F2 and infection: Within DK-P, those adolescents who had been vaccinated within 5–8 years before the infection had statistically significantly higher avidity across 3.5–8 M urea concentrations in comparison to those adolescents who had been vaccinated 9–13 years before infection (p < 0.01) (Figure 6), although they had similar anti-PT IgG concentrations (p = 0.710).