Your immune system is pretty amazing. If it encounters a pathogen it has met before, it will likely mobilize an antibody response to annilate the pathogen before you get sick and infectious. But some pathogens can undermine your defences: they evolve, so your immune system is less likely to recognize them on reencounter.
Influenza A virus is a good example. Changes in the viral genetic code can alter the shape of viral proteins (antigens) that your immune system uses to recognize the virus. Some genetic changes greatly affect antigen shape whereas others result in little or no antigenic change. During the last few decades, the influenza A H3N2 virus has undergone several major antigenic changes and many smaller ones. If antigens change only slightly by the time you're re-infected, then you may be partially immune to the new version of the virus. The more the antigens change, the less likely your immune system is to recognize the virus and the more susceptible you become.
This much is well known. What has been less clear is how:
- Viral genetic changes affect the spread of disease through the host population, given the profile of immunity built up during successive challenges with previous versions of the virus.
- Epidemic dynamics affect viral evolution.
Now Katia Koelle, Bryan Grenfell and collaborators have shown how viral evolutionary changes can account for observed epidemic patterns of influenza, and vice versa.
The researchers constructed a mathematical model for viral evolution and spread that allowed the degree of cross-immunity to depend on the extent of antigenic change.
They demonstrated that this simple model could account for many features of influenza A epidemics in humans, including lower attack rates during the second year after a large antigenic change.
The model also accounted for the observed "boom and bust" pattern of influenza virus evolution, whereby genetic diversity drops after a large antigenic change (a "cluster jump"), then rises during periods of antigenic stasis.
Science published a "Perspectives" article about this paper.
Written By: Katia Koelle, Sarah Cobey, Bryan Grenfell, & Mercedes Pascual
Journal: 314: 1898-1903
Journal Reference: 314: 1898-1903
Paper Id: 10.1126/science.1132745