The pharmaceutical giant Pfizer has begun testing a new coronavirus vaccine in the United States. The initial trial will involve 360 volunteers, and the first subjects have already received injections.

The vaccine was developed in a partnership between and the German biotech company . In addition to the U.S. trials, there will be some 200 patients enrolled in trials in Germany.

The candidate is known as an mRNA vaccine, and it is meant to contain the instructions for making viral proteins that can trigger an immune response in patients infected with the virus.

Previously, BioNTech has been developing mRNA-based treatments for cancer.

"Pfizer brings in expertise in infectious disease and vaccine development in vaccine manufacturing," says Philip Dormitzer, vice president and chief scientific officer for viral vaccines at Pfizer. "So it's a real partnership. ... Pfizer has the broad set of capabilities of major vaccine companies, a major pharmaceutical company, and BioNTech brings in real expertise in these RNA platforms."

There are multiple versions of the vaccine in the initial trials that began on Monday. The companies will advance the versions that appear safest and also produce the best immune response.

Even as they begin trials in humans, Pfizer is preparing to scale up its manufacturing capacity. The company says that is the only way it can be sure to have a productready as soon as possible to confront the ongoing pandemic.

"If you wait until you know how the vaccine works to start that, you're too late," Dormitzer says.

A vaccine developed by the biotech company and the National Institute of Allergy and Infectious Diseases is also based on mRNA technology. It began trials in humans last month.

Another vaccine, developed by Inovio, based on DNA technology, has also begun U.S. clinical trials. And vaccine candidates made by and the University of Pittsburgh are expected to begin clinical trials shortly. Last month, a vaccine by the University of Oxford began trials in the United Kingdom.

Johnson and Johnson, GSK and Sanofi have also announced vaccine plans.

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