Although Vice President Kamala Harris was infected by the coronavirus in April, it’s not impossible that she could become infected again — a prospect raised by her being with President Biden just two days before he tested positive.

According to her schedule for that day, Ms. Harris received the president’s daily briefing in the Oval Office. She also met with Olena Zelenska, the first lady of Ukraine, with Mr. Biden and others at the White House.

The prospect of both Ms. Harris, 57, and Mr. Biden, 79, contracting the virus could be unsettling, but both are fully vaccinated and have received two booster shots, which remain highly protective against severe disease. Ms. Harris received her second booster on April 1, and Mr. Biden on March 30.

Ms. Harris, who traveled to North Carolina on Thursday, said that she had spoken with Mr. Biden by phone and that he was “in good spirits.”

According to a White House official, Ms. Harris tested negative Thursday morning. On the advice of the White House medical team, she will remain masked, but her schedule will continue as planned.

She may not be in the clear. To account for the virus’s incubation period, many experts recommend taking a rapid test two to four days after a potential exposure, and taking at least two rapid tests about a day apart.

Ms. Harris previously tested positive for the coronavirus on April 26. In announcing her positive test, her office said she was not experiencing symptoms and would isolate at home. At that time, Ms. Harris was already fully vaccinated and had received two booster shots. She was prescribed the antiviral treatment Paxlovid.

At the time of her infection, the vice president had not been in close contact with Mr. Biden. She had spent several days in California and had not seen the president in person in eight days.

A month before that, Doug Emhoff, Ms. Harris’s husband, tested positive for the virus. Ms. Harris was not infected then, but while she isolated and continued to test negative, Mr. Emhoff’s positive test forced her to cancel an appearance at an event with Mr. Biden.

It is possible the vice president could contract the virus again. The antibodies that help protect against infection wane over time, and Omicron is more adept at dodging those antibodies than previous variants were.

Even a previous Omicron infection may not protect against a subsequent one. Although it is not clear what version of the virus Ms. Harris had in April, at the time, the BA.2 subvariant of Omicron was the dominant version in the United States.

Now, the BA.5 subvariant, which has spread even more quickly than previous versions, is causing a new surge of cases, including a spate of reinfections. The Centers for Disease Control and Prevention estimated last week that BA.5 was accounting for nearly 80 percent of new infections across the U.S.

“You can be previously infected — even as recent as the last couple of months — and have a very high rate of reinfection,” Dr. Ashish K. Jha, Mr. Biden’s coronavirus response coordinator, said in an interview last week.

A recent study from Qatar, which has not yet been reviewed by outside experts, suggests that BA.4 and BA.5 are better at evading antibodies from previous coronavirus infections than earlier versions of Omicron were.

However, people who were infected with an earlier version of Omicron should be better protected than those infected with other variants of the virus. According to the Qatar study, an infection with a pre-Omicron variant was 28 percent effective at preventing a subsequent infection with BA.4 or BA.5. A previous Omicron infection, however, was 80 percent effective at preventing a BA.4 or BA.5 infection.