F2 and infection: Based on the waning anti-PT antibodies after infection [11], an anti-PT IgG of ≥ 62.5 to ≥ 80 IU/mL is often used as the cut-off threshold indicating pertussis infection within 12 months, and cut-off values of ≥ 100 IU/mL and ≥125 IU/mL are used as evidence of a recent infection and acute infection, respectively [38,39].