Turn on the TV news, or look at a news website, and you’ll see charts, graphics, and dashboards that supposedly indicate the latest with COVID-19 – statistics revealing the number of tests, cases, hospitalizations and deaths, along with where they happened and whether they are rising or falling.

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Different stories are told depending on the dashboard. But one thing is certain: These indicators lag behind the actions we take, or don’t take, on COVID-19. As researchers who focus on public health, we can tell you that a fully accurate, real-time snapshot of the progress of the virus isn’t possible.

Some don’t get tested

There are many reasons for this. Here’s one: diagnostic testing data are incomplete. Someone infected with COVID-19 must first come in contact with the virus either through the air or (less likely) environmental surfaces. Symptoms show between two and 14 days later. But at least 40% of those infected will never manifest symptoms, or show such mild ones they don’t even suspect they have COVID-19. So they may never get tested, which means they won’t show up in the total number of tests, or the total number of cases.