Why is 'flu seasonal?
Transmission electron micrograph of an influenza virion. Image courtesy of the Public Health Image Library (image number 8430).
Every winter in temperate regions, there is an epidemic of influenza. The number of cases and deaths varies from year to year, but is usually substantial: hundreds of thousands of hospitalizations and tens of thousands of deaths in the US alone.
The influenza virus is notorious for its rapid evolution. Viral surface proteins (antigens) that the immune system recognizes to mount a rapid antibody response change from year to year. This antigenic drift means that the virus can reinfect and transmit from people who have been infected in previous years.
However, influenza viruses do not show any appreciable antigenic drift within 'flu seasons. So how do antigenically distinct viruses arise at the beginning of each 'flu season? Do they evolve locally or are they imported from elsewhere in the world?
A recent paper in PLoS Pathogens by Martha Nelson and Eddie Holmes of Penn State's Center for Infectious Disease Dynamics, and colleagues at the National Institutes of Health, supports the "imported from elsewhere" hypothesis. In an analysis of about 900 complete influenza genomes from Australia, New Zealand and the US, they found considerable evidence that viruses are moving around the world in complex and variable ways. In any given year, viruses appear to move from the southern to the northern hemisphere, and vice versa.
Like other studies, the Penn State researchers and their collaborators found no evidence for within-season in-situ evolution of the viruses. In any one place, multiple variants (clades) of the virus circulate in a given winter; some of these clades are more closely related to viruses found on a different continent than to isolates from the same place in the previous or following 'flu season.
The authors point out that their study highlights a need for increased sampling and surveillance of influenza virus diversity to understand the dynamics of viral evolution and consequent epidemics. In particular, there is currently little understanding of how year-round incidence of the virus in some tropical regions affects the diversity of viruses available to cause winter epidemics in temperate regions.