Participants who'd received the nasal spray vaccine were 36 percent less likely to get the flu than those who'd received a placebo.
The injection was 50 percent better in preventing the flu than the nasal spray, according to the study.
Though researchers did not test vaccines in children, nasal sprays may work just as well in children as injections, Monto said.
There are two different types of vaccines, usually referred to as live attenuated, which contain very weakened or modified live virus, and inactivated, which contain bits of dead virus.
Live attenuated influenza vaccine, which comes in nasal sprays, must replicate in the body in order to provoke the immune system to produce antibodies against the virus. In adults who may already have some immunity against that or other flu strains, the live attenuated virus may not be strong enough to cause that response, Monto said.
Children are more likely to lack antibodies to the virus. Without any natural immunity, the nasal spray may be equally effective, Monto said.
Live attenuated vaccines "must infect in order to protect," Monto said. "Adults, unlike children, have antibodies to the virus included in the vaccine. They are not infected by it and therefore are not protected. This would explain why the LAIV [live attenuated influenza vaccine] is highly effective in younger children."
In some cases, the live attenuated influenza vaccine may even provide enhanced protection, said Dr. Kenneth Bromberg, director of the Vaccine Research Center at the Brooklyn Hospital Center in New York City.
In addition to immunity involving antibodies, live attenuated vaccines can also provide cell-mediated immunity, an added type of immune response that can boost effectiveness.
"They are different approaches, and in the case of a well-matched strain, the inactivated vaccine worked better than the live attenuated," Bromberg
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