Pediatric infectious disease expert Dr. Lakshmi Panagiotakopoulos of the U.S. CDC resigned on Tuesday as co-leader of a working group that advises outside experts on COVID-19 vaccines and is leaving the agency, two sources familiar with the move told Reuters.

Panagiotakopoulos said in an email to work group colleagues that her decision to step down was based on the belief she is “no longer able to help the most vulnerable members” of the U.S. population.

In her role at the Centers for Disease Control and Prevention’s working group of the Advisory Committee on Immunization Practices, she co-led the gathering of information on topics for presentation.

Her resignation comes one week after Health Secretary Robert F. Kennedy Jr., a long-time vaccine skeptic who oversees the CDC, the Food and Drug Administration and the National Institutes of Health, said the COVID vaccine for healthy children and healthy pregnant women had been removed from the CDC’s recommended immunization schedule.