The Food and Drug Administration authorized the experimental treatment, called bamlanivimab, for use against mild-to-moderate Covid-19 in adults, including those who are 65 and older, and pediatric patients, the agency said in a statement.

Shares of Indianapolis-based Lilly gained as much as 5.2% in late trading on Monday. Through the close, the stock had advanced 8.3% so far this year.

The clearance gives doctors an option for tackling the virus in high-risk patients before they’re sick enough to require hospitalization. Other treatments that received the regulatory go-ahead, such as convalescent plasma, a component collected from the blood of recovered Covid patients containing immune factors, and Gilead Sciences Inc.’s antiviral remdesivir, are intended for use in severely ill Covid-19 patients.

The U.S. government will pay Lilly $375 million for 300,000 vials of the antibody treatment. The initial agreement is for delivery over the first two months following a regulatory green light. The U.S. also has the option to purchase an additional 650,000 vials through next June for as much as $812.5 million.

Now begins an even greater challenge: Keeping pace with demand.

“We’re in the middle of this surge in cases, so we have to continue to try to squeeze out as much supply as we can,” Chief Executive Officer David Ricks said in an interview. “We’re loading the trucks right now, so that they can move as fast as they can. We’ve made 88,000 doses that are being loaded tonight, and we about that much inside of a week shipping out to centers across the country.”

The drugmaker has a manufacturing partnership with Amgen Inc., among others, and plans to pursue more collaboration pacts in order to increase supply, Ricks said.

“It still might not be enough,” he said.

Bridge to Vaccine

Experimental antibody treatments could become a powerful component of the arsenal that doctors use to treat the coronavirus. Anthony Fauci, the top U.S. infectious-disease official, has referred to antibody-based medicines as a bridge to a vaccine. The class of treatments was thrust into the spotlight when President Donald Trump received an antibody cocktail made by Regeneron Pharmaceuticals Inc. after becoming infected with Covid-19.

The therapies, which rely on lab-made proteins that mimic the immune system’s ability to fight off the virus, also are being studied as short-term treatments that could be given to people such as nursing-home residents or staff who may have been exposed during a local outbreak to prevent them from getting sick.

Lilly approached the FDA for authorization of the single-antibody treatment it’s developing with Canadian biotech AbCellera Biologics Inc. last month. Meanwhile, U.S. regulators have yet to decide whether Regeneron’s antibody cocktail will get an emergency-use green light.

Lilly and closely held AbCellera’s monotherapy antibody treatment reduced the rate at which symptomatic patients were hospitalized or sent to emergency rooms compared with a placebo, according to interim study results released by the company in September. Trial results published in the New England Journal of Medicine showed the therapy reduced patients’ viral load at the middle of the three doses studied.

Complex Infusions

The U.S. will be responsible for allocating the therapy, Lilly said in a statement, and will make weekly decisions on where to send the product proportional to the confirmed number of cases in each state and territory over the previous seven days, based on data from the U.S. Department of Health and Human Services.

The antibody therapy should be administered “as soon as possible after a positive Covid-19 test and within 10 days of symptom onset,” according to the company. It will begin shipping the antibody therapy immediately to AmerisourceBergen Corp., which will distribute it as directed by the U.S. government.

Americans will have no out-of-pocket costs for the medicine, the company said, while acknowledging that health-care facilities may charge a fee for the product’s intravenous administration.

The arrival of monoclonal antibody therapies is welcome, but administering the drugs creates hurdles for hospitals and health systems, said Nancy Foster, vice president for quality and patient safety policy at the American Hospital Association.

The therapies require infusions for patients, delivered in the same way as chemotherapy medicines, but health systems can’t use the same facilities for Covid-positive patients and cancer patients with weakened immune systems, she said in an interview Monday before news of Lilly’s authorization.

Hospitals may have to find spaces “similar to but separate from” existing infusion centers, and dedicate staff and protective gear, she said. “You have to think through, how are we going to get the infusion therapy and patient and the caregiver all together in the same room,” she said.

Terminated Trial

Infectious disease experts say antibody drugs are most likely to be useful if they are given early on in the course of infection. That idea was bolstered on Oct. 26, when a government-sponsored trial of the Lilly antibody in hospitalized patients—who tend to be sicker—was terminated due to lack of efficacy.

The FDA said Monday that bamlanivimab is not authorized for patients who are hospitalized due to Covid-19 or require oxygen therapy after contracting the disease. It said the broader class of monoclonal antibodies may be associated with worse clinical outcomes when administered to hospitalized Covid-19 patients in need of high flow oxygen or mechanical ventilation.

Lilly is also studying a cocktail of two antibodies. It expects to approach regulators for authorization in November and seek full approval in the second quarter of 2021. Early results from a trial showed the combination reduced virus levels in patients with mild to moderate Covid-19 and cut the rate of hospitalizations and emergency room visits.

Other companies testing antibody treatments include Regeneron Pharmaceuticals Inc., AstraZeneca Plc and GlaxoSmithKline Plc and its partner Vir Biotechnology Inc.

–With assistance from Angelica LaVito and Robert Langreth.