We’re swarmed with coronavirus data – everything including new case counts, disease projections, positive tests, and hospital utilization.
But in the end, only one number matters, and that is Rt – the current reproductive rate. In other words, the number of people each person with the disease infects. Public health, the economy, and the speed of recovery really tie to whether the number of infected people is growing or shrinking.
The basics are pretty simple. If Rt = 1, each person infected with COVID will infect one other person. If Rt < 1, each person infected with COVID infects less than 1 person (in other words 100 people with COVID might only infect 50, if Rt = 0.5). If Rt > 1, each person with COVID infects more than one person.
If Rt < 1, the disease eventually fades away. If Rt > 1, the disease will continue to expand, potentially uncontrollable. If Rt hovers around 1, we’ll stay at status quo.
That’s why anyone paying attention should be concerned with New Jersey’s recent revelation that Rt is now at 1.35. Even New Jersey, which for several weeks had COVID largely under control, risks seeing increasing cases. With Rt above 1, those increases will start to compound, and the pandemic will get worse.
If you’re running a business depending on physical assets or public space, and you want to have a read on the future, Rt is really the number to watch. Even if cases are low, if Rt > 1, the disease will start to spread faster.
Forget the unemployment claims number or GDP – the economy is driven by the pandemic, and the best way to measure the pandemic’s trajectory is Rt. One would think CNBC would be reporting Rt every morning. It would not surprise me if some smart quant trader has built it into a model somewhere.
While the media has covered R, the coverage seems less than Rt’s importance warrants. Instagram founders Kevin Systrom and Mike Krieger have created a site, rt.live, to track this on a state by state basis.
While a lot of a disease’s transmission rates is tied to the biology, Rt is somewhat controllable. If everyone wears masks, Rt goes down. If people social distance, Rt goes down. If asymptomatic people with COVID hang out in bars or house parties without masks, Rt goes up.
When you look at rt.live on August 1, the data is mediocre. Few states are significantly below 1, and several are above 1.1.
The good news is the Rt of 2 or 3 that many states were seeing in March are well past. Even states with haphazard reopenings stayed below 1.5 during May and June and now as they take better precautions and the disease has started to run its course, Rt is back down near 1.
However being at 1 with a high number of daily new cases isn’t great either – it just means the plateau will be higher. So now, as a country, many states have thousands of new cases a day with limited prospects of seeing the numbers go down.
Despite early challenges, New York handled its shutdown and reopening well. It drove Rt down to 0.7, vastly reducing the disease’s spread. Then it reopened slowly, keeping bars and indoor dining closed in New York City. The net result is is low cases, Rt hovering slightly under 1, and at least outdoor activities open.
The tragedy is if policymakers in other states were watching Rt more closely, they may have tailored the reopening strategies (i.e., masks, limited indoor dining) to keep Rt near 1, thus avoiding their subsequent spikes. Perhaps, then we all wouldn’t be fretting over whether we can safely open our schools.
Why Rt isn’t the first number everyone is reporting baffles me. Maybe it is too hard to understand. But the bottom line is that until Rt goes significantly below 1 for a long period of time, COVID will not go away.
Even a vaccine won’t make COVID go away. It only helps drive down Rt due to more people being immune, which eventually reduces or eliminates the disease.
There is a country that is managing their COVID response explicitly by Rt, Germany. As early as April, Angela Merkel explained the concept succinctly. Not surprising, given the data and science based approach she has taken from the beginning – and Germany’s relatively effective handling of the pandemic.
The Great Refactoring 