The United States authorized an extra dose of COVID-19 vaccine for people with weakened immune systems overnight Thursday to Friday, as the country struggles to thwart the Delta variant.

Emergency use authorization for a third injection of Pfizer-BioNTech and Moderna vaccines was granted by the U.S. Food and Drug Administration (FDA) regulator. 

"The country has entered yet another wave of the COVID-19 pandemic, and the FDA is especially cognizant that immunocompromised people are particularly at risk for severe disease," said acting FDA Commissioner Janet Woodcock in a statement.  

The FDA said the additional dose was for solid organ transplant recipients or those with equivalent weakened immune systems.