Johnson & Johnson has backed off a previous pledge to deliver 24 million additional doses of its one-shot vaccine by the end of April amid the continued failure of its troubled contract manufacturer in Baltimore to win government manufacturing certification. With domestic production of the company’s vaccine still unapproved, the government slashed its national allocation of the J&J vaccine to states by 86 percent to just 700,000 doses next week, down from nearly 5 million, a cut that Maryland Gov. Larry Hogan (R) called “very concerning.” The cuts will make it harder across the country for much of April to...