F2 and pertussis: Moreover, a recent cross-sectional study conducted in Beijing compared the rate of undetectable anti-pertussis toxin (anti-PT) IgG antibodies (<5 IU/mL) in adults aged 20 to 39 years during two study periods (2010 and 2015/2016) and found that the rate of undetectable anti-PT IgG antibodies had significantly increased between the two periods, showing that the adult population were becoming vulnerable to pertussis [19].