Former Centers for Disease Control and Prevention Director Robert Redfield said Monday that he continues to believe that the COVID-19 pandemic probably leaked from a laboratory.

In an interview with Martha MacCallum on Fox News, Redfield said that during the past year and a half there has not been new evidence to indicate that the illness "evolved from nature." There has not been evidence that the virus is connected to any of the numerous animal species tested, but there has been increasing evidence to bolster the idea that it resulted from a lab leak, he said.

Redfield described himself as "disheartened" by the scientific community's failure to consider both pandemic origin theories in an open-minded manner, saying that he "was very rapidly sidelined, threatened, and you know really put on, sort of outed," due to his view that COVID-19 may have emerged from a lab.

The Trump-era CDC official said that COVID-19 has been extremely infectious among humans.

"This virus is replicating at a high capacity in humans. And as a consequence the risk for variant evolution is extremely high," he said.

Redfield pointed to the UK variant and the Delta variant and predicted that in the coming months another variant will be more infectious than the Delta variant.

"The one thing that we all can do that can really confront the evolution of this next variant, slow it down, is to really continue to get our population vaccinated. And more importantly I believe, although it's not getting as much discussion as I want, making sure that we're doing what we need to do to maintain the immunity in the vulnerable individuals," Redfield said.

He said it is apparent that the vaccine will not be "durable year after year" and it needs to be determined how to maintain immunity in people who got vaccinated early.

According to the CDC, 58.7% of the U.S. population ages 12 and older have been fully vaccinated, while 68.8% have received at least one dose. The vaccination rate is much higher among those 65 and older, with 80.4% of the U.S. population in that demographic having been fully vaccinated and 90.4% having received at least one dose.

Redfield said it is "imperative" to return K-12 students to in-person learning. He said that "a variety of negative public health consequences" occurred "as a consequence of doing virtual learning."

The CDC is currently recommending universal mask-wearing indoors for students, teachers, staff, and visitors at schools. Redfield said he has not "been able to review data that supports that recommendation."