In a severe blow to the Trump administration’s health agenda, a federal judge in Massachusetts on Monday blocked the government from implementing a series of decisions on vaccines made over the last year by Health Secretary Robert F. Kennedy Jr.

The ruling also reversed, at least for the time being, all decisions made by the panelists that Mr. Kennedy appointed to the Advisory Committee for Immunization Practices, which makes recommendations on which vaccines Americans should take. The court decision will prevent the committee from meeting later this week, as it was scheduled to do.

The judge’s ruling brought an abrupt halt to the major changes that Mr. Kennedy, who has long been skeptical of vaccines, had set in motion, upending national vaccine policy and making sweeping revisions to the recommendations for what shots are given and when. Those included cutting down the number of diseases covered by routine immunization, and restricting access to Covid vaccines, two pillars of Mr. Kennedy’s vaccine agenda.