A Cox proportional hazards model was used to calculate the mortality ratio for vaccinated and unvaccinated children. The overall mortality for any vaccine was nonsignificant (RR = 0.74, 95% CI 0.53–1.03). Receipt of BCG vaccine was associated with lower mortality (adjusted RR = 0.55, 95% CI 0.36–0.85), as was measles vaccine (adjusted RR=0.48, 95% CI 0.27–0.87). The mortality ratio for one dose of DTP vaccine versus none was 1.84 (95% CI 1.10–3.10), but the ratio for two to three doses was not significantly elevated (RR=1.38, 95% CI 0.73–2.61). The pattern was similar for OPV, with an elevated mortality ratio for one dose (RR = 1.81, 95% CI 1.07–3.05), and nonsignificant ratio for two to three doses (RR = 1.39, 95% CI 0.73–2.64).
The authors conclude that receipt of BCG and measles vaccines may have a protective effect against mortality, while receipt of a single dose of DTP and polio vaccines may carry a higher mortality risk compared with receiving no vaccinations. The results also suggest that DTP vaccine may negate the positive effects associated with BCG vaccine.
However, the interpretation of these findings warrants caution. The vaccination status of some children was unclear and more than a third of the children did not have records available. Many children may have been underimmunized, contributing to the increased mortality rates and reflecting limited access to health care. Vaccinated children were also more likely to receive health care than unvaccinated children, which may mean that getting vaccinated is associated with access to or use of other interventions that improve survival. Mothers of children vaccinated with DTP were younger than mothers of children vaccinated with BCG or measles vaccine, which means maternal age may have contributed to infant mortality risk. The adjustment procedure for potential confounders was also unclear. For the United States, the findings regarding BCG vaccine are not relevant since the vaccine is not routinely used in this country.
United States-Boston. In a case-control study, Burstein and Fleischer (1994) examined the relationship between vaccination and the risk of occult bacteremia. Cases and controls were patients treated in the emergency department at Children’s Hospital in Boston between November 1987 and December 1990. Cases were 54 children age 3 to 36 months who participated in a multicenter antibiotic study. Pathogens isolated from these children included S. pneumoniae, E. coli, S. aureus, H. influenzae, or Salmonella spp. The 108 controls were matched to cases according to age. Each case had two controls. One control group included febrile nonbacteremic children. The other group included nonfebrile children who were treated for symptoms not related to infectious diseases. Vaccination history, including DTwP, was obtained from medical records.
The authors found no significant difference between cases and controls for time since last vaccination of any type, or for time since last DTwP vaccination. Limitations of this study included weak statistical power. It was also unclear which vaccines, other than DTwP, the children received.