Objective: To examine determinants of thymus size at age 6 months and investigate whether thymus size at this age is a determinant of subsequent mortality.
Study design: Thymus size was measured by transsternal sonography in 923 6-month-old children participating in a measles vaccination trial in Guinea-Bissau.
Results: Thymus size was strongly associated with anthropometric measurements. Boys had larger thymuses than girls, controlling for anthropometry. Crying during sonography made the thymus appear smaller. Children who were not vaccinated with Bacille Calmette-Guérin (BCG) or were vaccinated with BCG in the preceding 4 weeks before inclusion into the study had larger thymuses. Children who had malaria or had been treated with chloroquine or Quinimax in the previous week before inclusion had smaller thymuses. Controlled for background factors associated with thymus size and mortality, small thymus size remained a strong and independent risk factor for mortality (hazard ratio = 0.31; 95% confidence interval = 0.18 to 0.52).
Conclusions: Small thymus size at age 6 months is a strong risk factor for mortality. To prevent unnecessary deaths, it is important to identify preventable factors predisposing to small thymus size.