Pharmaceutical giant Pfizer began testing multiple versions of an experimental coronavirus vaccine in healthy young people in the United States this week, a first step toward establishing the safety, dosage and most promising candidate to take into larger trials that will test effectiveness.

In an unusual trial design that signals the pressing need to find a vaccine against COVID-19, Pfizer is initially testing four versions of the vaccine, side by side. Typically, companies spend years on animal experiments and select a single promising candidate to put into human testing, but the drugmaker decided to create a flexible trial that could rapidly sift out the best option.

"The pandemic came upon us, fast and furious, and we didn't have a lot of time to do years of research," said Kathrin Jansen, head of vaccine research and development at Pfizer. "Because of the urgency and the crisis, we said, 'What can we do to shorten the development time for a vaccine?' "

Jansen said the goal is to have a vaccine ready for use in high-risk groups by the fall — an ambitious goal that echoes the timeline from a group at the University of Oxford in the United Kingdom. There are at least eight other vaccine candidates being tested in people worldwide, according to a tracker by the Milken Institute.

The vaccine is being developed with the German company BioNTech and uses a type of genetic material, called RNA, to teach a person's immune system to defend against the coronavirus.

The trial, initially centered at the University of Maryland and New York University, administered the first five vaccinations to people Monday. Four out of five people in the study will receive experimental vaccinations, and one will receive a placebo.

"This is a whole new world of doing these trials," said Kathy Neuzil, director of the Center for Vaccine Development and Global Health at the University of Maryland. "We've done many of these [vaccine] trials, but here we've done it with COVID precautions: screening everybody before they even come into the building, taking temperatures, masks, participants spread out in the room rather than everyone sitting together."

The trial will start with healthy, young people between the ages of 18 and 55. But as safety is established in that population, it will expand to an older group of study participants — people up to age 85 — because of the high risk they face from COVID-19, the disease caused by the coronavirus. Ultimately, researchers will enroll 90 people at each trial site, which will also include the University of Rochester Medical Center and Cincinnati Children's Hospital Medical Center.

That data will inform later-stage trials designed to test the effectiveness of the vaccine, which Jansen said could start this summer.

Michael Sikorski, 26, showed up at the clinic Monday knowing he was the fourth person in line to receive the vaccine in Maryland. Sikorski, in training to be a physician and a scientist, wore a mask, underwent temperature checks and got a nasal swab to confirm he wasn’t infected with COVID-19. He spent hours in a conference room in Baltimore with other volunteers. Sikorski qualified to get the vaccine but will have to return Wednesday for his shot.

His motivations are both "personal and global," said Sikorski, who happens to study the genomics of microbes that cause outbreaks.

“My parents are of the high-risk age, and my grandparents as well. I would love, both as a medical student and as a researcher of microbiology, to play an active role in coming up with a vaccine that could help,” Sikorski said. “And I also understand the importance of safe vaccine trials, to make sure these products are tested in healthy people first and closely monitored and slowly scaled up.”