As US coronavirus cases skyrocket, daily deaths are staying flat. The key is 'to keep the young people away from the old people,' one expert says.

• US coronavirus cases are surging in a majority of states. Most of these new cases are among young people.
• However, the national number of daily deaths reported remains relatively flat.
• Experts say increased testing and the rising number of young people getting sick are the primary explanations for that.
• The trend could continue if young people engaging in riskier behavior stay away from older people who are more vulnerable, an expert says.

The coronavirus is once again spreading rapidly in the US, with 38 states reporting increases in their daily case numbers.

But one key metric remains relatively unchanged: the US's number of daily COVID-19 deaths.

Whereas new daily infections jumped from around 20,000 in early June to well over 40,000 this week, deaths have stayed between 500 and 1,000 per day throughout the last month.

Dr. Jeffrey Klausner, a professor of epidemiology at University of California, Los Angeles, said the primary reason for this is the changing demographics of coronavirus patients. Young people account for a large portion of the new infections, and they tend to experience more mild, less fatal cases.

"Age is by far the strongest predictor of mortality," Klausner told Business Insider. Indeed, about 80% of US coronavirus deaths through mid-June were people over 65.

By now, young people understand that, so they're more willing to leave their houses and socialize with friends.

"The thing now is to keep the young people away from the old people," Klausner said.

A shift from older people to younger ones

Dr. Anthony Fauci, director of the National Institute of Allergy and Infections Diseases, told Congress last week that the US's new surge may simply be too recent for daily deaths to rise significantly.

"Deaths always lag considerably behind cases," Fauci said.

Generally, official case numbers reflect infections contracted about two to three weeks prior, since the virus incubates for an average of five days, then people usually don't seek a test until a few days after symptoms start. Then it takes additional time for test results to be processed and reported. COVID-19 gets most severe around week two of a patients' symptoms, and hospitalized patients tend to go in around day 10 or 11. If a patient dies some time after that, it can take several days for that to be reported and added to the official numbers.

Indeed, daily deaths have started rising in both Arizona and Texas, states with some of the highest new case growth. Still, there's reason to think the demographic differences between this surge and the US's prior peak could keep the share of deaths lower than before.

From February through May, the CDC reported that the median age of confirmed COVID-19 patients in the US was 48. But in the last few weeks, officials in southern and western states have said that age is trending downward.

In Florida, the median age of coronavirus patients fell from 65 in March to 37 at the end of last month, according to the Orlando Sentinel. Minnesota saw the median age of its coronavirus patients drop from 44 last month to 38 this week.

In Arizona, people between 20 and 44 accounted for nearly half of all coronavirus cases last week.

South Carolina, meanwhile, reported a 413% increase in new cases among people between the ages of 21 and 30 from June 21 to 28. Texas Gov. Gregg Abbot said in mid-June that residents under 30 were testing positive at a higher rate than before there, too.

"It's much, much different than early on in the epidemic, when most cases were older people and nursing home residents," Klausner said.

'Younger people are more mobile'

Public-health experts have offered a few possible explanations for this age shift. One is that testing has become much more accessible in the US — from June 1 to 8, the US conducted 3.2 million tests, whereas by March 8, fewer than 2,000 tests had been administered. So young people are likely getting tested at higher rates than before. The fact that more of their cases are mild also probably also explains why a smaller share of cases so far are fatal.

Additionally, young people seem to be giving up on social distancing more than older groups.

Mike Ryan, executive director of the WHO's health emergencies program, said last week that rising cases among young adults might "reflect the fact that younger people are more mobile and are getting out and taking advantage of the reduction in the restrictions of movement."

In recent weeks, several US college towns have confirmed clusters tied to parties and bars. More than 100 cases were connected to a bar in Lansing, Michigan, CNN reported, and more than 15o cases in Boise, Idaho were linked to people who went to bars and clubs without realizing they were carrying the virus, according to Kaiser Health News.

Still, COVID-19 can hit young people hard, too.

Younger patients represent a growing percentage of coronavirus hospitalizations. People between 18 and 49 comprised about 27% of hospitalizations during the week ending March 7, but 35% last week, CDC data shows. According to the Washington Post, Arizona, Arkansas, California, North Carolina, South Carolina, Tennessee, and Texas all reported record hospitalizations at the end of June.

The challenge now is keeping the number of deaths low.

Keeping coronavirus deaths low is a multi-generational effort

Unlike young people, Klausner said, many older people are still social distancing and taking other precautions, since they know they're high-risk.

"People over the age of 70 or 75, they're avoiding most people, or they're taking protective measures," he said. "Older people are saying, 'Hey, you know, I'm not going to attend this family event,' or, 'I'm gonna keep my distance where it's possible.'"

The problems, he said, arise when young people who are taking more risks interact with older people who are vulnerable to severe illness. Multi-generational households present a particular challenge.

"In multiple generational households, it may be difficult for people to practically social distance themselves," Klausner said.

A record 64 million people, or 20% of the US population, lived with multiple generations under one roof as of 2016, according to the Pew Research Center.

Plus, even if young people are willing to take the risks that come with meeting up at bars and parties, the essential workers serving them and interacting with them don't get to make that choice.

"They may be indirectly hurting people by infecting someone who then infects someone, who then infects someone who's vulnerable," Fauci said.

Aylin Woodward contributed reporting.