The northern and southern strains follow the cold seasons of the northern hemisphere (NH) and southern hemisphere (SH) respectively. The composition of the seasonal flu vaccine is determined by information released by the WHO (World Health Organisation) detailing the influenza strains that are predicted to be active and prominent during the months of May to October (SH) and October to April (NH). Flu vaccination is recommended once a year, but a repeat vaccination may be recommended if the strains vary significantly between the hemispheres.