Booster vaccination to help against avian influenza pandemic

H5N1 continues to pose a major
health risk to birds and humans. As of mid-June, more than 60 percent of the
more than 380 human cases have been fatal, and hundreds of millions of birds have
died or been culled to prevent the spread of the disease. Should the virus
evolve making human-to-human transmission more likely, a destructive global
influenza pandemic could result.

The cornerstone of planning
for such a possible pandemic is the development and distribution of effective
vaccines. Several vaccines have been developed, but as the virus continues to
mutate into genetically distinct lineages, or clades, the problem arises as to
whether vaccines based on an older clade will be effective against newer
versions. The new study is the first to report that giving one dose of a newer-clade vaccine to those
who were vaccinated previously with older versions is more effective than
giving only doses of the newer vaccine to unvaccinated subjects. The study,
conducted by Nega Ali Goji, MD, and colleagues from New York, Maryland, and
Alabama, gave a single booster dose of a vaccine based on a clade 1 H5N1 virus
circulating in Vietnam in 2004 to subjects who eight years earlier had received
two doses of a vaccine based on the original, clade 0 virus that appeared in
Hong Kong in 1997. Sixty-four percent had a positive immune response, which
compares favorably to the results of a previous study using two doses of the
clade 1 Vietnam virus, in which only 43 percent of those vaccinated had
a positive immune response.

The results not only support
the booster technique, but also show that even though the virus had mutated
since the initial vaccination, using it to boost an earlier vaccine is more
effective than simply vaccinating subjects with the most current vaccine. These
findings are important given the fact that influenza viruses are mutating
constantly. “These results suggest that one strategy for pandemic control
could involve prevaccination of some segments of the population prior to the
emergence of a pandemic so that effective protection could be achieved with a
single dose schedule if and when a pandemic emerges,” the authors wrote.
“If the finding that priming can result in enhanced responses to
single-dose vaccination schedules were confirmed, then pre-pandemic vaccination
programs could be considered, especially in populations of first responders,
health care workers, or the military. Such populations might then be able to be
effectively and rapidly vaccinated with a single dose of a vaccine specific for
an emerging pandemic if it were to occur.” 

In an accompanying editorial,
Gregory Poland, MD, of the Mayo Clinic College of Medicine, noted that some are
already looking to begin such prevaccination primers against H5N1 influenza.
For example, Japan is planning to immunize health care workers starting in
2009, and the U.S. Department of Defense is offering a vaccine to those in high
risk specialties. Dr. Poland pointed out that new studies are needed to investigate
different types of vaccine administration, deal with vaccinations that prevent
death but not infection and illness, search for more broadly cross-protective
influenza vaccines, and collect data on the vaccination of those who are not
healthy adults. Although, he said, “determining who should receive these
vaccines, when, and in what order and under what circumstances deserves
widespread debate,” he agrees that the findings of the study are novel, as
they “suggest that such a prime-boost strategy using vaccines derived from
different H5 clades, separated by years, may be worthwhile, immunologically
feasible, and safe.”

 The study is published in the 1
August issue of the Journal of Infectious Diseases, now available
 online.