Japan pharmaceutical company Shionogi & Co. has started human trials for its Covid-19 pill, according to The Wall Street Journal.

Shionogi, which helped develop the breakthrough cholesterol drug Crestor, is months behind Pfizer and Merck, which have begun later-stage trials of Covid capsules, WSJ reported. U.S.-based Pfizer has said its twice-daily pill might enter the market by the end of 2021.

"Our target is a very safe, oral compound, like Tamiflu; like Xofluza," WSJ quoted Isao Teshirogi, Shionogi's chief executive officer saying. He said the company's pill was designed to neutralize the virus five days after ingestion.

Shionogi's trials in Japan are expected to include around 100 healthy people, according to Teshirogi. He expects a larger trial comparing the drug with a placebo would begin later this year in Japan.

All three pharmaceutical companies want to fill one of the most critical gaps in preventing the pandemic. While researchers have demonstrated vaccines continue to be effective at preventing serious illness from identified strains of the virus, cases can still occur for those who receive their shots.

In April, the Telegraph reported Pfizer's antiviral pill. Pfizer, according to the publication, had been conducting trials in the U.S. and Belgium with adult volunteers taking pills designed to provide a "cure."

On CNBC's Squawk Box Pfizer CEO Albert Bourla said "it is accurate" to say the company is "working on" such therapies.

Existing treatments for people infected with the virus include Gilead Sciences Inc.'s infused antiviral medication Remdesivir, which is often administered in hospitals, as well as monoclonal antibodies.

Most direct antiviral monoclonal antibody therapies in development target the spike protein, which the virus uses to enter host cells, preventing viral attachment and entry into human cells.