(NEXSTAR) — So your pal is vaccinated against COVID-19, but you’ve yet to be immunized yourself. Is it safe to travel in a car with them?

According to William B. Greenough, a professor emeritus of medicine at Johns Hopkins School of Medicine, we don’t quite know yet.

Greenough, who’s known widely for his work on cholera, said there has yet to be a definitive study determining the effectiveness of the vaccine at halting transmission of the virus.

Though we know both the Pfizer and Moderna vaccines are 95% effective at preventing COVID infections, we don’t yet know if that means vaccinated individuals can be carriers.

“When someone is vaccinated, it’s very likely, but not proven, that they’re not going to carry or transmit the virus anywhere as much as an unvaccinated person,” Greenough said.

His guess — and it’s a guess, he stipulated — “is it’s probably fairly safe to travel with an immunized person.”

“It would be very safe, however, if the immunized person wore a mask,” he said.

Greenough recommends that anytime you’re in a car with someone outside your immediate household — vaccinated or not — wear a mask and avoid hugging, handshakes and other kinds of contact.

The story changes if you’re both vaccinated. On Monday, the Centers for Disease Control and Prevention released new guidelines suggesting that it’s safe for fully vaccinated people to gather without masks or social distancing.

A person is considered fully vaccinated two weeks after receiving the last dose of the vaccine.

The guidance is designed to address a growing demand as more adults are getting vaccinated and wondering if it gives them greater freedom to visit family members, travel, or do other things as they did before the COVID-19 pandemic swept the world last year.

“With more and more people vaccinated each day, we are starting to turn a corner,” said CDC Director Dr. Rochelle Walensky.

During a press briefing Monday, she called the guidance a “first step” toward restoring normalcy in how people come together. She said more activities would be approved for vaccinated individuals once caseloads and deaths decline, more Americans are vaccinated, and as more science emerges on the ability of those who have been vaccinated to get and spread the virus.

The CDC is continuing to recommend that fully vaccinated people still wear well-fitted masks, avoid large gatherings, and physically distance themselves from others when out in public. The CDC also advised vaccinated people to get tested if they develop symptoms that could be related to COVID-19.

The CDC guidance did not speak on people who may have gained some level of immunity from being infected, and recovering from, the coronavirus.

The Associated Press contributed to this report.