Nov 5 (Reuters) - Pfizer Inc"s (PFE.N) experimental antiviral pill to treat COVID-19 cut by 89% the chance of hospitalization or death for adults at risk of severe disease, the company said on Friday, as its CEO vowed to make this promising new weapon in the fight against the pandemic available globally as quickly as possible. The trial"s results suggest that Pfizer"s drug surpasses Merck & Co Inc"s (MRK.N) pill, molnupiravir, which was shown last month to halve the chance of dying or being hospitalized for COVID-19 patients at high risk of serious illness. Pfizer"s pill, with the brand name Paxlovid, could secure U.S. regulatory approval by the end of the year. Pfizer said it plans to submit interim trial results to the Food and Drug Administration (FDA) before the Nov. 25 U.S. Thanksgiving holiday. The trial was stopped early due to its high success rate. President Joe Biden said the U.S. government has secured millions of doses of Pfizer"s drug. "If authorized by the FDA we may soon have pills that treat the virus in those who become infected," Biden said. "The therapy would be another tool in our toolbox to protect people from the worst outcomes of COVID." Shares in Pfizer, which also makes one of the mostly widely used COVID-19 vaccines, rose 11% to close at $48.61. Merck"s fell 10% to close at $81.61. Shares of vaccine makers took a hit, with Moderna Inc (MRNA.O), Pfizer"s German partner BioNTech SE and Novavax (NVAX.O) all down 11-21%. Pfizer"s pill is given in combination with an older antiviral called ritonavir. The treatment consists of three pills given twice daily. It has been in development for nearly two years. The Pfizer and Merck pills are eagerly anticipated, with only limited options currently available for treating people sick with COVID-19. Full trial data is not yet available from either company. Pfizer is in active discussions with 90 countries over supply contracts for its pill, Chief Executive Officer Albert Bourla said in an interview. "Our goal is that everyone in the world would be able to have it as quickly as possible," Bourla said. Bourla added that for high-income countries Pfizer expects to price its treatment close to where Merck has priced its drug. Merck"s U.S. contract price is around $700 for a five-day course of therapy. For low-income countries, Bourla said Pfizer is considering several options, with the goal of "no barrier for them as well to have access." Merck"s pill was approved by British regulators in a world first on Thursday. Even with the potential offered by the Pfizer and Merck pills, preventing COVID-19 infections through broad use of vaccines remains the best way to control a pandemic that has killed more than 5 million people worldwide, including more than 750,000 in the United States, infectious disease experts said. "Vaccines are going to be the most effective and reliable tool that we have in this pandemic," said Dr. Grace Lee, professor of pediatrics at Stanford University School of Medicine. "These oral medications are going to augment our ability to really reduce the risk of severe disease, hospitalization and death, which is huge, but it won"t prevent infection." While more than 7 billion vaccine doses have been administered worldwide, that has covered only about half the world"s people. In the United States, 58% of all people, including 70% of adults, are fully vaccinated. Mizuho analyst Vamil Divan forecast a "very minor impact" from Pfizer"s drug on vaccination among people who do not want the vaccine or a booster shot as recommended by U.S. health regulators. "I think there"s a small percentage of people that may decide not to get vaccinated now that there are good treatment options," Divan said. MANUFACTURING GOALS Pfizer said it expects to manufacture 180,000 treatment courses by the end of this year and at least 50 million courses by the end of next year, including 21 million in the first half of 2022. Bourla said that, based on the better-than-expected trial results, Pfizer is considering potentially doubling next year"s manufacturing target. Antivirals need to be given as early as possible, before an infection takes hold, to be most effective. The planned analysis of 1,219 patients in Pfizer"s study examined hospitalizations or deaths among people diagnosed with mild to moderate COVID-19 with at least one risk factor for developing severe disease, such as obesity or older age. Among those given Pfizer"s drug within three days of symptom onset, the pill lowered the chances of hospitalization or death for adults at risk of developing severe COVID-19 by 89% compared to those receiving a placebo. Among these patients, 0.8% were hospitalized and none died by 28 days after treatment, compared to a 7% hospitalization rate and seven deaths in the placebo group. Rates were similar for patients treated within five days of symptoms: 1% of the treatment group was hospitalized, compared to 6.7% for the placebo group, which included 10 deaths. Pfizer said that represents 85% effectiveness at preventing hospitalization or death. Two other trials - one in people without underlying risk factors and another in people who have been exposed to the virus but are not yet infected - are continuing, with those results likely be available in the first quarter of 2022, Bourla said. Pfizer did not detail side any effects but said adverse events happened in about 20% of both treatment and placebo patients. Possible side effects include nausea and diarrhea. Reporting by Deena Beasley; additional reporting by Ankur Banerjee, Leroy Leo and Manojna Maddipatla in Bengaluru; Julie Steenhuysen, Trevor Hunnicut, Michael Erman and Francesco Guarascio Editing by Will Dunham, Anil D"Silva, Caroline Humer and Mark Potter