There are 3 types of herd immunity including 1) herd immunity of natural infections; 2) herd immunity from prophylactic vaccine; and 3) herd immunity of therapeutic vaccine. Characteristics of natural infections present periodical outbreaks every 3-10 years depending on base reproduction number (Ro) as herd immunity not requires 100% covered rate of infection to stop transmission. The Ro less than 2.0 will require a threshold less than 50% herd immunity to stop transmission. The Ro greater than 5.0 will require a threshold more than 80% herd immunity to stop transmission, and the Ro greater than 10 will require the threshold more than 95% herd immunity to stop transmission. As herd immunity of the natural infection is over the threshold, the infectious transmission will be blocked. The next outbreak won’t occur until more new children (population) are susceptible and the herd immunity goes down below the protective threshold. For instances, enterovirus 71 and dengue infections show periodical endemic or epidemic outbreaks every 5-10 years.
Characteristics of the cohort immunity for prophylactic vaccine usually present a full protection for young people but leave adults and elders beyond protection. For instance, MMR vaccination requires a second, even a third booster of vaccination to stop periodic outbreaks. Similarly adults may require more JE and Pertussis boost doses in adulthood to prevent re-emergence.? Recently, a novel herd immunity of therapeutic vaccine emerges through reviving infection immunity by adoptive immunity, adjuvant immunoactivation, activation of chimeric antigen receptors (CAR) or anti-immunorepression (anti-PD1 or anti-CTLA4). These strategies may revive immunity to interrupt infectious transmission from latent infections. Moreover, anti-immunorepression by anti-PD1, anti-CTLA4 or activation of CAR will also revive immunity of elders, who are usual breakages of herd immunity of common infections and emerging infections.