Estimation of the prevalence and contagiousness of undocumented novel coronavirus [severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2)] infections is critical for understanding the overall prevalence and pandemic potential of this disease. Here, we use observations of reported infection within China, in conjunction with mobility data, a networked dynamic metapopulation model, and Bayesian inference, to infer critical epidemiological characteristics associated with SARS-CoV-2, including the fraction of undocumented infections and their contagiousness. We estimate that 86% of all infections were undocumented [95% credible interval (CI): 82–90%] before the 23 January 2020 travel restrictions. The transmission rate of undocumented infections per person was 55% the transmission rate of documented infections (95% CI: 46–62%), yet, because of their greater numbers, undocumented infections were the source of 79% of the documented cases. These findings explain the rapid geographic spread of SARS-CoV-2 and indicate that containment of this virus will be particularly challenging.

Influenza A incidence peaks during winter in temperate regions. The basis for this pronounced seasonality is not understood, nor is it well documented how influenza A transmission principally occurs. Previous studies indicate that relative humidity (RH) affects both influenza virus transmission (IVT) and influenza virus survival (IVS). Here, we reanalyze these data to explore the effects of absolute humidity on IVT and IVS. We find that absolute humidity (AH) constrains both transmission efficiency and IVS much more significantly than RH. In the studies presented, 50% of IVT variability and 90% of IVS variability are explained by AH, whereas, respectively, only 12% and 36% are explained by RH. In temperate regions, both outdoor and indoor AH possess a strong seasonal cycle that minimizes in winter. This seasonal cycle is consistent with a wintertime increase in IVS and IVT and may explain the seasonality of influenza. Thus, differences in AH provide a single, coherent, more physically sound explanation for the observed variability of IVS, IVT and influenza seasonality in temperate regions. This hypothesis can be further tested through future, additional laboratory, epidemiological and modeling studies.virus survival ͉ vapor pressure ͉ droplet nuclei ͉ aerosol T here are 4 suspected modes of influenza virus transmission (IVT) (1): (i) transmission through direct physical contact with an infected individual, (ii) transmission via intermediate, often inanimate, objects (i.e., fomites), (iii) transmission via droplets expelled from infected individuals (e.g., by sneezing or coughing) that deposit on nasal or oral mucosa of a susceptible individual, and (iv) airborne transmission via expelled particles Ͻ2.5 m in radius, which are referred to as droplet nuclei and remain suspended in air as aerosols for extended periods of time. The relative importance of these 4 transmission modes remains a subject of much debate (1-7).A recent laboratory experiment examining influenza A transmission among guinea pig hosts suggests that airborne transmission may indeed be an important mode of IVT (8). In this study, groups of 4 infected and 4 susceptible hosts were maintained in separate cages at different temperature and relative humidity (RH) conditions. Transmission rates, measured as the percentage of susceptible hosts infected, were found to increase at lower RH. Two hypotheses were proposed to explain this relationship (8): (i) virus-laden droplet nuclei are more efficiently produced at lower RH because of increased evaporation of expelled droplet particles, such that more virus remains airborne longer; (ii) influenza virus survival (IVS) increases as RH decreases, such that the airborne virus remains viable longer at lower RH.Heretofore, analyses of the effects of humidity on both IVS and IVT have focused on RH as the metric for air moisture. RH is the ratio of the actual water vapor pressure of the air to the equilibrium, or saturation, vapor pressure of the air. Because saturation vapor pressure increases exponentially as te...

Here, the authors demonstrate that variations of absolute humidity explain both the onset of wintertime influenza transmission and the overarching seasonality of this pathogen in temperate regions.

Human influenza infections exhibit a strong seasonal cycle in temperate regions. Recent laboratory and epidemiological evidence suggests that low specific humidity conditions facilitate the airborne survival and transmission of the influenza virus in temperate regions, resulting in annual winter epidemics. However, this relationship is unlikely to account for the epidemiology of influenza in tropical and subtropical regions where epidemics often occur during the rainy season or transmit year-round without a well-defined season. We assessed the role of specific humidity and other local climatic variables on influenza virus seasonality by modeling epidemiological and climatic information from 78 study sites sampled globally. We substantiated that there are two types of environmental conditions associated with seasonal influenza epidemics: “cold-dry” and “humid-rainy”. For sites where monthly average specific humidity or temperature decreases below thresholds of approximately 11–12 g/kg and 18–21°C during the year, influenza activity peaks during the cold-dry season (i.e., winter) when specific humidity and temperature are at minimal levels. For sites where specific humidity and temperature do not decrease below these thresholds, seasonal influenza activity is more likely to peak in months when average precipitation totals are maximal and greater than 150 mm per month. These findings provide a simple climate-based model rooted in empirical data that accounts for the diversity of seasonal influenza patterns observed across temperate, subtropical and tropical climates.