Li Y, et al. - Through systematic analysis of 37,335 eligible studies from a systematic literature review of studies published between Jan 1, 2000, and Dec 31, 2017, the researchers determined the global patterns in monthly activity of influenza virus, respiratory syncytial virus, parainfluenza virus, and metapneumovirus. Two hundred and forty-six sites for influenza virus, 183 sites for the respiratory syncytial virus, 83 sites for parainfluenza virus, and 65 sites for metapneumovirus were involved. The influenza virus had distinct seasonal epidemics in winter months in most temperate sites but the timing of epidemics was more changeable and less seasonal with diminishing distance from the equator. The respiratory syncytial virus had distinct seasonal epidemics in both temperate and tropical regions, starting in the late summer months in the tropics of each hemisphere, reaching most temperate places in winter months. Influenza virus epidemics happened later than respiratory syncytial virus while no precise temporal order was seen in the tropics, in most temperate sites. Parainfluenza virus epidemics were frequently discovered in the spring and early summer months in each hemisphere. Metapneumovirus epidemics occurred in late winter and spring in most temperate places, however, the timing of epidemics was more different in the tropics. A shorter span in temperate sites and longer span in the tropics of influenza virus epidemics were noted. For the respiratory syncytial virus, the span of epidemics was alike across all sites, as it was for metapneumovirus. The parainfluenza virus had a more prolonged span of epidemics. This study was the first that presented global representations of the month-by-month activity of influenza virus, respiratory syncytial virus, parainfluenza virus, and metapneumovirus. In order to prognosticate the local onset month of influenza virus and respiratory syncytial virus epidemics, this model was concluded as effective. Therefore, for health services planning, the timing of respiratory syncytial virus passive prophylaxis, and the approach of the influenza virus and expected respiratory syncytial virus vaccination, the seasonality knowledge had significant implications.