(Reuters) - U.S. hospitals have been slow to embrace COVID-19 antibody drugs from Eli Lilly and Co and Regeneron Pharmaceuticals Inc that have been authorized to reduce the risk of hospitalization, U.S. officials said on Wednesday.

Demand for the therapies, which are given as a one-time intravenous infusion, has been "disappointing," and hospitals should be using them more, Operation Warp Speed chief advisor Dr. Moncef Slaoui said on a conference call. The government program is distributing and allocating the drugs.

Health systems say they have been slow to ramp up use of the antibody drugs due to extra levels of complexity during this pandemic - including requirements for quick diagnosis times and the need to isolate infectious patients.

The U.S. Department of Health and Human Services has so far delivered over 250,000 doses of Lilly's bamlanivimab and Regeneron's antibody combination of casirivimab and imdevimab to states and territories.

Both treatments have U.S. emergency use authorization for non-hospitalized COVID-19 patients at risk of becoming severely ill due to age or underlying health conditions.

The drugs are monoclonal antibodies, known as MAbs in medical shorthand, manufactured copies of proteins produced by the body to fight coronavirus infection.

The Regeneron cocktail was among the medicines used to treat U.S. President Donald Trump when he contracted COVID-19.

"We are using quite a bit. We've treated over 300 patients so far with bamlanivimab," said Katherine Perez, infectious disease pharmacist at Houston Methodist.

Her hospital system has set up five infusion sites across Houston and is capable of treating up to 35 patients a day.

Perez said Houston Methodist is mainly using bamlanivimab as Regeneron's drug is still being distributed using material from clinical trials, requiring pharmacies to mix several vials.

"Sometimes it takes eight vials to get one dose," she said.

Despite emergency use authorizations for both antibody drugs, the National Institutes of Health and the Infectious Diseases Society of America recommend against their routine use, saying that the benefits are uncertain.

ENTER THE 'MAB SQUAD'

Dr. Brandon Webb, infectious disease physician at Utah's Intermountain Healthcare, said his health center is working with researchers to gather real-world data on how well the drugs appear to be working.

Meanwhile, Intermountain has developed a risk stratification tool to match highest-need patients with antibody supplies.

Webb said the health system has trained a special team of providers, known as "the MAb Squad," who contact high risk patients as soon as their test results are positive to connect them with treatment at infusion centers across the state.

Slaoui said the U.S. is still discussing procuring additional antibody doses from both Lilly and Regeneron.

The U.S. has so far paid $1,250 per dose for 950,000 doses of Lilly's bamlanivimab and purchased 300,000 doses of Regeneron's antibody cocktail.