Average age at infection (A) is a useful summary measure indicating the arithmetic mean age at infection of all cases over some (often unspecified) period of time. For a childhood infection such as measles, it also provides an approximate indication of how narrow is the age window for vaccination which public health programs should aim for between the loss of passive immunity from maternal antibodies and the age at which the majority of children will already have been infected. The average age at infection will also provide some indication of the likely disease burden from an infection in which processes of morbidity or mortality are age-related. For infections provoking long-lasting immunity the average age at infection will change according to the evolution of the age profile of herd immunity in the population, so that it is likely to be higher when an infection is introduced into the population for the first time and lower as the infection becomes endemic in the population and many older individuals are immune following recovery from infection; the infectious agent is then primarily reliant on births to supply new susceptible individuals to the population. If an infection is characterized by epidemics repeated at longer intervals of time, the average age at infection will rise and fall accordingly. Note that the introduction of a vaccination program, through its impact on the age distribution of immunity, will itself change the average age at infection, and as infection becomes more rare as a result of a successful vaccination program, those who have not been vaccinated will themselves be less at risk of infection (herd immunity effect). If these individuals do eventually become infected it is likely to be at an older age than would otherwise have been the case in the absence of vaccination. Should morbidity increase with age it is therefore theoretically possible, dependent on circumstances, for vaccination to result in an increased burden of disease (Williams and Manfredi, 2004). Rubella infection constitutes a prime example of such a risk, being a mild infection in childhood, but should the average age at infection be delayed there is likely to be an increase in risk of infection for women in their fertile years, with a resulting risk of congenital rubella if infection occurs in the first trimester of pregnancy (Edmunds et al., 2000). Processes of demographic change whether a result of increase or reduction in the population growth rate or the onset of population decline, by changing the population age structure (and indirectly therefore the age profile of susceptibility), may also change the average age at infection (Williams and Manfredi, 2004). The arithmetic mean, of course, tells us little about the variance in ages at infection which may itself be important—if there is substantial variance in ages at infection, a substantial proportion of cases may occur in younger or older (perhaps much older) ages, so that when considering how changes in the age distribution of infection may affect age-dependent morbidity or mortality, median and percentile measures also become important.
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