Final answer:
The follow-up study on flu virus evolution highlights the rapid mutation and combination of flu strains, as seen in the H7N9 influenza, and the ongoing effort to produce annual flu vaccines in response to these evolutionary changes.
Step-by-step explanation:
The follow-up study for the evolution of the flu virus demonstrated the rapid mutation and reassortment abilities of flu strains, culminating in new threats such as the H7N9 influenza reported in China in 2013. This particular strain resulted from the combination of three different viruses, raising concerns due to its high mortality rate. The study underlines the importance of surveillance by organizations such as the World Health Organization and the Centers for Disease Control and Prevention.
With flu viruses evolving and adapting quickly, vaccines from previous years might not offer sufficient protection against new strains. Hence, scientists work diligently each year to predict the potential prevalent flu strains based on their evolutionary trends and accordingly develop effective vaccines. The annual flu shot is thus a direct response to the evolutionary progression of influenza viruses.
As for the Avian virus and the 1918 Spanish influenza, these viruses, among others, underscore the perpetual risk of pandemics due to their ability to infect the respiratory system with potential high fatality rates.