MANILA, Dec 23 (Reuters) - The Philippines on Thursday approved emergency use of Merck & Co Inc"s (MRK.N)COVID-19 treatment pill for adult patients at risk of developing severe illness as it tries to boosts the country"s arsenal of drugs to fight new variants. It also approved the Pfizer-BioNTech COVID-19 vaccine for children aged 5-11 so the vaccine, which will be administered in a paediatric formulation, could be administered once it becomes available, its Food and Drug Administration (FDA) chief said. FDA chief Rolando Enrique Domingo told a public briefing the benefits outweighed the risks and it was already being used for children in countries like the United States and Canada. In October, the FDA allowed 31 Philippine hospitals to use Merck"s molnupiravir, the world"s first oral antiviral medication for COVID-19, under a compassionate use permit even before approval for its wider use. The Southeast Asian nation has halved to three months the waiting time for a booster dose of COVID-19 vaccine in its battle to rein in the more infectious Omicron variant of coronavirus, which has forced a global tightening of curbs.