Доля непривитых детей возросла более чем вдвое. S. B. Omer, W. K. Y. Pan, N. A. Halsey, et al., Nonmedical Exemptions to School Immunization Requirements: Secular Trends and Association of State Policies with Pertussis Incidence, Journal of the American Medical Association 296 (2006): 1757–1763.
