Each seasonal influenza vaccine contains antigens representing three (trivalent vaccine) or four (quadrivalent vaccine) influenza virus strains: one influenza type A subtype H1N1 virus strain, one influenza type A subtype H3N2 virus strain, and either one or two influenza type B virus strains.Currently circulating influenza viruses in worldH1N1 (Type A): Cause of Swine fluQH2N2 (Type A)H3N2H5N1 (Type A): Cause of Avian influenza (Bird flu)QH7N9Type BInfluenza vaccinesEach seasonal influenza vaccine contains antigens representing three (trivalent vaccine) or four (quadrivalent vaccine) influenza virus strains:One influenza type A subtype H1N1 virus strainOne influenza type A subtype H3N2 virus strainEither one or two influenza type B virus strains.Influenza vaccines may be administered as an injection or as a nasal spray.The US Centers for Disease Control and Prevention recommend that everyone over the ages of 6 months should receive the seasonal influenza vaccine.Vaccination campaigns usually focus on people who are at high-risk of serious complications if they catch the flu, such as the elderly and people living with chronic illness or those with weakened immune systems, as well as health care workers.Most flu vaccines provide modest protection against contracting influenza, with the effect seasonably variable depending on antigenic drift.WHO recommends that influenza vaccines for use in the 2016 southern hemisphere influenza season contain the following viruses:An A/California/7/2009 (H1N1) pdm09-Iike virusQAn A/Hong Kong/4801 /2014 (H3N2)-like virusQA B/Brisbane/60/2008-like virusQIt is recommended that quadrivalent vaccines containing two influenza B viruses contain the above three viruses and a B/ Phuket/3073/2013-like virusQ.WHO recommends seasonal influenza vaccination for:Highest priorityPriority (in no particular order)* Pregnant womenQ* Children aged 6-59 monthsQ* ElderlyQ* Individuals with specific chronic medical conditionsQ* Health care workersQ">