As Merck & Co and Pfizer Inc prepare to report clinical trial results for experimental Covid-19 antiviral pills, rivals are lining up with what they hope will prove to be more potent and convenient oral treatments of their own.

Enanta Pharmaceuticals, Pardes Biosciences, Japan's Shionogi & Co Ltd and Novartis AG said they designed antivirals that target the coronavirus.

The designs also aim to avoid potential shortcomings, such as the need for multiple pills per day or known safety issues.

Infectious disease experts stressed that preventing Covid-19 through the wide use of vaccines remains the best way to control the pandemic. But they said the disease is here to stay and more convenient treatments are needed.

"We need to have...