US health officials have agreed to allow emergency use of a second antibody drug to help the immune system fight Covid-19, an experimental medicine that President Donald Trump was given when he was sickened last month.

The Food and Drug Administration (FDA) authorised use of the Regeneron Pharmaceuticals Inc drug to try to prevent hospitalisation and worsening disease from developing in patients with mild-to-moderate symptoms.

The drug is given as a one-time treatment through an IV.

The FDA allowed its use in adults and children 12 and over who weigh at least 88 pounds (40 kilograms) and who are at high risk of severe illness from Covid-19 because of age or certain other medical conditions.

Emergency authorisation allows use of the drug to start while studies are continuing to establish safety and effectiveness.

Early results suggest the drug may reduce Covid-19-related hospitalisation or emergency room visits in patients at high risk for disease progression, the FDA said.

Regeneron said that initial doses will be made available to roughly 300,000 patients through a federal government allocation program. Those patients will not be charged for the drug but may have to pay part of the cost of giving the IV.

Initial supplies will likely be vastly outstripped by demand as the US has surged past 12 million reported cases, with the country facing what health experts say will be a dark winter due to uncontrolled spread of the virus.

There is no way to know whether the Regeneron drug helped Mr Trump recover; he received a host of treatments and most Covid-19 patients recover on their own.

FDA regulators authorised the Regeneron drug using their emergency powers to quickly speed the availability of experimental drugs and other medical products during public health crises.

In normal times the FDA requires "substantial evidence" to show that a drug is safe and effective, usually through one or more large, rigorously controlled patient studies.

But during public health emergencies the agency can lower those standards and require only that an experimental treatment's potential benefits outweigh its risks.

The emergency authorisation functions like a temporary approval for the duration of the Covid-19 pandemic. To win full approval, Regeneron will have to submit additional research to fully define the drug's safety and benefit for patients.

The White House cast the decision as a victory for Mr Trump's efforts "to deliver cutting-edge treatments with highly promising results to protect the health and safety of the most vulnerable Americans," according to a statement from spokesman Michael Bars.