Have you herd? Indirect flu vaccine effects are critically important.

Seasonal epidemics of infl uenza virus infection occur frequently. These viruses infect the respiratory tract and typically cause fever, cough, and muscle and joint pain. In some individuals, particularly infants and elderly people, infl uenza infection can lead to severe illness or death. Seasonal infl uenza also places a strain on economic productivity due to employee absenteeism and exhaustion of health-care resources due to increased hospital admissions. Seasonal immunisation programmes are an important component of infl uenza prevention strategies, but several factors combine to make seasonal vaccination programmes less impactful than might be expected. First, viral mutation (also known as drift) is an ongoing process, resulting in vaccine mismatch with circulating strains in some years and therefore decreased vaccine effi cacy. Furthermore, infl uenza epidemiology is unfortunately characterised by dissociation between populations most likely to have severe disease (infants and elderly people), and populations most likely to benefi t from vaccination (older children and adolescents—ie, those aged 2–16 years). Older children and adolescents seem to be important for the early propagation of infl uenza in populations, and the effi cacy of infl uenza vaccines is greater in these populations than in elderly people, but they are generally not prioritised for immunisation because of their lower risk of morbidity and mortality from this infection. This paradoxical state of aff airs has led some researchers to suggest that the best way to minimise the burden of seasonal infl uenza would be to focus immunisation programmes on children, adolescents, and young adults, with benefi t accrued by elderly people and infants as a result of herd immunity. Herd immunity is the additional protection provided to both unvaccinated and vaccinated individuals in a population, and results from the inability of successfully vaccinated individuals to propagate disease. Such an approach would be supported by both observational data, and more recent randomised trials suggesting that protection of all age groups results when children and adolescents are immunised against infl uenza. Such an approach might raise ethical questions: individuals who bear the discomfort and risk of immunisation provide the benefi t of herd immunity to others at the extremes of age, who are less likely to benefi t from direct immunisation. Although such an approach would be consistent with a utilitarian optimum, and with a communitarian approach to health and medicine, it might be a diffi cult sell in a health context that embraces an individualistic focus on health-care decision making. In The Lancet Public Health, David Hodgson and colleagues provide a model-based cost-eff ectiveness analysis of emerging live-attenuated infl uenza virus (LAIV)-based seasonal infl uenza programmes targeting people aged 2–16 years. Although the effi cacy of LAIV had been the subject of controversy, the most recent randomised trial showed effi cacy equivalent to that seen with a more traditional trivalent subunit vaccine, providing the same herd eff ects. Intranasal administration of LAIV has the benefi t of making parenteral injection unnecessary, a benefi t that can be readily appreciated by parents of young children. Importantly, and in accordance with best practices, Hodgson and colleagues use a well calibrated dynamic mathematical model that allows them to evaluate the cost-effectiveness of vaccinating one group in the population (aged 2–16 years) on disease risk in another (elderly people). They project that LAIV programmes focused on young individuals are in themselves highly cost-effective health interventions. However, their analysis highlights the importance of herd effects for decision makers: a rapidly implemented LAIV programme focused on children aged 2–16 years is actually more effective at preventing disease in elderly people than a more slowly implemented programme that includes both LAIV and direct immunisation of elderly people themselves. This finding is because a rapid, early focus on young individuals, in whom the vaccine is more effective, stops the influenza epidemic in its tracks. Furthermore, when young individuals are vaccinated, seasonal vaccination programmes targeted at low-risk elderly individuals might cease to be cost-effective (depending on the notoriously murky definition of Lancet Public Health 2017