Pfizer (PFE  ) on Tuesday said prescriptions for its oral antiviral COVID-19 treatment increased nearly 10-fold in the United States since February as more locations recieve supply.

Chief Executive Albert Bourla told investors in an earnings call on Tuesday that more than 79,000 patients for the U.S. were treated with Paxlovid in the week ended April 2022, up from about 8,000 patients for the week ended February 25, 2022. Bourla said that now more than 33,000 locations in the U.S. have supply of Paxlovid, with 77% of recent COVID cases occurring within 5 miles of those health care locations.

The U.S. Food and Drug Administration (FDA) authorized Paxlovid on an emergency use basis in December for adults and adolescents ages 12 and older who were infected with COVID and at risk of severe illness from that infection.

Since then, however, supply of the antiviral treatment was constrained, according to Bourla. To combat the limited supply, Pfizer has increased production to 6 million courses produced through the end of March, Bourla said, and the company is on track to manufacturer 30 million course in the first half of the year and 120 million by the end of 2022.

Pfizer reported in its first quarter earnings on Tuesday $1.5 billion in sales for Paxlovid and reaffirmed its guidance of $22 billion in sales for the year.

The FDA also authorized Merck's (MRK  ) rival antiviral, molnupriavir, shortly after Pfizer's, but molnupriavir is only authorized for use in adults ages 18 and older. In clinical trials, Paxlovid was proven to be more effective than molnupriavir, reducing the risk of hospitalization or death by 90% in adults with COVID infection at risk of developing severe illness. Molnupriavir was found to be only 30% effective.

Paxlovid currently has 90% market share compared to molnupriavir in the U.S. retail pharmacy, long-term care and mail order markets, Bourla said. The antiviral is also authorized in more than 60 countries, he added.

Paxlovid is a five day treatment consisting of three tablets (two nirmatrelvir pills and one ritonavir tablet) taken twice per day as soon as a patient is diagnosed with COVID. Nirmatrelvir works by inhibiting an enzyme the COVID virus uses to reproduce, and ritonavir (a commonly used HIV drug developed by Pfizer) slows a patient's metabolism so nirmatrelvir can remain active longer.