Health and Human Services (HHS) Secretary Xavier Becerra, following a tense exchange with Republican Kentucky Sen. Rand Paul at a hearing Thursday, wouldn’t apologize for his past remarks suggesting COVID-19 survivors who don’t get vaccinated are like “flat-earthers.”

Becerra blamed “flat-earthers” who “choose to peddle fiction” for the increase in COVID-19 infections among unvaccinated people during an online forum Sept. 21 hosted by the Foundation for a Healthy Kentucky, the Courier-Journal reported. “Flat earthers — meet delta,” he said.

Paul went after Becerra during his appearance before the Senate Health, Education, Labor, and Pensions Committee for “completely ignoring the science on natural immunity … despite the large body of scientific evidence” in favor of naturally acquired immunity.

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“Do you want to apologize to the 100 million Americans who suffered through COVID, survived, have immunity, and yet you want to hold them down and vaccinate them? Do you want to apologize for calling those people flat-earthers?” Paul asked.

“We follow the facts and the science at HHS,” Becerra responded, dodging the senator’s question. “We use the expertise of the medical professionals and the scientists at HHS to make decisions, and we rely on what is on the ground showing results.”

“Except for the dozens and dozens of studies — in fact most, if not all, studies show robust immunity from getting the disease naturally,” Paul interjected.

“You’re selectively doing this because you want us to submit to your will,” he continued. “You have no scientific background, no scientific degrees, and yet you aren’t really concerned about the 100 million Americans who had the disease. You just want to tell us: do as you’re told. That’s what you’re telling us. You want to mandate this on all of us.”

The efficacy of natural immunity, or protection from a disease through prior infection, has been the subject of public health debates during the pandemic. Some health experts and media outlets have argued that natural immunity is a “misleading” term or “irrelevant” against vaccination. (RELATED: Sen. Ron Johnson Demands Answers On Study Of COVID-19 Natural Immunity)

But a number of scientific studies have indicated that recovery from COVID-19 grants a fairly robust level of natural immunity against future infection, particularly in younger people. Two studies in May concluded that natural immunity in individuals who have recovered from COVID-19 could last more than one year after infection.

A landmark Israeli study published in August found that vaccinated individuals who never had COVID-19 were 13 times more likely to receive a breakthrough infection than unvaccinated individuals who had been infected and recovered.