Problems encountered in the delivery and storage of OPV in an African country.

Southern Africa may be developing into another poliomyelitis-free zone. Apparent control has been achieved in spite of suboptimal coverage, absence of supplementary strategies, vulnerability to importation and major outbreaks of other waterborne pathogens such as typhoid. A number of serological studies have demonstrated significant susceptibility, especially in rural populations, and studies of three southern African epidemics have shown, in two of them, that epidemics can occur in spite of relatively high levels of serological immunity and vaccine coverage if the burden of virus is sufficiently great. Two studies comparing vaccine coverage to serological immunity have shown that only 78% of fully immunized children in one study and 81% in the other study developed antibodies to all three types of poliomyelitis. In two investigations where vaccine samples were recalled for potency testing from peripheral clinics, almost a half and almost a third, respectively, of vaccine vials had titres below WHO recommended cut-off limits. Major logistical and organizational difficulties in storage and transport of vaccine under cold chain conditions were present in South Africa. These are being rectified and energetic attempts are being made, aided by a strong governmental commitment to improve vaccine coverage and to institute mass immunization campaigns. Nevertheless the need for the development of a more stable OPV for developing and tropical countries still remains a major objective in the global eradication of poliomyelitis.