The Nonspecific Effects of Vaccines and the Expanded Program on Immunization

There is now clear evidence that the simplistic conventional model of immunization is invalid [1]. We can no longer assume that a vaccine acts independently of other vaccines, or that it influences only infections caused by the target disease. Strong evidence from randomized trials suggests that bacillus Calmette-Gue´rin vaccine (BCG) reduces mortality from infections other than tuberculosis and that measles vaccine reduces mortality from infections other than measles [1–4]. However, there is worrying evidence that wholecell diphtheria-tetanus-pertussis vaccine (DTP) may increase mortality from infections other than diphtheria, tetanus, or pertussis in high-mortality areas [1, 3–8]. These nonspecific effects of BCG, measles vaccine, and DTP are generally stronger in girls, appear to be maximal in the first 6 months after immunization, and are largely determined by the most recent vaccine administered [1].