One-Shot Wonders

The first round of Covid-19 vaccines may reach the U.S. before year-end, but getting them widely distributed to an entire country will be no easy feat.

The leading vaccine candidates from Pfizer Inc. and partner BioNTech SE as well as Moderna Inc. face a number of hurdles. Both inoculations use a relatively new technology, engineered messenger RNA, that so far has needed a follow-up booster shot to make the vaccinations effective.