In 13 years at the Centers for Disease Control and Prevention, Dr. Fiona Havers crafted guidance for contending with Zika virus, helped China respond to outbreaks of bird flu and guided safe burial practices for Ebola deaths in Liberia.

More recently, she was a senior adviser on vaccine policy, leading a team that produced data on hospitalizations related to Covid-19 and respiratory syncytial virus. To the select group of scientists, federal officials and advocates who study who should get immunizations and when, Dr. Havers is well known, an embodiment of the C.D.C.’s intensive data-gathering operations.

On Monday, Dr. Havers resigned, saying she could no longer continue while the health secretary, Robert F. Kennedy Jr., dismantled the careful processes that help formulate vaccination standards in the United States.

“If it isn’t stopped, and some of this isn’t reversed, like, immediately, a lot of Americans are going to die as a result of vaccine-preventable diseases,” she said in an interview with The New York Times, the first since her resignation.

Dr. Havers, 49, cited an escalating series of attacks on federal vaccine policy by Mr. Kennedy. Three weeks ago, the health secretary announced in a minute-long video on X that the agency would no longer recommend Covid-19 vaccines for healthy children or pregnant women.

Last week, he fired all 17 members of the agency’s Advisory Committee on Immunization Practices, saying without evidence that the group was beset with conflicts of interest and that a clean sweep was needed to restore public trust.