The new finding settles a key debate on what the virus dues during the summer, when it is not afflicting people.
"Nobody really knows why flu is a winter disease in the temperate regions and more continuous in the tropics. The big question is, 'Why is flu seasonal,'" says Edward Holmes, professor of biology at Penn State.
The flu virus in the Northern Hemisphere evolves some time before the start of the winter infection season, changing enough to evade the previously primed immune system. Just before summer, the virus disappears, only to resurface the next fall with a completely different genetic makeup.
However, scientists have so far been clueless about what happens to the virus between two successive winters.
According to Holmes, the key question is whether the virus settles into a dormant state waiting for the right cues of temperature and sunlight to reactivate, or whether it migrates to viral reservoirs in the tropics, from where it is later reintroduced.
Scientists believe that regions like Southeast Asia, where humans and animals live in close proximity, may be the permanent melting pot where viruses continually circulate and exchange genetic information.
Holmes and Penn State graduate student Martha Nelson worked with their colleagues at the National Institutes of Health to analyse the influenza A virus genomes of 900 virus samples from New Zealand, Australia and New York state dating between 1998-2005.
The researchers found that the genomes of 52 viruses from New York were closely related to viruses that circulate during the winter (April to October) in New Zealand and Australia.
They said that the mixed family trees of viruses from both the north and south suggested that there exists widespread viral traffic across the equator each season, which contributes significantly to new epidemics in both hemispheres.
"If the viruses had been dormant, samples from successive seasons in each region would only be closely related to other viruses of that same region. The fact that they are, instead, interspersed clearly tells us that the viruses are seasonally migrating across both hemispheres," said Holmes, who is also affiliated with Penn State's Center for Infectious Disease Dynamics (CIDD).
The researchers are, however, still unsure where and when the viruses are evolving to beat the immune system. Martha Nelson believes that during the summer, the virus changes its entire genetic makeup in the tropics, where it is found year-round.
"But we cannot say for sure at this point. To test this theory, we need viral samples from the tropics," Nelson said, expressing hope that samples from these regions will be available in the next few years.
The researchers are also unsure why the virus seems to die out during summer and what exactly triggers its return.
"It could be anything from human migration, aspects of climate, levels of sunlight, seasonal susceptibility of people or a combination of all these and more factors. That is another big question," said Holmes, whose work is funded by the National Institutes of Health.
The study published in PLoS Pathogens states with certainty that multiple viruses arrive in New York state each season.
Holmes says that the best way to protect communities is to have an extremely good system of disease surveillance in place, and to develop universal vaccines that can protect against all kinds of influenza virus.