For weeks now, America’s leaders and its public have been obsessed with one set of numbers: How many people have died? How many confirmed cases? And in what states?

But to understand why experts are so alarmed and what may be coming next, the public needs to start paying attention to a whole other set of numbers: How many ventilators do we have in this country? How many hospital beds? How many doctors and nurses? And most importantly, how many sick people can they all treat at the same time?

Consider the ventilators

For those severely ill with a respiratory disease such as covid-19, ventilators are a matter of life or death because they allow patients to breathe when they cannot on their own.

In a report last month, the Center for Health Security at Johns Hopkins estimated the United States has a total of 160,000 ventilators available for patient care (with at least an additional 8,900 in the national stockpile).

A planning study run by the federal government in 2005 estimated that if the United States were struck with a moderate pandemic like the 1957 influenza, the country would need more than 64,000 ventilators. If we were struck with a severe pandemic like the 1918 Spanish flu, we would need more than 740,000 ventilators — many times more than are available.

The math on hospital beds isn’t any better

The United States has roughly 2.8 hospital beds per 1,000 people. South Korea, which has seen success mitigating its large outbreak, has more than 12 hospital beds per 1,000 people. China, where hospitals in Hubei were quickly overrun, has 4.3 beds per 1,000 people. Italy, a developed country with a reasonably decent health system, has seen its hospitals overwhelmed and has 3.2 beds per 1,000 people.

The United States has an estimated 924,100 hospital beds, according to a 2018 American Hospital Association survey, but many are already occupied by patients at any one time. And the United States has 46,800 to 64,000 medical intensive-care unit (ICU) beds, according to the AHA. (There are an additional 51,000 ICU beds specialized for cardiology, pediatrics, neonatal, burn patients and others.)

A moderate pandemic would mean 1 million people needing hospitalization and 200,000 needing intensive care, according to a Johns Hopkins Center for Health Security report last month. A severe pandemic would mean 9.6 million hospitalizations and 2.9 million people needing intensive care.

Now, factor in how stretched-thin U.S. hospitals already are during a normal, coronavirus-free week handling usual illnesses: patients with cancer and chronic diseases, those walking in with blunt-force trauma, suicide attempts and assaults. It’s easy to see why experts are warning that if the pandemic spreads too widely, clinicians could be forced to ration care and choose which patients to save.

No one knows how bad it will be

This is where we need to say that no one knows how bad this is going to get. But, as many experts have pointed out, that is part of the problem.

“The problem with forecasting is you have to know where you are before you know where you’re going and because of the problems with testing, we’re only starting to know where we are,” said Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security.

The speed at which the number of U.S. cases is rising hints we are headed in a bad direction.

But because so much is still unknown, exactly how bad could range widely. It will depend largely on two things: The number of Americans who end up getting infected and the virus’s still-unknown lethality (its case-fatality rate).

One forecast, developed by former CDC director Tom Frieden, found that infections and deaths in the United States could range widely. In a worst-case scenario, but one that is not implausible, half the U.S. population would get infected and more than 1 million people would die. But his model’s results varied widely from 327 deaths (best case) to 1,635,000 (worst case) over the next two or three years.

This is why experts have been yelling so much about testing, social distancing and hand washing

“Slowing it down matters because it prevents the health service becoming overburdened,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. “We have a limited number of beds; we have a limited number of ventilators; we have a limited number of all the things that are part of supportive care that the most severely affected people will require.”

The sooner you interrupt the virus’s chain of transmission, experts say, the more you limit its climb toward exponential growth. It’s similar to the compounding interest behind all those mottos about invest when you’re young. Early action can have profound effects.

That math is also why so many health officials, epidemiologists and experts have expressed frustration, anger and alarm over how slowly the United States as a country has moved and is still moving to prepare for the virus and to blunt its spread.

This post has been updated to include more recent numbers on U.S. hospital beds from the American Hospital Association, including the number of specialty intensive care beds available.

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