Answer to critical thinking question 1Explain in easily understood language that:
- Herd immunity is the resistance to infectious disease exhibited by the population as a whole and high levels of herd immunity hinder transmission because epidemics are possible only when an infectious agent spreads throughout a susceptible population.
- Before vaccination was introduced, outbreaks of measles and whooping cough occurred cyclically every few years, sporadic cases arising in between. A large number of infections occurring simultaneously was possible only when an entire cohort of children lacking immunity had built up through a lack of previous exposure, the number of infections waning once they had all been infected. In community health terms, sporadic infection is significant as epidemics are possible as long as individuals lacking immunity remain.
- The aim of immunization programmes is to induce acquired immunity in all susceptible people, preventing the development of outbreaks.
- It is generally believed that over 90 per cent of the population must be immune to the pathogen causing a disease before a state of herd immunity can exist.
- Since the media coverage of the (unproven) research about MMR vaccine side-effects the number of children vaccinated has fallen and the number of cases of measles has increased.