"The incentive: About $3,000 to compensate for their time"

To begin finding out, Memoli first developed a laboratory-grown copy of the H1N1 flu strain and sprayed different amounts into volunteers’ noses until he found the right dose to trigger mild flu. He hopes eventually to test the harsher H3N2 strain, too.

Now he’s infecting two groups – people with low antibody levels and those with high levels. Some were recently vaccinated, and some weren’t. He’ll compare how sick they get, how long they’re contagious and how the immune system jumps into action.

Called a human challenge study, this kind of research hasn’t been performed with flu viruses in the U.S. for more than a decade, before scientists had ways as sophisticated to measure what happens.

“It’s all going to add up to a better understanding of what you need to have to be protected against the flu,” said Dr. John Treanor, a flu specialist at the University of Rochester Medical Center who is closely watching the work.