With the nation’s top public health officials as their audience, Senate Republicans on Thursday voiced their displeasure with the Biden administration’s new broad-reaching vaccine mandate for large businesses.

“I’m just saying it’s a tough sell to tell people who have had COVID that they’re now required to be vaccinated by the federal government. I believe you have a very difficult sell, “Sen. Richard Burr, the top Republican on the Senate Health Committee, told the heads of the Biden White House COVID response team, CDC Director Rochelle Walensky and chief medical adviser Dr. Anthony Fauci, in a hearing.

The new mandate, which was announced in September but only became effective on Thursday, will apply to nearly 100 million U.S. workers and will require them to either get vaccinated or submit to weekly COVID tests. The mandate will go into effect on January 4, following the holidays.

Both Fauci and Walensky backed the mandate, citing 745,000 American deaths and thousands more every week, the vast majority of which are among the unvaccinated.

During the hearing, Burr was one of several Republicans who argued that unvaccinated Americans who have had COVID-19 should be exempt from the new mandate because they now have “natural” immunity to the virus, an argument made by business organizations opposed to the mandate.

While studies have shown that people retain an immune response to the virus after recovery, it is unclear how long that immunity lasts and it does not appear to be as robust as vaccination immunity. According to Wallensky and Fauci, a recent CDC study found that people who developed “natural” immunity through infection were five times more likely to develop COVID-19 than people who were fully vaccinated.

Nonetheless, Burr argued that health care professionals were already facing a shortage of workers and couldn’t afford to lose any more because they refused to comply with mandates. Workers are required to get vaccinated under the mandate for health care workers, which applies to any facility that receives federal funding through Medicare or Medicaid, with no option to do weekly tests instead.

The data on how long natural immunity lasts is “murkier,” according to Walensky, but the CDC can track how long vaccines work with certainty, citing a recent study that found more infections in people who had COVID than in those who were vaccinated.

“CDC recommendations suggest that if you are vaccinated after being previously infected, you have a more durable, robust, and known immune response, and those are our recommendations,” Walensky said. Sen. Lisa Murkowski, a Republican from Alaska, also expressed concern about worker shortages to public health officials.

“When we talk about vaccines and vaccine hesitancy, we need to talk to those who are putting in place some of these standards that are causing an extraordinary issue and problem within our workforce in my state,” Murkowski said.

Both Fauci and Walensky defended the mandates, which the Biden administration hopes will put a stop to COVID cases that have disrupted everything from education systems to supply chains and the economy.

“We know the most disruptive thing in our workforce is to have a COVID outbreak and to have workers in that workforce come down with COVID infection, severe disease, and in some cases death. Vaccination as we’ve seen, decreases your risk of infection by sixfold, decreases your risk of hospitalization and death by tenfold even during this delta surge. So there’s absolutely a public health priority to get people vaccinated and to continue the important prevention and mitigation strategies including masking to keep them safe,” she said.

Fauci said he did not have a model on hand, but had “a very firm and confident answer” to the first question.