US pharmaceutical giant Pfizer and German biotechnology company BioNTech have begun testing an experimental coronavirus vaccine on humans in the United States, according to an announcement from the companies on Tuesday.

The companies’ coronavirus vaccine program is called BNT162. Study participants in the program in Germany were given doses of the vaccine last week and now the US trial — at NYU Grossman School of Medicine in New York and the University of Maryland School of Medicine — is underway, the companies have revealed.

The program’s Phase 1/2 study is designed to test the safety, effectiveness and best dose level of four mRNA vaccine candidates and is to be evaluated in a single, continuous study, the companies said.

The first participants in the first stage of the study will be healthy adults ages 18 to 55, according to the announcement.

Worldwide race to develop a vaccine: Pfizer isn’t the only company with a potential Covid-19 vaccine program this far along.

In April, scientists at Oxford University’s Jenner Institute in the United Kingdom began testing its vaccine on humans and, depending on the trial results, says the vaccine could be ready as early as September.

In the US, the National Institutes of Health also started testing in humans, becoming the first to do so in March.

The World Health Organization says that eight vaccine programs are in the clinical trial phase and more than 100 others are in preliminary phases.