Jim O’Neill, a top deputy to Health Secretary Robert F. Kennedy Jr., will serve as acting director of the Centers for Disease Control and Prevention, according to an administration official who requested anonymity to discuss a personnel change that has not been formally announced.

The director of the nation’s top public health agency, Susan Monarez, was fired by President Donald Trump late Wednesday because she wasn’t “aligned with the president’s mission” and refused to resign, according to the White House press secretary. Monarez is fighting to keep her job, and her lawyers say she was targeted for standing up for science.

Four other agency leaders resigned on Wednesday. “We knew … if she leaves, we don’t have scientific leadership anymore, ” one of the officials, Dr. Debra Houry, told The Associated Press on Thursday. “We were going to see if she was able to weather the storm. And when she was not, we were done.”

How could the changes at the CDC affect average Americans?

The clash at the CDC was apparently triggered by conflict over changes in policy regarding recommendations for COVID-19 vaccinations that could make getting a shot more complicated for people.

The Food and Drug Administration approved updated COVID-19 shots on Wednesday but limited their use for many Americans — and removed one of the two vaccines available for young children. The new jabs from three manufacturers are approved for all seniors, but the FDA narrowed their use for adults and children to those with high-risk health conditions, such as asthma or obesity.

The changes present new barriers to access for millions of Americans, who would have to prove their risk, and others who may want the shots but suddenly no longer qualify.