Against a backdrop of justified unrest across the US - as different places ‘open up’ to a greater or lesser extent, there remains a lot of confusion about what to expect if the effective reproductive number R(t) (also Re or just R) ticks above 1. incoming 1/n

There seems a tendency to think that this will lead to an imminent second wave, or nothing much because of the better weather. In fact what we expect is something between the two. If R is greater than 1 that means exponential growth, but not an *immediate* surge 2/n

We have to take account of the time between infections. The serial interval (the time between cases in a transmission chain) is about 5-6 days. So if R=1.1 and you have 10 infections now, in about a week you expect them to have transmitted to about 11 people. Not a big jump 3/n

That's not a lot more, but a change like that’s not something you’re gonna notice that quick, especially with the sketchy surveillance we have right now 4/n

Test Trace and Isolate is important for controlling disease, it is not a way to measure how much transmission is going on right now, or in the future. If there's enough disease going on there may well be more contacts than contact tracers available to test them 5/n

Let’s look at it instead in terms of the doubling time. How long until you will see twice as many new infections each week? Well depending on the serial interval, for R=1.1 it could be a month or so. Remember there’s no adequate surveillance of transmission in the community 6/n

But the larger R gets, the shorter the doubling time. Remember it take time for infections now to show up in the ERs, and so even relatively large shifts in R will not be immediately apparent 7/n

The places with the worst surveillance, will be worst placed to see anything coming their way. Meaning that they could be building a powderkeg of infection and know nothing about it because they’re not looking 8/n

We know where that leads (NYC, NJ, MA more to come). Let’s not do that again. Let’s be ready to take action to control the pandemic *before* it gets bad, not after. It's not going away, and even if there's a respite in summer, winter is coming 9/end