title: Sex-differential effect on infant mortality of oral polio vaccine administered with BCG at birth in Guinea-Bissau. A natural experiment.
abstract: BACKGROUND
  The policy to provide oral polio vaccine (OPV) at birth was introduced in low-income countries to increase coverage. The effect of OPV at birth on overall child mortality was never studied. During a trial of vitamin A supplementation (VAS) at birth in Guinea-Bissau, OPV was not available during several periods. We took advantage of this "natural experiment" to test the effect on mortality of receiving OPV at birth.
  
  
  METHODS
  Between 2002 and 2004, the VAS trial randomised normal-birth-weight infants to 50,000 IU VAS or placebo administered with BCG. Provision of OPV at birth was not part of the trial, but we noted whether the infants received OPV or not. OPV was missing during several periods in 2004. We used Cox proportional hazards models to compute mortality rate ratios (MRR) of children who had received or not received OPV at birth.
  
  
  RESULTS
  A total of 962 (22.1%) of the 4345 enrolled children did not receive OPV at birth; 179 children died within the first year of life. Missing OPV at birth was associated with a tendency for decreased mortality (adjusted MRR = 0.69 (95% CI = 0.46-1.03)), the effect being similar among recipients of VAS and placebo. There was a highly significant interaction between OPV at birth and sex (p = 0.006). Not receiving OPV at birth was associated with a weak tendency for increased mortality in girls (1.14 (0.70-1.89)) but significantly decreased mortality in boys (0.35 (0.18-0.71)).
  
  
  CONCLUSIONS
  In our study OPV at birth had a sex-differential effect on mortality. Poliovirus is almost eradicated and OPV at birth contributes little to herd immunity. A randomised study of the effect of OPV at birth on overall mortality in both sexes is warranted.
authors.affiliation: Bandim Health Project, Statens Serum Institut, Copenhagen, Denmark. cb@ssi.dk
authors.forename: Christine Stabell
authors.initials: CS
authors.lastname: Benn
authors.forename: Ane Baerent
authors.initials: AB
authors.lastname: Fisker
authors.forename: Amabelia
authors.initials: A
authors.lastname: Rodrigues
authors.forename: Henrik
authors.initials: H
authors.lastname: Ravn
authors.forename: Erliyani
authors.initials: E
authors.lastname: Sartono
authors.forename: Hilton
authors.initials: H
authors.lastname: Whittle
authors.forename: Maria
authors.initials: M
authors.lastname: Yazdanbakhsh
authors.forename: Peter
authors.initials: P
authors.lastname: Aaby
mesh_terms: D001500:BCG Vaccine / Q000627:therapeutic use*; D005260:Female; D006169:Guinea-Bissau; D006801:Humans; D007223:Infant; D007226:Infant Mortality; D007231:Infant, Newborn; D008297:Male; D011055:Poliovirus Vaccine, Oral / Q000009:adverse effects* / Q000627:therapeutic use*; D016016:Proportional Hazards Models; D015203:Reproducibility of Results; D012737:Sex Factors; D016896:Treatment Outcome; D014801:Vitamin A / Q000494:pharmacology; D014802:Vitamin A Deficiency / Q000188:drug therapy
chemical_list: D001500:BCG Vaccine; D011055:Poliovirus Vaccine, Oral; D014801:Vitamin A