By Deena Beasley (Reuters) – Rival antiviral pills from Pfizer Inc and Merck & Co that demonstrated efficacy in trials of adults with COVID-19 who are at high risk of serious illness are now both in use. The drugs are being studied to see if they can prevent infection in people exposed to the virus. Here is an explanation of the differences in the two pills. Which of the new pills works better? Trial data provided by the two companies suggest that Pfizer has the more effective pill. Pfizer in December said final trial results showed that its treatment reduced the chance of hospitalization or d…