Seasonal influenza is a common respiratory tract infection caused by human seasonal influenza viruses. The influenza viruses could be transmitted through respiratory droplets, and could be latent for 1-4 days. At the onset of influenza, the infected individual will manifest symptoms including malaise, runny nose, headache, coughing, fever and sore throat and etc. In Hong Kong, seasonal influenza is usually prevalent in months like from January to March and from July to August. Quadrivalent influenza vaccine is eligible for individuals aged 6 months or above, and is advised to be taken for comprehensive protection against influenza.