Injectable vaccine better than intranasal for seasonal flu in adults

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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 new report today 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.’ But he noted that it is not yet clear whether the same findings will 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 around the beginning of 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 originally developed by epidemiologist Hunein ‘John’ Massaab, have been studying its efficacy for several years. Earlier results have suggested that the injectable vaccine was better, but the results have not been 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 the fall of 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 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. Monto said that they are now compiling data to determine whether that is the case but that ‘the differences are not striking.’

Karen Lancaster, a spokesperson for MedImmune, 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 some hope for use of vaccines against swine flu, according to Monto. 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, who have been exposed to few viruses. Because the swine flu is so sharply different from the seasonal viruses of past years, even adults may have little resistance to it and the swine flu vaccine is likely to be more efficient than expected, he said.

-- Thomas H. Maugh II

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Photo credit (top): Harry Cabluck / Associated Press

Photo credit (bottom): Keith Srakocic / Associated Press