Two companies considered to be advanced in their pursuit of a coronavirus vaccine, Pfizer (PFE -0.19%) and BioNTech (BNTX -0.45%), have selected one candidate for advancement to a phase 2/3 clinical study. The two companies, which are collaborating on the vaccine effort, said Monday in a joint press release that they have chosen BNT162b2, one of four candidates they were testing.

The selection was made after evaluating data from phase 1/2 clinical trials of all four candidates. Pfizer and BioNTech said they also consulted with the Food and Drug Administration's (FDA) Center for Biologics Evaluation and Research and other regulators on the choice.

A medical professional using a microscope.

Image source: Getty Images.

The companies said that "[p]reliminary clinical phase 1/2 data from nearly 120 patients demonstrated a favorable overall tolerability profile for BNT162b2, as compared to BNT162b1, with generally mild to moderate and transient (1-2 days) systemic events, such as fever, fatigue, and chills, and no serious adverse events."

Pfizer and BioNTech added that they have finalized the phase 2/3 study protocol, again incorporating responses from healthcare regulators around the world. As many as 30,000 people will be enrolled in this clinical trial, and it will consist of a diverse population of adults aged 18 to 85. It will take place in 39 U.S. states and in countries such as Argentina and BioNTech's native Germany.

All of the Pfizer-BioNTech coronavirus vaccine candidates work by using messenger RNA (mRNA) to teach the body's cells how to make antibodies that can attack the COVID-19 virus.

The fortunes of the two companies were mixed in trading on Monday. Pfizer fell marginally by 0.3%, while BioNTech outpaced many peer biotech stocks and the top equities indexes by rising 2.8%.