Childhood mortality after oral polio immunisation campaign in Guinea-Bissau

Abstract

Though previous studies have suggested a non-specific beneficial effect of oral polio vaccine (OPV), there has been no evaluation of the mortality impact of national polio immunization days. On the other hand, studies examining the effect of OPV and diphtheria–tetanus–pertussis (DTP) vaccines, which are usually administered together in routine immunisation programmes in low-income countries, have found no beneficial or even a negative effect on infant survival. In 1998, we used the opportunity of two national immunisation days to examine the impact of OPV administered alone on survival for the 6103 children less than 5 years of age in the Bandim Health Project's study area in Guinea-Bissau. Survival was ascertained through regular surveillance from March 1998 until the beginning of the war on June 7, 1998, the end of 1998, or the end of 1999, respectively. The child register was linked with a register for the only paediatric ward in Bissau to determine the risk of hospitalisations. Among children under 5 years of age, 82% had received 1 or 2 doses of polio vaccines during the campaign. Though polio vaccination during the campaign was associated with slightly lower mortality, this difference was not significant for all children under 5 years of age (mortality ratio (MR) = 0.46 (0.18–1.15)). However, oral polio vaccination was associated with a beneficial effect for children under 6 months of age at the time of the campaign, the mortality ratio being 0.09 (95% CI 0.01–0.85) in the 3 months before the war controlling for significant background factors, including routine immunizations, antenatal consultations, and arm circumference. The polio-vaccinated children aged 0–5 months had fewer hospitalisations than children who had not been polio vaccinated (RR = 0.27 (0.10–0.76)). With longer follow-up to December 1998 or December 1999, the difference in mortality gradually disappeared, the MR for polio-vaccinated children being 0.61 (0.32–1.14) and 0.83 (0.51–1.34), respectively. Among children aged 6–59 months of age, measles vaccine was associated with a 56% reduction in mortality (MR = 0.44 (0.28–0.69)) and no effect of oral polio vaccine was measurable in this age group. The effect of polio vaccine among children less than 6 months of age could be due to selection bias but might also represent a non-specific beneficial immune stimulation and there is nothing to suggest that OPV might have a negative effect on infant survival. Studies of the possible non-specific effects of oral polio vaccine are warranted before OPV is withdrawn.

Introduction

When oral polio was introduced in the 1960s, a Chilean study observed a significant reduction in mortality and suggested that Sabin's oral polio vaccine virus limited other enteroviruses [1]. In studies conducted in the Soviet Union in the 1950s and 1960s, Voroshilova found marked reduction in morbidity associated with the use of non-pathogenic enterovirus vaccines including oral polio vaccine (OPV) [2]. If such effects were indeed true, it would add to the value of national polio immunisation days but would also question the health impact of dropping oral polio vaccination in the later stages of polio eradication [3]. Surprisingly, the effect of national polio immunization days on morbidity and mortality has not been examined previously.

The major reason for undertaking the present study was the observation that routine immunisations may have important non-specific effects for childhood survival [4], [5], [6], [7], [8]. BCG and measles vaccine have been associated with marked reduction in mortality, which cannot be explained by the prevention of measles or tuberculosis [4], [5]. However, diphtheria, tetanus, pertussis (DTP) and OPV have been associated with no reduction or slightly increased mortality [4], [5], [7], [8]. Since DTP and polio vaccines are usually administered together at 6, 10, and 14 weeks of age in the immunisation programme in low-income countries, it is difficult to distinguish the effects of these vaccines. We therefore used the opportunity of national polio immunisation days in 1998 in Guinea-Bissau to assess the effect on child survival of oral polio vaccine administered alone.