“I said, ‘I am an RNA scientist. I can do anything with RNA,’” Dr. Karikó recalled telling Dr. Weissman. He asked her: Could you make an H.I.V. vaccine?

“Oh yeah, oh yeah, I can do it,” Dr. Karikó said.

Up to that point, commercial vaccines had carried modified viruses or pieces of them into the body to train the immune system to attack invading microbes. An mRNA vaccine would instead carry instructions — encoded in mRNA — that would allow the body’s cells to pump out their own viral proteins. This approach, Dr. Weissman thought, would better mimic a real infection and prompt a more robust immune response than traditional vaccines did.

It was a fringe idea that few scientists thought would work. A molecule as fragile as mRNA seemed an unlikely vaccine candidate. Grant reviewers were not impressed, either. His lab had to run on seed money that the university gives new faculty members to get started.

By that time, it was easy to synthesize mRNA in the lab to encode any protein. Drs. Weissman and Karikó inserted mRNA molecules into human cells growing in petri dishes and, as expected, the mRNA instructed the cells to make specific proteins. But when they injected mRNA into mice, the animals got sick.

“Their fur got ruffled, they hunched up, they stopped eating, they stopped running,” Dr. Weissman said. “Nobody knew why.”

For seven years, the pair studied the workings of mRNA. Countless experiments failed. They wandered down one blind alley after another. Their problem was that the immune system sees mRNA as a piece of an invading pathogen and attacks it, making the animals sick while destroying the mRNA.

Eventually, they solved the mystery. The researchers discovered that cells protect their own mRNA with a specific chemical modification. So the scientists tried making the same change to mRNA made in the lab before injecting it into cells. It worked: The mRNA was taken up by cells without provoking an immune response.