Which Covid-19 Numbers Really Matter

This is what to pay attention to, according to an epidemiologist

For a layperson, national, state, and local Covid-19 data pages seem pretty straightforward (high numbers are bad! low numbers are good!) — until you really start thinking about what this data means for your health and your behaviors. Which numbers are actually important? How do you interpret an increase or decrease in Covid-19 cases or test positivity rates? And how should the data inform what actions you should — or shouldn’t — be taking? To get some clear guidance on how to parse it all, Elemental spoke to Eleanor Murray, ScD, assistant professor of epidemiology and co-director of the Epidemiology Covid-19 Response Corps at the Boston University School of Public Health.

This interview has been edited and condensed for clarity.

Elemental: Where is the best place to find Covid-19 data for your area? And how local should you be going?

Eleanor Murray: For most places, your state health department is a good first stop, but many states are so large that this won’t be specific enough. Local data from your city (or from your metropolitan area in larger cities like New York) will be more useful in those states. Your local health department may have a website that provides these numbers, or your state health department website might provide local numbers as well.

Aside from just generally staying up to date about what’s happening in your area, when is it useful to look at local Covid-19 data?

Local Covid-19 data is most useful when you are making decisions about whether to engage in higher risk activities, such as indoor dining, attending religious services in person, or visiting friends or family in their homes. When Covid-19 rates are low in your area, these may be reasonable to do (potentially with some safety precautions such as masks), whereas during a surge they should be avoided.

What are the most important numbers to look at? What’s the first number you check for your area every day?

I look at a couple of numbers: What are the per capita new case counts and — most importantly — how do they seem to have been changing over the past few days, and what are the test positivity rates? Test positivity rates can give you an indication of whether there is enough testing being done. We want to see those numbers below 2%–5% to indicate that there is not an outbreak happening and that sufficient testing is being done. Numbers closer to 10% indicate that there is under-testing, and numbers around 20% or above indicate a substantial surge in cases is probably happening. If cases and positivity are increasing or decreasing then that’s a good indication of whether your area is high or low risk at this time.

How do you tell if things are “good” or “bad”? How big of an increase or decrease really matters?

The trickiest part about assessing the numbers is that we are always playing catch up. Deaths tell us about infections that happened a month or even more in the past, hospitalizations tell us about infections from a few weeks ago, and even cases can be misleading since people can be infectious before they have symptoms. We also see a lag in reporting, so that the numbers reported today aren’t actually the final numbers for today — over the next few weeks today’s numbers will change. This means that even if you thought cases were going down, once you get a few weeks out you can look back at the graphs and realize they were actually going up. Because of this, it’s generally a good idea to take the maximum precautions suggested by the past few weeks of case data.

If you’re considering traveling, what should you know about where you’re coming from and where you’re going, and how might those numbers affect your plans?

When traveling, you definitely want to avoid moving between high and low case count regions, because this could lead to causing a new surge somewhere. In general that means you should really only travel if the cases are low where you are and have been low for the past couple weeks — and then only travel to and through other low case count areas! If cases are high where you are or where you want to go, it’s inadvisable to travel where at all avoidable.

How should you read/interpret the death rate and the hospitalization rate? Do those matter on a day-to-day basis for a layperson?

Hospitalization and death rates are really more of a measurement of how poorly the government is doing in controlling this pandemic. Doctors have gotten a bit better at treating patients who need to be hospitalized but in general the risk of death from Covid-19 is pretty close to where it was. Changes in the death rate over time mostly reflect changes in who is getting infected rather than any indication of disease severity. And hospitalization and death lag infection by so much that we really should not be looking at those numbers to think about how much risk we might be at on a day-to-day basis.

Editor and writer. Past: Elemental, Real Simple, Refinery29, SELF. Certified personal trainer; prenatal and postnatal exercise specialist. Cat & person mom.

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