The 1918 influenza pandemic (the “Spanish Flu”), by some estimates, killed as many as 100 million people in a very short period of time. The 2009 “Swine Flu” pandemic didnt kill so many, but it spread rapidly and widely across the globe. Despite that difference, it turns out the two viruses responsible for these pandemics have some important similarities.
Influenza virus has a protein on the surface called hemagglutinin, or HA, which is used to attach to host cells, allowing the virus to then enter and replicate. HAs change rapidly, which is partly why influenza keeps coming back. When HA changes, your antibodies dont recognize it so well, so you get sick again. It turns out that the HAs of 2009 and 1918 are similar on both the sequence and structure level. There is a small patch on the HA protein that is 95% identical between 1918 and 2009 but only 70% identical to seasonal strains. Looking only at the 3D structure, among all influenza HAs, the 2009 HA is most similar to the 1918 HA. The 1918 and 2009 HAs also lack glycosylation at the tip, while seasonal influenza viruses HAs are sugary.
Why is that interesting? An unusual pattern was noted in the 2009 pandemic: elderly people were not as affected as younger people, the reverse of what is usually seen with influenza. It was proposed that perhaps some people still had immunity to the 1918 virus, which continued to circulate for many years after 1918, and that immunity was cross-protective. A recent study shows that this indeed seems to be the case. Mice immunized with the 1918 virus are protected against the 2009 virus. The converse is also true: if you immunize mice with the 2009 virus, they are protected against the 1918 virus. That’s pretty impressive when you consider that one season’s vaccine might not protect you from next season’s virus. It seems the immune system cant really tell the difference between these viruses. Note that it also tells us how long immunity can last! The next question is, how and why has this HA structure come back?