If an individual has been infected with a similar strain within the past few years, then antibodies might neutralize the infecting virions. Anti HA antibodies such as IgA and IgG have been shown inhibit uncoating of the virus. However, if preexisting antibodies do not protect against the infection, activated macrophages and natural killer cells help recover from the infection. A polyvalent inactivated vaccine is developed each year depending on the most common strains of influenza present during the last season. Common side effects from the vaccine include local tenderness, and less commonly fever and malaise for a day.
Efficacy is highest in the young and lowest in the old due to the idea of original antigenic sin.
An influenza epidemic . If you got your annual influenza vaccine, then you win.