At a mean TIV coverage rate of 83% (range, 53–100%), indirect protection of non-recipients of the influenza vaccine had a protective effectiveness of 61% (95% confidence interval, 8–83%; P = .03). The overall protective effectiveness (direct and indirect protection) check details was estimated to be 59% (95% CI, 5–82%; P = .04). Bearing in mind that this randomised controlled study was over a single season, used TIV rather than LAIV and targeted a slightly narrow age range, the estimate of indirect protection is consistent with that estimated in this paper. The long-term

impact of vaccination on the dynamics of influenza transmission depends in part on the degree of cross protection between different strains, MAPK inhibitor imparted by the vaccine. This analysis has highlighted the potential importance of herd immunity in preventing influenza in high risk groups. A long-term programme of vaccination may, however, alter the breadth of this herd immunity. The influenza virus evolves away from the herd host immune protection by a process of antigenic shift

and drift [42] and [43]. Each individual host immune system comprises a repertoire of immunities to strains that had previously infected that individual. This natural immunity is long term and has some level of cross-protection against strains not previously experienced by that individual. Thus the natural herd immunity of a population is based on the collective experience of influenza over the last 50 years and is cross-protective to varying degrees against other related strains as well. It can be assumed that vaccine induced immunity is less cross-protective and possibly shorter

lived than natural immunity, although studies of the duration of immunity in naturally exposed individuals and from time series data have proved inconclusive [44] and [45]. If an effective seasonal influenza vaccination strategy were in place for 50 years, the herd immunity of the population will comprise the collective experience of annual influenza vaccination over the last 30 or so years (as the immunity from 30 to 50 years will have waned and natural infection would have been rare). This new herd immunity however will be at a high level, but its antigenic scope may be narrower than the natural herd immunity counterpart, possibly leading to an increased susceptibility to strains that have undergone antigenic drift or shift. Strains that have undergone antigenic shifts have the potential to cause pandemics, as was observed in 2009. These emerging strains typically infect and cause morbidity in younger individuals than those responsible for seasonal influenza [46] and [47]. With the emergence of A(H1N1)v following the 2009 pandemic, this shift in the age distribution of infection towards younger individuals is likely to increase the direct benefits of paediatric vaccination.