Shares of Pfizer Inc. /zigman2/quotes/202877789/composite PFE +0.03% were up 0.6% in premarket trading on Thursday after the drugmaker disclosed it is paying BioNTech /zigman2/quotes/214419716/composite BNTX +5.68% $185 million for their COVID-19 vaccine development collaboration, with additional possible future milestone payments up to $563 million. BioNTech's stock gained 8.6% before the market opened. The program, which was first announced by the companies in March, aims to put a vaccine candidate into clinical trials in April. Separately, Pfizer had also told multiple media outlets that it is testing an unnamed investigational therapy as a treatment for people sickened by the novel coronavirus. Year-to-date, Pfizer's stock is down 11.7%, while BioNTech's stock has jumped 32.7%. The S&P 500 /zigman2/quotes/210599714/realtime SPX +0.48% has fallen 14.9%.