Naturally Acquired Yellow Fever Immunity in Muzo, Colombia


In the years 1936 to 1938 a study was made of the immunity to yellow fever of the human population in the Municipio of Muzo, a typical jungle yellow fever area in the State of Boyacá, Colombia. Census data were taken on 4,965 persons, and two separate collections of serum specimens were made prior to the use of 17D vaccine in the municipio.

In the “first bleeding,” 2,129 blood specimens were obtained. Because the mice used in the neutralization tests in the early part of the study were not uniformly susceptible to yellow fever, an excessive amount of virus was employed, and as a consequence special rules had to be set up for the interpretation of the test results; the most important of these was that inconclusive results were interpreted as positive. On this basis, the crude percentage positive of the first bleeding sera was 34.0, and that of 1,479 persons born in Muzo, 34.6. The proportion of positives increased with age and length of residence in Muzo, but there was an unexpectedly high percentage of positives in the 10–14 age group. While the number of positive males was appreciably larger than that of females, especially in the older age groups, the proportion of female positives to male was relatively high and suggests that transmission of yellow fever may not infrequently occur outside of forests. This hypothesis is supported by the observation that Haemagogus spp. have been taken biting well outside the forests and even within houses.

One to two years after the first bleeding, a second bleeding was done on 1,462 of the original donors. Net conversion rates (conversions less reconversions) for males and females were 12.8 per cent and 6.7 per cent, respectively. As would be expected, the rates were highest in adult males. In females the pattern by age group was irregular.

The data do not permit a conclusion as to whether yellow fever is enzootic or epizootic in Muzo.

Author Notes

The Rockefeller Foundation, New York 20, N. Y.

Lederle Laboratories Division of American Cyanamid Company, Pearl River, New York.

Consultant on Nuclear Energy Affairs, The Rockefeller Foundation, New York 20, N. Y.