WEDNESDAY, Sept. 23, 2009 (HealthDay News) -- If you have the choice between a seasonal flu vaccine that comes in a nasal spray or an injection, go for the injection, new research shows.
In a study of adults tracked over one flu season, vaccines made from inactivated, or "killed," flu virus -- the injectable form -- provided better protection against the seasonal flu than vaccines made from live attenuated virus, the type of vaccine available in a nasal spray.
"The nasal spray vaccine is effective but isn't as effective as the injected vaccine," said lead study author Arnold S. Monto, an epidemiology professor at the University of Michigan School of Public Health. "But it's better to get some vaccine than no vaccine, so if you're averse to getting an injection, get the nasal spray."
The researchers stressed that their findings, published in the Sept. 24 issue of the New England Journal of Medicine, applied to seasonal flu vaccine efficacy in adults only. The same may not hold true for children, who may respond just as well to a nasal spray vaccine, or for the H1N1 swine flu vaccines that are on the way.
Researchers gave 1,952 adults aged 18 to 49 either an injected flu vaccine, a placebo injection, a flu vaccine nasal spray or a placebo nasal spray during the 2007-2008 flu season. That season, vaccines were well-matched to the predominant flu in circulation, according to the study.
Participants were reminded each week to come in for an exam and lab tests if they showed signs of respiratory illness. About 6.1 percent of participants, 119 in all, got the flu, mostly influenza A/H3N2.
Among those who got sick, participants who'd received the injected flu vaccine were 68 percent less likely to have the flu compared to those who'd received a placebo, according to the study. The flu virus was confirmed using lab tests.
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 said. "There could be situations, such as with a mismatched strain, in which the live attenuated could perform just as well, or perhaps even better, than the injected."
And what about for H1N1 swine flu vaccines?
With the swine flu, vaccines made with live attenuated virus may work just as well as injected vaccines because children and most adults have no antibodies or natural immunity to the new strain, Monto noted.
"We are totally susceptible," he said. "That's why we are having these school outbreaks with high attack rates."
The U.S. Centers for Disease Control and Prevention has more on seasonal flu.