Seasonal Viruses May Flow from Tropical Sources

Influenza A viruses, seen with an electron microscope.Wellcome Library, London.

Influenza virus strains that cause seasonal flu in temperate climates may emerge anew from tropical regions each year, according to a new study. Understanding how flu viruses evolve and spread is essential for making more effective future flu vaccines.

Each year, influenza is estimated to cause 250,000 to 500,000 deaths worldwide, according to the World Health Organization. Influenza type A, the most potent version of the virus, is responsible for seasonal epidemics in temperate climates during winter months. Researchers design flu vaccines to present our immune system with a part of the virus so that our bodies can recognize the real virus when it tries to infect us. But viruses accumulate mutations quickly so that our immune systems no longer recognize them. New strains of the virus thus emerge, requiring new flu vaccines to combat them.

To better understand how viruses evolve, a team of international researchers performed an in-depth comparison of 1,302 individual influenza A viral genomes. The viruses came from 2 different subtypes that were collected over 12 years from New York State (northern hemisphere) and New Zealand (southern hemisphere). The research team included scientists from NIH’s National Institute of Allergy and Infectious Diseases (NIAID) and Fogarty International Center (FIC). The work was also supported by NIH’s National Institute of General Medical Sciences (NIGMS). Their results were published online on April 16, 2008, in Nature.

As expected, the researchers found that each flu virus epidemic peaked during the winter months. Genes encoding for proteins with related functions in the virus seem to evolve together in sets. The researchers also found that one of the viral surface proteins has the highest rate of mutation, making it an unlikely candidate for future vaccine development, as it would be difficult for scientists to stay one step ahead of the changes.

The patterns of virus evolution that the scientists observed suggested that the viruses present in low-levels in temperate climates during warmer months weren’t the ones that lead to flu epidemics. The influenza viruses that caused epidemics seemed to originate from "source" populations where flu viruses persist year-round, such as the tropics of South-East Asia. Each winter season, the researchers hypothesize, viruses flow from tropical climates to temperate climates in the northern and southern hemispheres. Once epidemics have run their course in a temperate area, new viruses come from the tropics to take their place the following flu season.

Studies of more influenza virus samples from the tropics will be needed to confirm the idea that seasonal flu epidemics are caused by viruses flowing from tropical climates to temperate ones. Researchers hope that by understanding these trends in virus evolution, they will be better able to predict how to make vaccines before new influenza viruses even arrive in temperate climates.

—by Vanessa C. McMains