Overall, we note that the immunization schedule was well respected for all children in both countries, with a median time between doses close to 30 days.
While we observed in Cameroon a protection rate of 92%, corresponding with previously reported protection rates for hepatitis B vaccination in both developing and developed countries 
, we were surprised by the low protection level (58%) in Senegal. Moreover, in Senegal, the proportion of children responding to the vaccine varied dramatically over time with a low protected rate in 2006–2007 (43%).
Faced with these unexpected results, we sought to verify the hypothesis of lower vaccine immunogenicity, by measuring the response to the diphtheria antigens associated in the same vial. Antibodies against diphtheria antigens persist several months after vaccination 
. In the present study, the detection of antibodies was performed using EIA (IgG testkit, Genzyme, Germany) in nineteen blood samples from Senegalese children vaccinated less than one year after vaccination: nine responder children (anti-HBs+) and ten vaccinated and non-responder children (anti-HBs–). Low anti-diphtheria antibody response was significantly associated with the lack of anti-HBs antibodies (p
0.036), but the low value of correlation (r
0.48) impaired any definitive conclusion. The low value of the coefficient between anti-diphtheria and anti-HBs antibodies is probably due to the small number of cases.
Several possible explanations may account for these results. First, there may exist problems with storage conditions in Senegal, since frequent power outages may provoke lapses in backup electrical systems and compromise the cold chain 
. Second, there may be a quality problem with the pentavalent QuinvaxemTM
(Crucell) vaccine. In 2011, for instance, Quinvaxem production temporarily ceased because of sterilisation problems 
. A third explanation may be related to children's nutritional status, which is much more severe among Senegalese children than among Cameroonian children. The small number of children with moderate or severe malnutrition in our study, especially in Cameroon, led to the result that when both variables (nutritional status and country) were introduced simultaneously in a multivariate model, only country remained significantly associated with antibody response. However, the percentages of protected children were lower in the two countries among those children with moderate or severe malnutrition. These results are in accordance with recent studies that reported no immune response difference between healthy children and those with compromised nutritional status 
Although we cannot definitively explain the reason(s) for anti-HBV vaccination failure in Senegal, the striking disparity between our results, based on anti-HBs antibody levels, and vaccination card registrations demonstrates a critical need for monitoring accurately vaccine delivery and coverage. Current vaccination coverage surveys are based mainly on an assessment of immunisation cards 
. Yet our results, although not based on a representative sample of Senegalese or Cameroonian child populations, reveal a significant problem in vaccine response in Senegal that present official surveys cannot detect. This response problem may extend well beyond hepatitis B: the same children who have not developed an immune response to the HBV vaccine are also at risk for diphtheria, tetanus, pertussis and Haemophilus influenzae
Current evaluations of vaccination programmes, particularly in resource-poor countries, necessitate supplemental and regular biological monitoring, to ensure vaccine quality and storage and to verify that vaccine recipients are genuinely protected. We would also call for further studies on larger populations in countries that participate in the EPI, so as to investigate more fully the vaccines and their storage and delivery. These results make evidence the need for collaboration between the Expanded Programme on Immunization and national programmes to control Hepatitis B. Such measures should constitute a clear global public health priority.