Injectable vaccines containing inactivated viruses prevent about 50% more seasonal flu in healthy adults than the intranasal vaccine containing a weakened virus, according to a report published online Wednesday in the New England Journal of Medicine.
"We have two effective vaccines," said Dr. Arnold S. Monto of the University of Michigan, who led the study. "In children, I would prefer FluMist [intranasal vaccine], and in adults, injected, based on the data we have. However, an adult who does not want a shot should take FluMist."
He noted that it was not yet clear whether the findings would hold for the pandemic H1N1 influenza virus, commonly known as swine flu.
FluMist will be the first vaccine available for swine flu, with at least 3.4 million doses arriving in early October. Overall, FluMist, manufactured by MedImmune Corp. of Gaithersburg, Md., will account for about 42 million of the roughly 270 million doses of swine flu vaccine ordered by the federal government.
Multiple studies have shown that the intranasal vaccine works better against seasonal flu in children. Three large head-to-head studies showed that those who received FluMist had 35% to 53% fewer cases of flu than those who received injectable vaccines. But the results in adults have been much more inconsistent, with some studies showing FluMist to be better and others showing injectables to be superior.
Researchers at the University of Michigan, where the inactivated-virus vaccine was developed by epidemiologist Hunein "John" Maassab, have been studying its efficacy for several years.
Earlier results suggested that the injectable vaccine was better, but they were not conclusive -- in part because the seasonal flu outbreaks of recent years have been mild.
For the current study, Monto and his colleagues enrolled 1,952 healthy adults in fall 2007. Healthy adults were chosen because they are the only group for which seasonal flu vaccine is not recommended and are thus the only group that could ethically be given placebo vaccines. Half were given either FluMist or an injectable vaccine produced by France's Sanofi-Pasteur, and half were given a placebo.
By the end of the January-April 2008 flu season, the injectable vaccine had protected 68% of recipients, while the intranasal vaccine had protected 36%. That's 50% fewer. Many people believe that even when the vaccine does not prevent infection, it minimizes symptoms of the virus.
Karen Lancaster, a MedImmune spokeswoman, noted that "other studies have indicated that FluMist works just as well as -- or, in some cases, better -- than the flu shot. As a result, in adults, the jury is still out."
The difference in response between adults and children may offer hope for vaccines against swine flu, Monto said.
The intranasal vaccine must infect the nasal passages to produce a protective immune response. Adults who have been exposed to a variety of influenza viruses probably have more antibodies in their nasal passages than do children.
But because the swine flu is so sharply different from the seasonal viruses, even adults may have little resistance to it and thus the swine flu vaccine will probably be more efficient than expected, he said.