There's lots of talk about when to re-open the US border. Here's an illustration of the numbers of #COVID19 infected travellers we'd expect to see crossing into #Ontario daily, assuming different levels of incidence in the US and numbers of cross-border travellers. 1/n

Right now the number of daily border crossings is low (probably >1000 and <5000) and the US reported 70,000 cases yesterday, so we'd expect to see a handful (~2-9) of COVID cases cross into Ontario each day based on the current situation. 2/n

We'd expect about 30% of those cases to show symptoms, while the rest would be incubating/pre-symptomatic/asymptomatic. We'd catch these extra cases with compliance to 14-day quarantine. 3/n

You can see though, that as the number of travellers into the province increases, we're looking at lots of imported cases each day. Once case burden in the US declines (e.g. 1000-10,000 case per day), we can ease border restrictions while keeping imported case numbers low. 4/n

Important to note that this is just looking at cases arriving in the province. Any of these imported cases that are not identified/isolated could potentially spark outbreaks with large numbers of downstream infections. 5/n

These estimates are based on a simple model, not accounting for heterogeneity in risk across the US or among those who choose to travel, but can give you a sense of the current risk at the border. You can play around with it here: https://art-bd.shinyapps.io/importation_risk/ if interested. /end