social distancing, treating ill individuals, or vaccination) required to slow transmission of the disease. Therefore, the magnitude of R plays an important role in the selection and aggressiveness of countermeasures (e.g. The value of R characterizes the final number infected in the absence of an intervention in homogeneously mixed populations, the herd immunity threshold, and, when coupled with the generation time, defined as the interval between infections in two consecutive generations, or the serial interval, defined as the interval between the onset of symptoms in two consecutive generations), the speed with which the disease spreads in the population. In contrast, the effective reproduction number (R E) is calculated in a population with underlying immunity and accounts for a population’s reduced susceptibility to infection. If R is calculated in a population entirely susceptible to infection (or where an assumption about population susceptibility to infection is made), then R is known as the basic reproduction number (R 0). The population susceptibility to the infection also affects the interpretation of R. Some examples include estimating R using the growth rate of the epidemic, the epidemic curve’s size and shape, the final attack rate, or by direct observation of disease transmission from one generation to the next. Many methods to calculate R have been described that allow for the use of epidemiologic data from different epidemic time points. R describes on average how many persons a case will infect, and a value of R greater than 1 indicates that the infection may grow or persist in the population while a value of R less than 1 indicates that the infection will decline in the population, although exceptions exist. An important transmissibility parameter identified is the reproduction number (R), which is defined as the average number of secondary cases generated per typical infectious case. Recognizing that the characteristics of future pandemics will be difficult to predict given the mutability of the influenza virus and the range of morbidity and mortality experienced in previous pandemics, an approach to the early assessment of influenza pandemics has been developed relying on standardized measures of transmissibility and clinical severity. Four influenza pandemics have occurred since the beginning of the 20 th century and have ranged widely in transmissibility and clinical severity. These major shifts can result in the introduction of novel influenza viruses into the human population to which humans have little or no immunity, causing pandemics. Influenza viruses are constantly changing either through the collection of minor point mutations or through major antigenic shifts. In the United States between 5% and 20% of the population are infected with influenza every year, resulting in between 3,000 and 49,000 influenza-associated deaths. Continued monitoring of R during seasonal and novel influenza outbreaks is needed to document its variation before the next pandemic.Īnnual influenza epidemics occur worldwide and cause substantial morbidity and mortality. These R values represent the difference between epidemics that are controllable and cause moderate illness and those causing a significant number of illnesses and requiring intensive mitigation strategies to control. Four studies reported six novel influenza R values. The median R value for seasonal influenza was 1.28 (IQR: 1.19–1.37). Twenty-four studies reported 47 seasonal epidemic R values. The median R value for 2009 was 1.46 (IQR: 1.30–1.70) and was similar across the two waves of illness: 1.46 for the first wave and 1.48 for the second wave. Fifty-seven studies reporpandemic R values. Four studies reported seven 1968 pandemic R values. Six studies reported seven 1957 pandemic R values. The median R value for 1918 was 1.80 (interquartile range : 1.47–2.27). Twenty-four studies reported 51 R values for the 1918 pandemic. Ninety-one papers were retained, and an additional twenty papers were identified from the references of the retained papers. We retained and summarized papers that estimated R for pandemic or seasonal influenza or for human infections with novel influenza viruses. We conducted a systematic review to summarize published estimates of R for pandemic or seasonal influenza and for novel influenza viruses (e.g. The potential impact of an influenza pandemic can be assessed by calculating a set of transmissibility parameters, the most important being the reproduction number (R), which is defined as the average number of secondary cases generated per typical infectious case.
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