Dozens of service members at Lackland Air Force Base in Texas have fallen ill with the flu in the weeks since Defense Secretary Pete Hegseth rescinded the vaccine mandate, sources familiar confirmed to CBS News. 

"Over the last three weeks, the 37th Training Wing, in close coordination with the 59th Medical Wing, has been managing a localized influenza outbreak among trainees at Basic Military Training," an Air Force spokesperson told CBS News in a statement. 

The New York Times first reported on the outbreak at the training wing in Texas.  

The unit has implemented mitigation measures to isolate and treat symptomatic trainees, according to the Air Force spokesperson, and is monitoring trainees who may have been exposed. The trainees who are symptomatic are receiving antiviral medications such as Tamiflu. 

In April, Hegseth issued a memo that rescinded the decades-long mandate and said in an accompanying video that the Pentagon was "seizing this moment to discard any absurd, overreaching mandates that only weaken our warfighting capability."

In a statement on Thursday, Chief Pentagon spokesman Sean Parnell said that under the updated guidance, the Under Secretary of Defense for Personnel and Readiness granted exceptions to this policy for the departments of the Army, Navy, Air Force, and National Security Agency and Defense Health Agency if there were needs to make the vaccine mandatory.

"The decisions were based upon thorough risk assessments and are designed to maximize operational readiness, lethality, and force generation, while safeguarding at-risk populations. The Army, Navy, Air Force, NSA, and DHA are responsible for implementing the ETPs," Parnell said in the statement, referring to exceptions to policy. "The Department remains committed to the health and readiness of our warfighters and civilian personnel."

The sources familiar confirmed that only about 40% of Air Force trainees have opted to take the flu vaccine since the mandate was rescinded. 

The 37th Training Wing at Lackland is the largest training wing in the Air Force and provides the basic military training of all enlisted recruits entering the Air Force, Space Force, Air Force Reserve and Air National Guard, according to an Air Force fact sheet. 

On Wednesday, the unit in a release said Keon McDaniel, a trainee who was in his sixth week of basic military training, died following a medical emergency. He died June 16 at Brooke Army Medical Center, where he had been transported on June 12 for care related to the emergency. 

The cause of the medical emergency is currently under investigation, the release said.