVID-19 spread is steadily increasing across the US here. Now less than 20 States have reproduction numbers below 1.0.
It is relatively early in the upswing with the US going above Reff=1.0 in the second week of September so we feel it is too early for definitive predictions. But the longer it persists above Reff=1.0 the more likely the US will follow European States and the United Kingdom into a second wave, as the Northern Hemisphere prepares for colder weather.
The US reporting of new daily cases by State and by County means our model can provide the most sophisticated analysis available. Canada and Australia provide State and Provincial data, but is not down to the county level, or its jurisdictional equivalent. Europe States have this data but it is not being distributed by the European Centre for Disease Control.
Our model calculates Reff fairly accurately down to a population of 300,000 people so the more localised the data the better to understand the disease spread and the more information for politicians and policymakers to address the threat. As an example, take Utah which as a State currently has Reff=2.3 and climbing. The county data below shows the challenge is in two counties: Salt Lake and Utah. Salt Lake City and Provo city is where there are issues.
Provo is a college town and so we can speculate that this surge is due to the commencement of the new academic year and students arriving from far and wide without defences that University of Illinois at Urbana Champaign (UIUC) has established. See here for more information on the UIUC Shield program.
You can see below in the screenshot and here that 46,000 returning students to UIUC from all over the world probably created the conditions for the spike in late August. The rapid testing and self isolation program deployed on campus probably got that surge quickly under control. [Caveat: Our comments on Provo and Champaign counties are not based on a rigorous study and should be considered anecdotal only.]