The top US health agency has permanently fired 600 employees at the Centers for Disease Control and Prevention (CDC), according to a union representing the workers, BBC reported.

Many of the staff were already on paid administrative leave as a part of mass layoffs proposed by Health Secretary Robert F Kennedy Jr in April, according to the American Federation of Government Employees (AFGE). 

The move from the US Department of Health and Human Services finalises employee firings across the agency, including the Division of Violence Prevention and the Office of Equal Employment Opportunity, the union said.

The layoffs come two weeks after a gunman opened fire at the CDC's headquarters in Atlanta, according to BBC.