Seroconversions in unvaccinated infants: further evidence for subclinical measles from vaccine trials in Niakhar, Senegal

Background Increases in measles antibodies without rash-illnesses have been documented in previously vaccinated children exposed to measles cases. The phenomenon has been incompletely evaluated in young unvaccinated infants w ith immunity of maternal origin. Methods Monthly cohorts of newborns were prospectively randomized to vaccin e and placebo control groups during a trial of high-titre vaccines in Niakh ar, Senegal. Measles antibodies were assayed in blood samples of enrolled c hildren collected at 5 months old, when controls received a placebo injecti on, and at 10 months, when the placebo group was given measles vaccine. Int ensive prospective surveillance for measles was conducted throughout the tr ial. Results One-fifth (n =53) of the placebo controls seroconverted, with known exposure to a measles case in only three of them. None of the seroconverte rs developed a measles-like rash. Sixteen-fold or greater increases in titr es were noted in about one-quarter of them. Compared with placebo controls who did not seroconvert, seroconverters were more likely to have had exposu re to a measles case and to travel, more likely to be boys than girls, and had significantly lower baseline antibody titres. Measles was endemic in th e study area throughout the trial. Seroconversions did not adversely effect subsequent nutritional indices or mortality. Conclusions Although laboratory errors and inadvertent injection of vaccine rather than placebo may have played some role, they do not fully explain t he above observations, which are consistent with subclinical measles in the seroconverters. The possible role of subclinical measles in occult transmi ssion, its potential effect on the type and duration of subsequent immunity , and its impact on response to primary vaccination need to be determined.