Vaccination status and sequence of vaccinations as risk factors for hospitalisation among outpatients in a high mortality country.

BACKGROUND: Most developing countries are implementing the WHO immunisation programme. Although vaccines reach most children, many modifications of the recommended schedule are observed in practice. We investigated the association between vaccination status and risk of hospitalisation in Guinea-Bissau. METHODS: From May 2003 to May 2004, all consultations of children less than five years of age at the outpatient clinic of the paediatric ward at the national hospital in Bissau were registered. For each consultation, information was collected about the child's name, sex, age and socio-cultural conditions, as well as diagnosis and whether the child was hospitalised. Information about vaccinations was also registered from the child's vaccination card. We analysed the association between vaccination status and risk of hospitalisation in age intervals according to the pre-dominant vaccines. We particularly emphasised the comparison of those who had received the recommended vaccination for the age groups and those who were delayed and only had the previous vaccinations. We also examined those who had received the vaccines out of sequence. RESULTS: Information about vaccinations was available for 11,949 outpatient children of whom 2219 (19%) were hospitalised. Among children less than 3 months of age, unvaccinated children compared to BCG children had as expected a higher risk of hospitalisation; controlled for important determinants of hospitalisation, the hospitalisation risk ratio (HRR) was 1.99 (95% CI 1.37-2.89). In contrast, there was no difference in the HRR for children aged 1½-8 months who were delayed and had only received BCG compared to those who as recommended had received diphtheria-tetanus-pertussis (DTP) vaccine after BCG (HRR=1.10 (0.77-1.59)). In the age interval 9-17 months of age, children who were delayed and had only received DTP had significantly higher risk of hospitalisation compared with children who as recommended had measles vaccine (MV) as the most recent vaccination (HRR=1.39 (1.16-1.66)). Having received DTP after MV (HRR=1.60 (1.15-2.24)) or MV and DTP simultaneously (HRR=1.51 (1.16-1.97)) was also associated with higher risk than MV only as most recent vaccination. In contrast, the children aged 18-59 months who as recommended had received a DTP booster after MV did not have lower risk of hospitalisations compared with children who were delayed and had received only MV (RR=0.90 (0.75-1.07)). After 9 months of age, there was a significant difference in the female-male HRR for children who had MV (HRR=0.85 (0.72-1.00)) or DTP (HRR=1.08 (0.96-1.22)) as most recent vaccination (p=0.02, test of interaction). CONCLUSION: Following the recommended vaccination schedule for BCG and MV is associated with a reduced risk of hospitalisation but this is not the case for DTP and booster DTP. Receiving DTP simultaneously with MV or after MV is associated with increased risk of hospitalisation. Vaccines have sex-differential effects on the risk of hospitalisation.