Anthony S. Fauci, the country’s leading infectious-disease expert, said Monday that he’s “cautiously optimistic” about a potential coronavirus vaccine now entering the last phase of testing.

Speaking on CNN, Fauci said he briefed President Trump on Monday on the 30,000-person Phase 3 trial just launched for the vaccine candidate being developed by biotech company Moderna in collaboration with the National Institutes of Health. Pharmaceutical giant Pfizer also announced a 30,000-person trial of its coronavirus vaccine candidate Monday.

Fauci cautioned that there’s no “guarantee” of a working vaccine but noted that a Phase 1 trial suggested the Moderna candidate created antibody responses in volunteers comparable to what is seen in people who have recovered from covid-19 — a “hallmark” of success.

Here are some significant developments:

  • Robert C. O’Brien, Trump’s national security adviser, has tested positive for the coronavirus, the White House said Monday. O’Brien, the highest-ranking administration official known to have tested positive, has “mild symptoms."
  • As many as 14 Miami Marlins players and coaches have tested positive for the novel coronavirus, an outbreak that has come three days into the MLB season.
  • Warner Bros. is pushing ahead with the release of “Tenet,” the anticipated, if embattled, Christopher Nolan movie.
  • About 4,000 federal employees are seeking disability compensation on the grounds that they contracted the coronavirus at work.
  • A physician who headed the intensive care unit at Baltimore’s Mercy Medical Center has died of the novel coronavirus, a hospital spokesman said Monday.