By Deena Beasley and Carl O’Donnell

(Reuters) -An antiviral pill developed by U.S. drugmaker Merck & Co could half the chances of dying or being hospitalized for those most at risk of contracting severe COVID-19, with experts hailing it as a potential breakthrough in how the virus is treated.

If it gets authorization, molnupiravir, which is designed to introduce errors into the genetic code of the virus, would be the first oral antiviral medication for COVID-19.

Merck and partner Ridgeback Biotherapeutics said they plan to seek U.S. emergency use authorization for the pill as soon as possible and to make regulatory applications worldwide.

“An oral antiviral that can impact hospitalization risk to such a degree would be game changing,” said Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security.

Current treatment options include Gilead Sciences Inc’s infused antiviral remdesivir and generic steroid dexamethasone, both of which are generally only given once a patient has already been hospitalized.

“This is going to change the dialogue around how to manage COVID-19,” Merck Chief Executive Robert Davis told Reuters.

Existing treatments are “cumbersome and logistically challenging to administer. A simple oral pill would be the opposite of that,” Adalja added.

The results from the Phase III trial, which sent Merck shares up more than 9%, were so strong that the study is being stopped early at the recommendation of outside monitors.

Shares of Atea Pharmaceuticals Inc, which is developing a similar COVID-19 treatment, were up around 20% on the news.

Shares of COVID-19 vaccine makers Pfizer Inc and Moderna Inc were off more than 2% and 14%, respectively.

Michael Yee, a biotechnology analyst at Jefferies, said the share move indicated that investors believe “people will be less afraid of COVID and less inclined to get vaccines if there is a simple pill that can treat COVID.”

Pfizer and Swiss drugmaker Roche Holding AG are also racing to develop an easy-to-administer antiviral pill for COVID-19. For now, only antibody cocktails which have to be given intravenously are approved for non-hospitalized patients.

A planned interim analysis of 775 patients in Merck’s study looked at hospitalizations or deaths. It found that 7.3% of those given molnupiravir were hospitalized and none had died by 29 days after treatment, compared with hospitalization of 14.1% of placebo patients. There were also eight deaths in the placebo group.

“Antiviral treatments that can be taken at home to keep people with COVID-19 out of the hospital are critically needed,” Wendy Holman, Ridgeback’s CEO, said in a statement.

‘WORK WITH ALACRITY’

Scientists welcomed the potential new treatment to help prevent serious illness from the virus, which has killed almost 5 million people around the world.

“A safe, affordable, and effective oral antiviral would be a huge advance in the fight against COVID,” said Peter Horby, a professor of emerging infectious diseases at the University of Oxford.

In the trial, which involved patients from around the world, molnupiravir was taken every 12 hours for five days.

The study enrolled patients with laboratory-confirmed mild-to-moderate COVID-19, who had symptoms for no more than five days. All patients had at least one risk factor associated with poor disease outcome, such as obesity or older age.

Merck said viral sequencing done so far shows molnupiravir is effective against all variants of the coronavirus including the highly transmissible Delta, which has driven the recent worldwide surge in hospitalizations and deaths.

It said rates of adverse events were similar for both molnupiravir and placebo patients, but did not give details.

Merck has said data shows molnupiravir is not capable of inducing genetic changes in human cells, but men enrolled in its trials had to abstain from heterosexual intercourse or agree to use contraception. Women of child-bearing age in the study could be pregnant and also had to use birth control.

Merck said it expects to produce 10 million courses of the treatment by the end of 2021, with more coming next year.

The company has a U.S. government contract to supply 1.7 million courses of molnupiravir at a price of $700 per course.

Davis said Merck has similar agreements with other governments, and is in talks with more. Merck said it plans a tiered pricing approach based on country income criteria.

Merck has also agreed to license the drug to several India-based generic drugmakers, which would be able to supply the treatment to low- and middle-income countries.

Molnupiravir is also being studied in a Phase III trial for preventing infection in people exposed to the coronavirus.

Merck officials said it is unclear how long the FDA review will take, although Dean Li, head of Merck’s research labs, said, “They are going to try to work with alacrity on this.”

(Reporting by Deena Beasley; Additional reporting by Josephine Mason, Editing by Lincoln Feast, Kirsten Donovan, Alexander Smith and Bill Berkrot)