Herd immunity describes a form of immunity that occurs when the vaccination of a significant portion of a population (or herd) provides a measure of protection for individuals who have not developed immunity. Herd immunity theory proposes that, in contagious diseases that are transmitted from individual to individual, chains of infection are likely to be disrupted when large numbers of a population are immune to the disease. The greater the proportion of individuals who are immune, the smaller the probability that a susceptible individual will come into contact with an infectious individual. Immunity may be obtained naturally by contracting and recovering the disease, or attempts can be made to build temporary immunity via vaccination. Individuals who are not immune could be indirectly protected by individuals who are immune, as the latter will not contract and transmit the disease between infected and susceptible individuals. Mathematically, a 'herd immunity threshold' can be calculated for each disease which indicates the percentage of the population that needs to have immunity (either via previously contracting the disease or via an effective vaccination) in order to slow or prevent the spread of the disease. Theoretical herd immunity thresholds for vaccine-preventable diseases range from 83 – 94%.
There are issues with the use of the current USA vaccination schedule to attain herd immunity:
The SmartVax philosophy encourages research to find out why vaccine-injuries occur and what makes a child susceptible to vaccine-injury. Given the potential that children susceptible to vaccine-injuries could also be prone to be non-responders to vaccines, this research could yield new understanding into how vaccines can be designed that are both safer and more effective for this subgroup. Even with safer and more-effective vaccines, there may still be a portion of this subgroup that might need exemption due to risk of vaccine-injury. SmartVax research will provide knowledge that could lead to screening techniques to identify the subgroup needing exemption. More-effective vaccines will mean that a higher percentage of children can be exempted while still maintaining herd immunity for the population, and safer vaccines will mean that there is a lower pool of children that need to be exempted. The net result is that with a SmartVax approach to research and vaccine policy, the USA could dramatically reduce vaccine-injuries and have improved protection against infectious diseases (such as attaining herd immunity for pertussis, which is not feasible today with the current less-effective vaccine).
 John TJ, Samuel R (2000). "Herd immunity and herd effect: new insights and definitions". Eur. J. Epidemiol. 16 (7): 601–6. doi:10.1023/A:1007626510002. PMID 11078115.
 History and Epidemiology of Global Smallpox Eradication From the training course titled "Smallpox: Disease, Prevention, and Intervention". The CDC and the World Health Organization. Slide 16-17.