Measles Vaccination in Guinea-Bissau. Strategies to Reduce Disease Burden and Improve Child Survival

BACKGROUND:

Two dose schedules:

After the failure of the high-titre measles vaccine, which was to be administered at 6 months of age it was suggested to perform trials of early two-dose measles vaccination schedules to lower the age of vaccination. The standard titre SW measles vaccine has been shown to be associated with a non-specific beneficial effect on child mortality that cannot be ascribed to the protection against measles. The two measles vaccines most commonly used in the Expanded Programme on Immunization (EPI) are the standard titre SW and the standard titre EZ vaccine. Surprisingly, these two vaccines have never been compared in a randomised trial with child mortality as end-point, and it is not clear which would be most suitable for use in a two-dose vaccination schedule.

From 1995 to 2002, all children in the BHP study area were included in an early two-dose measles vaccination trial from 6 months of age. The children were randomised to either a one-dose group receiving an inactivated polio vaccine (IPV) at 6 months of age and a measles vaccine at 9 months of age, or a two-dose group receiving two doses of measles vaccine at 6 and 9 months of age. For the first 6 months of the trial, the standard-titre EZ measles vaccine was used, for the rest of the trial the standard-titre SW vaccine was used. Results from the trial showed that an early two-dose schedule increased coverage considerably and provided better protection against measles among infants than the recommended one-dose at 9 months of age schedule. The EZ and the SW vaccine were used in two different cohorts - so a direct comparison was not possible, but the EZ vaccine seemed to boost a secondary immune response better than the SW vaccine. Further, the SW vaccine was less able to induce a protective level of antibodies when used from 6 months of age than the EZ vaccine. The preliminary data that we have on long-term vaccine efficacy supports that the SW vaccine might be less suitable than the EZ vaccine for use in very early measles vaccine schedules (unpublished data). On the other hand, trials performed during the 1980's in Mexico and Bangladesh have shown that administration of the standard-titre EZ vaccine to infants as young as 4-6 months of age gave good seroconversion results.

The Global Measles Strategic Plan 2001-2005 developed by the WHO and UNICEF states that all children should be guaranteed a second opportunity for measles vaccination either through campaigns or routine immunisation. The best strategy for protection of infants living in overcrowded urban African societies and who run a great risk of contracting measles before the recommended age of immunisation, will probably be early vaccination followed by a second vaccination later in childhood. If, in a vaccination campaign strategy, administration of the first vaccine is postponed this could dramatically increase the measles incidence in children below the age of vaccination. To this add that vaccination campaigns have not yet been evaluated in Africa in terms of childhood mortality relative to the routine schedule. Some children might get two, three or even more measles vaccines in an uncontrolled and not necessarily beneficial manner. Thus finding the best schedule and vaccine strain for use in a routine two-dose schedule seems a better approach for this region.