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Two weeks ago, the message from blood banks and hospitals was dire: Mass closures and disintegrating blood drives were creating a potential death spiral for perishable supplies nationwide as the coronavirus pandemic surged.

Amid a respiratory-infection outbreak that has little to do with blood, doctors pondered the horror of treating patients who need transfusions with no blood to replenish their bodies. “I am looking at the refrigerator that contains only one day’s supply of blood for the hospital,” a Michigan doctor warned on March 17.

But pleas to the public to help prevent a crisis by volunteering to donate blood en masse appear to have worked, giving hospitals and blood banks some relief. Still, there is growing concern that unexpected developments during the pandemic could again threaten the supply.

“People have come out in significant numbers. It has been incredible to watch,” Paul Sullivan, senior vice president of the American Red Cross, said Tuesday in an interview, describing nationwide supplies as “adequate” for the time being.

“We say that cautiously, because we don’t know what will come,” he said.

Initial fears were prompted by a wave of closures of schools, businesses and places of worship, effectively ending the most crucial mover of blood — big public drives that supply the Red Cross with more than 80 percent of its stock, with blood banks providing similar numbers.

The collection of about 350,000 pints of blood slated for thousands of Red Cross drives was canceled from mid-March through April, Sullivan said, driving worry that a once-reliable stream will not soon return.

The emergency call for donors led to waves of volunteers braving public places to give blood in their communities. Reduced capacity has led to a glut of donors, sending some Red Cross appointments into late April.

Blood blanks have stressed the need to stretch donations into the future. Yet as the virus continues its deadly march, potential donors and blood bank staffers could be taken out of commission.

“Donors are calling us now, and we are scheduling appointments two to eight weeks away,” said Rick Axelrod, president and chief executive of Lifestream Blood Bank, which serves 80 hospitals in Southern California.

Individual donors have filled the gaps left by big drives, and with demand dropping, supplies are good “right now,” Axelrod said — with a caveat.

“It’s ‘right now’ because we’re really worried about four weeks from now,” he said.

Donation centers have transformed their spaces, ensuring that donor beds are six feet apart, metering appointments and cleaning donor stations more often — and more visibly — to quell concerns and protect donors and staffers, Sullivan said.

A refreshed supply was aided by other consequences of the viral outbreak.

Elective surgeries have dipped as hospitals conserve manpower, and the number of trauma patients has declined because people are driving less and having fewer workplace accidents, said Baruch Fertel, an emergency physician at the Cleveland Clinic in Ohio.

The hospital itself solicited donations, creating a mini-surge of supply, and algorithms developed amid potential shortages help doctors manage their stock, Fertel said. But even with the falling number of trauma cases, women are still hemorrhaging in childbirth, people are still being shot, and cancer patients still need transfusions.

“If we keep focus on donations, and judicious use, we can stay out of trouble,” Fertel said. “But we’re not out of the woods. Folks who are healthy and recovered should consider giving blood.”

Peter Paige, the chief physician executive for the Jackson Health System in Miami, said his doctors have not been in the habit of wasting blood, but concern about a supply disruption has led to a “heightened level of awareness as we prepare for potential decrease,” he said.

Blood is perishable by nature, and the expiration of products extracted from a pint of blood varies widely. Red blood cells last about 42 days, Sullivan said, and plasma can be frozen.

But platelets, the tiny cells in blood that help it clot — a lifeline for trauma patients and chemotherapy treatments — last only five days. The real number after collection and transport is about three days, Sullivan said, making constant donations critical.

The uncertainty has cast a pall over several blood banks that say that while supplies are adequate for now, accelerating coronavirus infections could increasingly limit would-be donors’ ability to get to blood drives.

Diversifying pockets of donors has become critical, said Nicholas Canedo, a spokesman for We Are Blood, a blood bank that serves 40 hospitals in Central Texas. Donors have to wait two months to give blood again, so the bank’s fleet of buses has been instrumental in reaching untapped communities.

“We’re all very inspired and feeling pretty good about the community reaction so far,” he said. “But we don’t know what the situation will be like in the next weeks and months, and whether that will affect donor turnout.”

Some blood banks have been overwhelmed with support.

The New York Blood Center, part of a conglomerate that supplies hospitals across 13 states, said that supplies were “excellent” and that it even turned away prospective donors in New York City, asking them to come back later, given the center’s limited capacity. Even then, replenishment of reserves is not guaranteed, said Andrea Cefarelli, the center’s senior executive director.

“Hopefully we don’t have to sound the alarm three weeks from now,” she said.

But even after the coronavirus pandemic, another crisis may loom, given the virus’s deadliness among older people. Consider the frequency of donors by age to be shaped like a horse saddle, Sullivan explained. At one end, people older than 65 make up a big percentage of donors, with a drop in the middle.

The other rise is among high school and college students, who turn out in a big way on campuses. When blood banks ask donors where they first gave blood, donors most often say in the military or in high school, Axelrod of Lifestream said.

But school closures now may snuff out an entire generation’s first time experiencing the satisfaction of possibly helping to save lives by giving blood.

“People have said, ‘Why not recruit high school kids, they don’t have school,’” Axelrod said. “But the problem is, they didn’t donate, so we don’t have their names.”

To find a blood donation center, visit the American Red Cross or an independent blood center.