title: Survival bias in observational studies of the impact of routine immunizations on childhood survival.
abstract: OBJECTIVE
  In situations with vaccination coverage high enough to control pertussis, observational studies have reported divergent effects of diphtheria-tetanus-pertussis (DTP) vaccination on childhood survival. We examined whether this could be because of methodological differences.
  
  
  METHODS
  Some studies of the impact of DTP updated information on vaccination retrospectively (retrospective updating approach) while others kept vaccination status fixed for the time between follow-up visits (landmark approach). First, we conducted simulations with these approaches to investigate the impact of different mortality levels, vaccination incidence rates, intervals between data collection visits, and the proportion of children whose vaccination card had not been seen after death. Second, we re-analysed data from Guinea-Bissau using the retrospective updating approach.
  
  
  RESULTS
  In simulations, the retrospective updating approach produced rate ratio (vaccinated/unvaccinated) estimates biased towards zero because of a differential misclassification which created survival bias as risk-free observation time was allocated to the vaccinated group. The landmark approach produced rate ratio estimates biased towards one. Biases increased with interval between data collection visits and incidence of vaccination, but were unaffected by the underlying mortality level. Survival bias increased with the proportion of dead children whose vaccination status could not be updated. The re-analysis of data from Guinea-Bissau changed the estimated impact of DTP from an 84% higher mortality using the landmark approach to a 37% lower mortality using the retrospective updating approach.
  
  
  CONCLUSIONS
  The apparent contradiction between DTP studies could be because of methodological differences. To assess child survival associated with routine vaccinations, studies minimizing the effect of biases are warranted.
authors.affiliation: Bandim Health Project, Danish Epidemiology Science Centre and Statens Serum Institut, Copenhagen, Denmark. hjn@ssi.dk
authors.forename: Henrik
authors.initials: H
authors.lastname: Jensen
authors.forename: Christine Stabell
authors.initials: CS
authors.lastname: Benn
authors.forename: Ida Maria
authors.initials: IM
authors.lastname: Lisse
authors.forename: Amabelia
authors.initials: A
authors.lastname: Rodrigues
authors.forename: Per Kragh
authors.initials: PK
authors.lastname: Andersen
authors.forename: Peter
authors.initials: P
authors.lastname: Aaby
mesh_terms: D001500:BCG Vaccine / Q000009:adverse effects; D015982:Bias; D002648:Child; D046688:Child Mortality; D003198:Computer Simulation; D015721:Diphtheria-Tetanus-Pertussis Vaccine / Q000009:adverse effects*; D006169:Guinea-Bissau / Q000453:epidemiology; D006801:Humans; D007223:Infant; D007226:Infant Mortality; D007231:Infant, Newborn; D008954:Models, Biological; D012107:Research Design; D012189:Retrospective Studies; D016019:Survival Analysis; D013997:Time Factors; D014611:Vaccination / Q000009:adverse effects
chemical_list: D001500:BCG Vaccine; D015721:Diphtheria-Tetanus-Pertussis Vaccine