(Reuters) -The European Union’s drug regulator is set to decide whether to approve Pfizer’s COVID-19 pill at the end of this month, before doing a final review of Merck’s similar but less effective drug in February, a source with knowledge of the matter said. Late last year, the European Medicines Agency (EMA) gave guidance to member states on how to use both antiviral pills as emergency treatments, while it carried out rolling reviews to help member states decide on quick adoption ahead of any formal EU-wide approval. The EU watchdog said the panel of experts reviewing new drugs was meeting f…