In early 2020, as a new deadly coronavirus began spreading around the world, Pfizer Inc assembled what it called a “SWAT team” of scientists and chemists to identify a potential treatment to fight Covid-19.

The US pharmaceutical giant, which had begun exploring a vaccine, also wanted to produce a pill that could stop the infection from progressing, similar to how the widely-used Tamiflu drug fights influenza. The team scoured Pfizer’s library of molecules looking for unused compounds to help jumpstart the process, and quickly identified a promising candidate.

More than a year later, Pfizer has yet to embark on large-scale human trials of a Covid-19 oral treatment - something it says it hopes to start by July.

Pfizer and its rivals, including...