V. Evaluation of Cross-Immunity against Type 1 Dengue Fever in Human Subjects Convalescent from Subclinical Natural Japanese Encephalitis Virus Infection and Vaccinated with 17D Strain Yellow Fever Vaccine
To compare the micro-enzyme-linked immunosorbent assay (ELISA) done with cultured Plasmodium falciparum as antigen with the indirect fluorescent antibody test (IFA), 261 sera collected in Vietnam and 351 samples eluted from filter papers collected in Honduras were examined by both methods of measuring antibody to P. falciparum. Thick blood films were also examined from persons sampled in Honduras. Discordance between the two tests was observed in 23.0% of the Vietnam sera and in 29.4% of the samples from Honduras. The major differences observed occurred in younger persons (<30 years in the Vietnam specimens and <16 years of age in those from Honduras); in these age groups seropositivity rates by ELISA were significantly higher than those found by IFA. However, even in older persons in whom the seropositivity rates observed with the two tests were similar, many of the specimens were positive by one test but not both. Fifteen persons had slide-demonstrated infection with P. falciparum; all 15 were positive by IFA (reciprocal titer ⩾20) but only 12 were positive (⩾80) by the ELISA. Two of 7 children aged 1–5 years with P. falciparum infections had a negative ELISA test, as did 1 of 6 children 6–15 years of age. These results suggest the measurement of both shared and dissimilar antibodies by the two serologic tests. Further work is necessary to determine why the ELISA is negative in some persons with slide-proven infections.