Mr Biden was asked by a reporter during a press conference on Friday why he still wears a mask when he walks out to the podium since he and virtually everyone around him have received the coronavirus vaccine.
"Because I'm worried about you," he said. "That's a joke. It's a joke."
Mr Biden then went on to explain his real reason for wearing masks despite being vaccinated and being largely surrounded by other vaccinated people.
"Why am I wearing the mask? Because when I'm inside it's still good policy to wear the mask. That's why," he said.
The president said he's so used to not wearing his mask outside that he sometimes forgets to put it back on when he walks away from the podium.
When Mr Biden referenced "good policy," he was likely referencing the Center for Disease Control's mask guidelines for fully vaccinated individuals.
According to the CDC, it is still recommended for fully vaccinated people to continue wearing masks inside in order to protect individuals who may not be vaccinated.
"In indoor public spaces, the vaccination status of other people or whether they are at increased risk for severe COVID-19 is likely unknown," the CDC guidelines states. "Therefore, fully vaccinated people should continue to wear a mask that fits snugly against the sides of your face and doesn’t have gaps, cover coughs and sneezes, wash hands often, and follow any applicable workplace or school guidance."
Continued mask use is also important to stave off the spread and development of mutant strains of the coronavirus that could prove resistant to the already-developed vaccines.
Brandon Guthrie, an epidemiologist at the University of Washington, told Vox that the potential for variants to arise makes mask usage a still important element of countering the pandemic.
“These variants — especially ones that have mutations that make them more transmissible — can pop up and then spread quite quickly,” Mr Guthrie said. “If you combine that with a variant that could evade the current vaccines, you’re not going to get a lot of advance warning. Which means a widespread outbreak, including some people who had already gotten their shots, could get going before we were able to contain it.”