This Perspective discusses the following new study published in PLOS Medicine: Yu H, Alonso WJ, Feng L, Tan Y, Shu Y, et al. groups most at-risk of severe outcomes and often a top priority for national vaccination programs. Therefore, good knowledge of likely temporal trends in the risk of influenza contamination is a necessary prerequisite for the design of optimal vaccination programs. In this week’s PLOS Medicine, Ccile Viboud and colleagues  present an extensive analysis of sentinel virological surveillance of influenzas A(H3N2) and B from China with the objective of obtaining epidemiological patterns that support the design of the country’s first national influenza vaccination program. The authors use time series of viral isolation data from a network of sentinel hospitals, finding strong evidence for key epidemiological features of the incidence of influenza subtypes. Rather than relying on syndromic definitions or excess mortality, these biologically robust outcomes identify the patterns of circulating strains with high specificity. Despite variability KLHL11 antibody in both FRAX486 the propensity of individuals to seek treatment and the likelihood of them being tested, virological surveillance data accurately describe the timing of peak incidence, the duration of elevated incidence (the influenza season), and periods when influenza is usually absent (provided testing levels are high year-round). In many temperate populations such as the United States, knowledge of epidemiological patterns of influenza incidence has facilitated the robust design of vaccination programs : incidence is strongly seasonal, with a very low risk of infection during the summer. The vast majority of infections are focused in a 6C8 week period in the winter months. Therefore, vaccination programs that are expected to last 6 weeks are initiated 12 weeks prior to the expected start of the season (the beginning of October in the Northern Hemisphere and the beginning of April in the Southern Hemisphere). At lower latitudes, patterns are far less clear . Equatorial populations such as Singapore report almost constant year-round incidence of influenza-like illness , while some subtropical locations, such as Hong Kong, exhibit weak biennial cycles, with their seasonality characterized primarily by a clear off-season . A study of influenza patterns in Brazil, a country with a FRAX486 large population spanning a wide range of latitudes, revealed wave-like dynamics originating in the less populated equatorial region and FRAX486 travelling out towards larger temperate populations (based on excess pneumonia and influenza mortality) . In their study, Viboud and colleagues were able to individual China into three epidemiological zones for influenza A(H3N2). In the temperate FRAX486 north, incidence peaked sharply during January and February, while in the tropical south, a longer epidemic with a lower peak was observed during April and May. The regions in the middle latitudinal zone exhibited biannual cycles with smaller incidence peaks temporally aligned with their northern and the southern neighbors. Intriguingly, there were clear differences in the spatial patterns of influenza B compared with those of influenza A. There was little evidence of biannual cycles for influenza B, with the timing of the single peak each year closely correlated with latitude: epidemics occurred first in the north and then progressed steadily to the south. Perhaps most striking, the authors also found that the proportion of samples positive for influenza B increased from less than 20% in the northernmost provinces to almost 50% in the southernmost provinces. These observations point to fundamentally different circulation patterns between influenzas A(H3N2) and B.