Within the best-fitting parameter regimes, incidence periodicity for each modelled strain is basically biennial with strains dominating alternate years. While real strain dynamics are clearly more complex than this (figure 1), observed patterns do show a tendency for a particular strain not to dominate in successive years. Within the model, cross-immunity generates a negative correlation between strains, causing them to alternate in successive years. For strong cross-immunity, both strains become antigenically similar and goodness of fit falls off rapidly.