Characteristics of hepatitis A outbreak and effect of "herd immunity barrier" in population with high immunity

Among 3 communities with a high level of immunity of hepatitis A in Xi'an, the immunity before epidemic (pre-epidemic prevalence) ranged between 75.8% and 923% while the attack rate varied from 14.7 ‰ to 57.1 ‰ and the epidemic lasted from 10 to 31 days. In contrast, among 3 communities with a low level of immunity, the immunity before epidemic ranged between 30.0% and 57.1% while the attack rate varied from 140.6‰ to 4444 ‰ and the epidemic duration was from 10 to 117 days. There was a very significant negative association between the attack rate and the immunity (P0.01). Authors considered that the immunity up to a given level acquired by natural infection could 'establish a herd immunity barrierin a community just as in the case after vaccination. In the community with a high level of immunity, hepatitis A is characterized by lower attack rate and shorter epidemic duration. These characteristics are associated with the level of immunity and the effect of herd immunity barrier. But the effect was relative, hence the corresponding control strategies are discussed in the paper.