The Washington PostDemocracy Dies in Darkness

A quarter of those who contract the coronavirus may not show it — meaning it may be closer than you think

The Financial District of New York on Monday. (Gabby Jones/Bloomberg News)

Speaking to NPR on Monday, Centers for Disease Control and Prevention Director Robert R. Redfield explained that his agency continued to learn new information about the coronavirus, which is thought to have originated in China late last year and is spreading quickly in the United States.

“One of the [pieces of] information that we have pretty much confirmed now is that a significant number of individuals that are infected actually remain asymptomatic. That may be as many as 25 percent,” Redfield said. “That’s important because now you have individuals that may not have any symptoms that can contribute to transmission, and we have learned that in fact they do contribute to transmission.”

Those who ultimately do show symptoms, he added, might actually transmit the virus for two days before symptoms appear.

The upshot? Lots of people may be infecting others without even knowing that they’re sick. Lots of people.

As of writing, there are nearly 200,000 confirmed cases of the virus in the United States.

The country has focused primarily on testing those with symptoms and/or a known relationship to someone else who has been confirmed to have contracted the virus. That means that relatively few of the cases above are asymptomatic carriers.

If we assume that the number of confirmed cases accurately reflects the number of symptomatic cases in the country — not a fair assumption, given the slow pace of testing — and if we further assume that there is one asymptomatic case for every three cases that have been confirmed, we already have more than a quarter of a million cases in the country.

In the abstract, this is a subtle distinction. A jump from 0.06 percent of the country’s population to 0.08 percent doesn’t seem like a particularly broad expansion. But consider what it might mean on a local level.

Johns Hopkins University has been compiling county-level data on the number of confirmed cases. Grouped by the scale of cases, the spread of the virus looks something like this:

Introduce an estimate of those who are asymptomatic, and the increase even on local levels becomes obvious.

We can look at this another way. Darker colors on the map below indicate a higher population of individuals who have contracted the virus. When we transition from the hard count provided by Johns Hopkins University to a broader estimate of possible cases that include those without symptoms, even places without many cases become obviously denser.

The Washington Post’s Chris Ingraham estimates that about 6 percent of the country lives in a county in which there have been no confirmed cases of the virus. The other 94 percent demonstrably lives in relative proximity to someone who has tested positive.

It’s unlikely that the 6 percent who don’t live in affected counties have no exposure to the virus, given that no counties in the country are surrounded by impermeable barriers (to the best of my knowledge). Adding in the assumption that a quarter of those who have contracted the virus may not demonstrate symptoms — may not themselves even know — and proximity to the coronavirus narrows further. Even in the counties with no confirmed cases, there are probably cases.

This isn’t meant to be alarming. It should instead be a reminder that assumptions about the likelihood of possible exposure are limited by the fact that exposure may never obviously present itself.

You are being asked to distance yourself from other people because you may not know which people can give you the virus — and because you may not know that you’re giving other people the virus, either.