Adds details about CDC advisory group discussions, upcoming meeting in paragraphs 12-15

By Julie Steenhuysen, Michael Erman and Dan Levine

June 3 (Reuters) - Pediatric infectious disease expert Dr. Lakshmi Panagiotakopoulos resigned on Tuesday as co-leader of a U.S. CDC 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, said the COVID vaccine for healthy children and healthy pregnant women had been removed from the CDC's recommended immunization schedule. Kennedy oversees the CDC, the Food and Drug Administration and the National Institutes of Health as leader of the Department of Health and Human Services.

The move was a departure from the usual process in which ACIP experts meet and vote on changes to the immunization schedule or recommendations on who should get vaccines before the agency's director made a final call. The committee had not voted on the changes announced by Kennedy and the CDC does not yet have a permanent director.

Two days after Kennedy's announcement, the CDC published a vaccine schedule online saying that COVID-19 vaccines remain an option for healthy children aged 6 months to 17 years when parents and doctors agree that it is needed.

It had previously recommended updated COVID vaccines for everyone aged 6 months and older, following the guidance of the panel of outside experts.

Two sources said Panagiotakopoulos did not include a specific reason for her departure. Panagiotakopoulos did not return requests for comment.

"Unfortunately for me, this is a personal decision," Panagiotakopoulos wrote in an email to members of the working group that was read to Reuters by a source who received it.

"My career in public health and vaccinology started with a deep-seated desire to help the most vulnerable members of our population, and that is not something I am able to continue doing in this role," she wrote.

The committee is scheduled to meet on June 25-27 and is expected to deliberate and vote on recommendations for use of COVID-19 vaccines, according to one of the sources who was not authorized to speak publicly.

In its last ACIP meeting on April 15-16, Panagiotakopoulos presented data to the panel on Moderna's MRNA.O COVID-19 vaccine and potential options for the 2025-26 vaccination schedule. Those include maintaining the current broad policy or narrowing recommendations for certain populations.

The working group had been leaning toward narrowing use of the vaccine, the source familiar with the group's discussions said. But, the source added, the experts likely would recommend broader use for certain at-risk populations, including very young children and pregnant women who are at high risk of COVID complications.

A positive, broad recommendation for these groups sends a stronger message to providers and insurance companies that these populations should be vaccinated, and such vaccines should be covered for those uses, the source said.

Two sources said the working group is continuing to meet despite last week's surprise changes in the immunization schedule, and is preparing to make recommendations to the full group of ACIP advisers at their upcoming meeting.

(Reporting by Julie Steenhuysen in Chicago, Michael Erman in New York and Dan Levine in San Francisco; Editing by Caroline Humer and Bill Berkrot)