WASHINGTON (AP) — The U.S. is poised to get a third vaccine against COVID-19, and health officials are girding for questions about which one is best. If cleared for emergency use, the Johnson & Johnson vaccine would offer a single-dose option that could help speed vaccinations. The challenge will be explaining how protective the J&J shot is after the astounding success of two-dose vaccines from Pfizer and Moderna. Those shots were found to be about 95% effective against symptomatic COVID-19. The numbers from J&J are not that high, but they are not an apples-to-apples comparison. Regulators say it strongly protects against serious illness.