H5N1 continues to pose a major health risk to birds and humans and it is feared that if the virus evolves making human-to-human transmission a reality, it may result into a destructive global influenza pandemic.
The only solution to avert this possibility 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 current study has become the first to state that 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.
Conducted by Nega Ali Goji, MD, and colleagues from New York, Maryland, and Alabama, the study 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.
This showed that 64 percent of subjects had a positive immune response, which compares favourably 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.
Not only do the findings support the booster technique, but they 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," wrote the authors.
They added: "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 A. Poland, MD, of the Mayo Clinic College of Medicine, noted that some are already looking to begin such prevaccination primers against H5N1 influenza.
Poland also stated 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.
The study is published in the August 1 issue of The Journal of Infectious Diseases, now available online.