Despite a continuous search for safer and more immunogenic vaccines, adverse reactions still occur. Adverse reactions to vaccines are generally mild; severe events resulting in death or permanent damage are rare. In every instance, the benefits of preventing the disease far outweigh the risks of vaccination. In the early days of vaccine development, a number of accidents were associated with faulty production. Most recent problems encountered with the use of vaccines are due to programmatic errors. Because of the large number of doses administered it is probable that there will be some temporal and merely coincidental association between adverse events and vaccine administration. Immunisation has a direct protective effect for the individual and an indirect effect on herd immunity for the community. The major goal of postmarketing surveillance is the early detection of and appropriate response to adverse events in order to curtail a negative impact on immunisation programmes. Risk-benefit analyses for immunisation are faced with a number of potential difficulties; definition of the risks and benefits themselves, individual versus community risks and benefits, and the continuously evolving nature of risks with changes in disease epidemiology. Based on risk-benefit studies, for an individual just as for the community, it may not always be of interest to use the vaccine with the lowest complication rate. Good immunisation programmes should help to decrease the risk of adverse effects.