Human immunodeficiency virus (HIV) transmission models that include variability in sexual behavior over time
have shown increased incidence, prevalence, and acute-state transmission rates for a given population risk profile.
This raises the question of whether dynamic variation in individual sexual behavior is a real phenomenon that can be
observed and measured. To study this dynamic variation, we developed a model incorporating heterogeneity in
both between-person and within-person sexual contact patterns. Using novel methodology that we call iterated filtering
for longitudinal data, we fitted this model by maximum likelihood to longitudinal survey data from the Centers
for Disease Control and Prevention’s Collaborative HIV Seroincidence Study (1992–1995). We found evidence for
individual heterogeneity in sexual behavior over time. We simulated an epidemic process and found that inclusion of
empirically measured levels of dynamic variation in individual-level sexual behavior brought the theoretical predictions
of HIV incidence into closer alignment with reality given the measured per-act probabilities of transmission.
The methods developed here provide a framework for quantifying variation in sexual behaviors that helps in understanding
the HIV epidemic among gay men.