Clinical poliomyelitis is uncommon in the native population of Liberia, but is relatively much more frequent in foreign visitors. The serum neutralization test against poliomyelitis viruses was performed, using the serum of 80 Liberians of various ages, and indicated a rapid accumulation of sero-immunity, up to an average level of about 90 per cent by 5–6 years of age. This pattern of sero-immunity is very comparable to the patterns found in other studies of under-sanitated areas, and indicates an abundance of poliovirus and infection at a very early age.
The pattern of sero-immunity in Liberia is markedly different from that in an American city studied by Melnick and Ledinko and perhaps typical of the population of foreigners in Liberia. This suggests that the foreign adult visitors may include a significant number of non-immunes who are at hazard in an under-sanitated area. The use of prophylactic vaccination against poliomyelitis is not now justified in the native Liberian population, but may be in the foreign visitor to Liberia.