Final answer:
In a campaign about seasonal influenza, the example of secondary prevention is encouraging vaccination, as it aims to reduce the impact of the disease by building immunity.
Step-by-step explanation:
In a campaign about seasonal influenza, an example of secondary prevention is encouraging vaccination. Secondary prevention aims to reduce the impact of a disease that has already occurred. Vaccination helps by building immunity to a virus, preventing outbreaks, and reducing symptoms. While measures such as promoting hand hygiene and public health education are important, they are typically considered primary prevention strategies because they aim to prevent the disease from occurring in the first place. Providing antiviral medications and educating about symptoms may help reduce the severity and spread of influenza once it has occurred, but the only option listed that falls into the category of secondary prevention is vaccination.