Long-awaited data is in: A fourth dose of the coronavirus vaccine reduces the risk of getting severely ill. But triple-vaccinated people already are so well protected from hospitalization that a fourth dose offers relatively little real-world benefit, experts say. The second booster drops the risk from exceedingly small to … even smaller. But is that enough for the U.S. to launch another ambitious round of immunizations? “For the majority of healthy younger adults, I don’t believe a fourth vaccine makes sense,” said Dr. Warner Greene of the Gladstone Institutes in San Francisco. “Three doses p…