Nasal vaccines are “the only way to really circumvent person-to-person transmission,” said Jennifer Gommerman, an immunologist at the University of Toronto. “We can’t live forever sheltering vulnerable people and boosting them so that their antibody levels stay artificially high.”

Nasal vaccines have been shown to protect mice, ferrets, hamsters and monkeys against the coronavirus. A new study last week offered powerful evidence in support of their use as a booster.

An intranasal booster induced immune memory cells and antibodies in the nose and throat, and strengthened protection from the initial vaccination, the researchers reported.

“Our approach is to not use a nasal vaccine as a primary vaccination, but to boost with nasal vaccine, because then you can leverage the existing immunity that’s already created,” said Akiko Iwasaki, an immunologist at Yale University who led the study.

When she and her colleagues used a mix of proteins from the new coronavirus as well as the related SARS virus, their experimental nasal vaccine seemed capable of fending off a broad range of coronavirus variants.

“There’s some flexibility, and there might be more resilience against the virus,” said Dr. Gommerman, who was not involved in the work. “And because we don’t know what the virus will do next, that’s awfully appealing.”