The Impact of Population Immunity on Antigenic Drift During Large Epidemics and Small Outbreaks
Population immunity is described as the number of individuals in a given population that is sufficient to prevent the transmission of a disease from the infectious to the susceptible. Although high population immunity usually prevents epidemics, we have seen many epidemics that were caused by infectious agents such as the Severe Acute Respiratory Syndrome (SARS) and Influenza type A H1N1 viruses. Transmissibility of a disease is defined as the probability of transmission from an infectious person to a susceptible individual and is an important factor in disease spreading. This study describes a realistic population contact model which simulates the spread of disease to determine if population immunity decreases the antigenic drift level, or keeps it low. This putative effect would result in transmission rates being kept low and thus preventing the spread of disease during epidemics. The results show that population immunity plays a significant role in keeping antigenic drift low during most disease spreading cases, but not during epidemics. However, low transmissibility was effective in keeping antigenic drift low even during epidemics. The study concluded that high population immunity is still important to prevent epidemics, but once immune escape occurs, lowering transmissibility of a disease becomes the significant factor in preventing further disease spread.