This causes a challenge for distributing the drugs, since they are only authorized for people who are not hospitalized, yet must be infused intravenously in a clinic or hospital.
Regeneron has suggested that the people who benefit the most from the treatment are those who have not yet mounted an antibody response and who also have high viral loads -- but learning who those people are would require separate tests that are not routinely given to patients who test positive for the virus. Company executives have acknowledged that at first, such tests may not be available, and the emergency authorization notes that people at high risk include those who are over 65, or have underlying conditions such as obesity or diabetes.
Getting the treatment to the right people will require quick turnarounds in testing, as well as coordination among federal, state and hospital officials -- many of the same challenges that have complicated the U.S. response to the pandemic.
The distributor AmerisourceBergen will deliver Regeneron's treatment on a weekly schedule based on the number of Covid-19 cases in each state. The federal government plans to work with state health officials to determine which hospitals and clinics should receive it.
In a call with reporters this month to outline how the Eli Lilly treatment would be distributed, Janet Woodcock, a top federal drug official, said the administration was working with hospitals and infusion companies, and acknowledged the logistical complexity of a drug that takes an hour to administer, followed by an hour of observation. The hospitals and clinics will also have to figure out how to safely treat infectious patients without exposing other people to the virus.
"We're all going to need to get the word out that people at high risk have a therapeutic option now as outpatients, because until this point people have been told to stay home unless they get very sick," she said.
The emergency use authorization, or E.U.A., was previously an obscure corner of regulatory law that mostly escaped broad public attention. But during the pandemic, it has become a centerpiece of the administration's health policy: Since February, the agency has granted hundreds of emergency authorizations related to Covid-19, many for diagnostic tests and others for personal protective equipment, blood-purification devices, ventilators and therapies.
In signing off on the emergency authorizations, F.D.A. scientists in the agency's Office of Infectious Diseases had to weigh the need for clear evidence that the treatments worked with the increasing desperation for useful drugs as the pandemic accelerated again in the United States. Unlike a full F.D.A. approval, which requires a rigorous vetting of clinical trial data showing a drug is safe and effective, an emergency authorization simply requires the potential benefits of a drug to outweigh its risks.
The emergency authorization for Regeneron may still have the unintended effect of complicating the role of clinical trials in proving the treatment's effectiveness for different age groups. If the drug becomes available more broadly, fewer people may want to sign up for clinical trials and risk the possibility that they may receive a placebo.
Regeneron's clinical trials are continuing in hospitalized patients as well as in studies looking at whether the treatment can prevent infections in people who have been exposed to someone with Covid-19.