''Those who got the inactivated vaccine [the traditional flu shot] were half as likely to get the flu as those who got the live attenuated vaccine [the flu nasal spray]," says the study's lead author Arnold Monto, MD, a professor of epidemiology at the University of Michigan School of Public Health in Ann Arbor. The study is published in the New EnglandJournal of Medicine.
Despite those results, Monto and other experts say that getting vaccinated against seasonal flu -- whether by the traditional injection or the newer nasal spray -- is better than getting no vaccine at all.
Flu Shot Beats Flu Spray: Study Details
For the study, Monto and his colleagues enrolled 1,952 healthy men and women, all aged 18 to 49, in the fall of 2007. They randomly assigned the participants to receive either the traditional vaccine, the nasal spray, or placebo, without the participants knowing if they were getting placebo or the actual vaccine.
They tracked the participants throughout the 2007-2008 flu season, and suspected cases of influenza -- marked by muscle aches, headache, nasal congestion, nausea, and vomiting -- were confirmed either by cell culture of the virus or by a highly sensitive test known as a PCR assay, or both.
Flu Shot Beats Flu Spray: Study Results
At the end of the study, 119 people had confirmed influenza. Those who got the injection had a 50% reduced chance of getting influenza compared to the group that got the nasal spray.
When the researchers looked only at the vaccines' effectiveness in preventing influenza A, the more common flu to infect the participants, the injection was 72% effective but the nasal spray just 29%.
Too few cases of influenza B were found to compare the vaccines, according to Monto. Funding for the study was provided by the National Institute of Allergy and Infectious Diseases and an unrestricted grant from Sanofi Pasteur, which makes the flu shot vaccine. MedImmune provided the nasal spray vaccine. Monto has received lecture fees from Sanofi Pasteur.
Flu Shot Beats Flu Spray: Study Implications
In recent years, studies comparing the two types of vaccines have produced mixed results, Monto says. ''Definitive studies have been done in children looking at the same comparison and they have found the live attenuated [nasal flu spray] outperforms the inactivated, and that's for children under age 6."
Why is that? ''The nasal spray vaccine works best in bodies which have produced fewer antibodies," Monto speculates. The virus in the nasal spray is attenuated, or weakened, and it must infect the nasal passages to trigger an immune response, the production of antibodies, which protects against infection by the virus.
Children typically have not been exposed to as many strains of the flu virus, so they have not built up as many antibodies in their body as adults have. So the nasal spray may have a harder time infecting adult nasal passages, as they may have been exposed to similar flu virus strains in the past and already have antibodies to them, Monto speculates.
Is his study likely to be the last word for adults aged 18 to 49? ''We think the study is at least definitive when circulating viruses are relatively similar to the ones that have been circulating before," Monto tells WebMD. With seasonal influenza, the viruses change from year to year, and experts must predict which strains will be prevalent in deciding how to formulate the vaccine each year.
Flu Shot vs. Spray: Other Opinions
The new study ''is going to surprise some people, I think," says Zachary Rubin, MD, a hospital epidemiologist at Santa Monica -- UCLA and Orthopaedic Hospital, Santa Monica, Calif., and an assistant clinical professor of medicine, division of infectious diseases, at the David Geffen School of Medicine, University of California Los Angeles.
Because of the studies finding the nasal spray better at protecting against influenza in children, he says people may have assumed the same might be true in young adults.
Based on the new study results, Rubin says, "I think young adults 18 to 49 should perhaps consider getting the shot instead of the spray.'' (The CDC says the spray is an option for healthy people aged 2 to 49 if they are not pregnant and don't have certain health conditions.)
If a spray is somehow the only option, however, Rubin says the important thing is to get immunized.
The study results mirror those published earlier this year by a team from the Armed Forces Health Surveillance Center in Silver Spring, Md., says Jose Sanchez, MD, the leader of the influenza team at the center. "It reiterates the evidence that we have seen in the military where inactivated vaccine -- the flu shot -- outperforms the intranasal vaccine," he says.
In that study, published in the Journal of the American Medical Association, researchers tracked the effectiveness of the nasal vaccine and compared it with the injected vaccine among military personnel and found that the injection was associated with fewer medical visits related to flu compared to the nasal spray.
Sanchez and Monto say it's important to emphasize that the comparison studies compared vaccines for seasonal influenza, not for current H1N1 or "swine flu." Whether adults could respond well to a nasal spray H1N1 vaccine, since the strain is new to both children and adults, isn't yet known.