Background: During 2005, the surveillance system of the Chilean Immunization Program detected an increased number of adverse reaction notifications associated to diphtheria, pertussis and tetanus whole-cell vaccine (DPT), coincidently with a change in the vaccine manufacturer.
Aim: To compare the reactogenicity of two DPT formulations (vaccines 1 and 2) in 18-month-old infants and 4-year-old children.
Material and methods: Severe adverse reactions to DPT were studied at the emergency room of two hospitals of Santiago in a case-control study (110 cases and 171 controls, who consulted for other causes). Simultaneously the incidence of total adverse reactions (mild and severe) for vaccine 1 and 2 was estimated in a cohort of 1,017 children vaccinated in an ambulatory health center of the same area. The formulation of DPT received by all participants was verified, as well as the temporal relation with consultation or symptoms referred by their caregivers.
Results: There was a greater probability of consulting at the emergency rooms for severe adverse reactions among children who received vaccine 1 (odds ratio (OR) =7.1; p <0.001), being greater among 4-year-old children (OR =18.9; p <0.001). Coincidently, in the cohort of vaccinated children, those who received vaccine 1 had a higher risk of presenting adverse reactions (RR =2.9; p <0.001), being high fever the commonest.
Conclusions: We confirmed that vaccine 1 was associated to a higher risk of adverse reactions, especially among 4-year-old children. These results allowed the sanitary authority to adopt an informed decision. The usefulness of observational studies in vaccine adverse reactions is confirmed.