But even if the president were now immune to the coronavirus, he may not remain so, scientists warn. The president’s unique treatment may have prevented his body from making the antibodies necessary for long-term protection.

The monoclonal antibodies he received were produced by the drug company Regeneron and will wane in a matter of weeks, as the synthetic molecules are known to do. Without replenishment, this decline may leave Mr. Trump even more susceptible to the virus than most patients who have recovered from Covid-19, several experts warned.

Moreover, the steroid treatment the president received early in the course of his illness suppresses the body’s natural immune response, including its propensity to make antibodies of its own.

“He may be not protected the second time around, especially because he didn’t develop his own antibodies,” said Akiko Iwasaki, an immunologist at Yale University.