A good strategy for developing a universal flu vaccine may be to try to mimic the body's natural immune response to a pandemic flu virus, a new study suggests.
The researchers wanted to examine changes in the immune system that result from a lifetime of exposure to flu viruses, so they analyzed blood samples from 40 people, ages 35 to 70, over a 20-year period.
Participants who had been exposed to two particular strains of pandemic flu viruses — H2N2 in 1957, and H1N1 in 1977 — had increased levels of special immune proteins called broadly neutralizing antibodies.
Broadly neutralizing antibodies target a region of the flu virus called the "stalk,", which varies little between flu strains (in contrast, another region, called the "head," changes frequently).
It's thought that boosting levels of these special antibodies could be the key to creating a universal flu vaccine. However, people normally don't produce high levels of broadly neutralizing antibodies when they're exposed to a seasonal flu virus, because the body makes it a priority to produce antibodies against the virus head.
It's only when a virus is very different from what people have encountered in the past — as is the case with pandemic strains, which have very different heads — that the body increases production of broadly neutralizing antibodies against the stalk, said study researcher Mathew Miller, a postdoctoral fellow in microbiology at Mount Sinai School of Medicine in New York.
"If we mimic the natural situation that occurs when a pandemic strikes using a vaccine approach, then we may be able to generate a universal flu vaccine," Miller said.
Levels of broadly neutralizing antibodies increased modestly over time in the study participants, and were highest among those who'd been exposed to more than one pandemic. Levels of broadly neutralizing antibodies were 3.8-fold higher in those who had been exposed to both H2N2 and H1N1, compared with those exposed to only H1N1, the study found.
The finding suggests a strategy for making a universal flu vaccine: create a vaccine that contains flu viruses with very different heads, but highly similar stalks, Miller told LiveScience.
The study also found that levels of antibodies against the flu virus head increased over time for each of the pandemic flu viruses. This increase occurred despite the fact that participants were exposed to the pandemic flu strains only once.
This finding is important because, previously, researchers didn't really know how long immunity against pandemic viruses lasted.
"The observation that we're constantly boosting our levels of antibodies to strains we've encountered in the past suggests that, during the lifespan of a given generation, it’s very unlikely that a strain similar to one that they've seen earlier in their lives will cause a major pandemic," Miller said.
The study is published today (Aug. 14) in the journal Science Translational Medicine.