7.4 Vaccine safety
Our views on the safety of vaccines have changed enormously since they were first introduced. Consider variolation, which preceded vaccination for smallpox (Section 1). The high fatality rate from smallpox meant that the 2–3 per cent risk of death associated with variolation was considered acceptable in the seventeenth century. Nowadays however, the risk of adverse reactions is a primary consideration, partly because vaccination is usually performed on people who are well, so there is a requirement for very high safety standards.
The production of vaccines is subject to rigorous quality controls to ensure that every batch delivers the same potency, does not induce harmful autoimmune or inflammatory responses, and is free from harmful contaminants. Nevertheless, accidents have occurred in which pathogens survived in supposedly ‘killed’ whole-cell vaccines and live attenuated strains have occasionally reverted to pathogenicity (as the Polio Case Study illustrated). Vaccines are usually produced by growing pathogenic or attenuated strains in laboratory animals or in tissue cultures derived from their cells, and animal proteins may be added to tissue cultures as nutrients.
What additional challenges does this pose for vaccine safety?
Quality checks must ensure that no ‘animal’ viruses, immunogenic proteins or DNA from the vaccine donor or growth medium are present in the vaccine.
The new DNA vaccines, the use of viral or bacterial ‘gene vectors’ and the possibility that prions may contaminate vaccines, add further levels of complexity. (New challenges for vaccine quality control are reviewed in a WHO report by Dellepiane et al.,2000.)
Most people tolerate the minor discomfort of an injection and the possibility of local swelling, slight fever or other minor symptoms in the short term following a vaccination, but more extreme or long-lasting side-effects are unacceptable. No medical procedure carries a zero risk and vaccinations are no exception, but in particular groups, such as people with immunodeficiencies, the risk is higher and vaccination with live attenuated strains may be inadvisable. Even when a vaccine has a good safety record, the perception of risk and concerns about adverse reactions can have a major impact on vaccine uptake.