Bordetella pertussis isolates which do not express some of acellular pertussis vaccine (aPv) antigens, e.g. pertactin (PRN), have been increasingly reported in countries using aPvs. In Finland, primary pertussis vaccination with whole-cell vaccine was replaced by aPv containing pertussis toxin (PT) and filamentous hemagglutinin (FHA) in 2005 and then by aPv containing PT, FHA, and PRN in 2009. We aimed to study alterations in the expression of FHA, PRN, and PT, three antigens included in aPvs and adenylate cyclase toxin (ACT) not included in current aPvs, among Finnish isolates collected during 1991–2020.

Of 904 isolates collected by the Finnish Reference Laboratory for Pertussis during 1991–2020, 302 were randomly included. An adapted, monoclonal antibody based, antigen expression ELISA, including the culture of B. pertussis in Stainer-Scholte medium, was performed to quantify the expression of ACT, FHA, PRN, and PT of each isolate. ACT activity was also measured for 16 isolates. Arbitrary units were used for comparing levels of each antigen expression of isolates grouped in every five years.

Following the implementation of aPv in 2005, B. pertussis isolates exhibited a 1.75-fold increase for FHA (p < 0.001) and a 1.5-fold increase for ACT (p < 0.0041) expression until 2020. No FHA or ACT deficient isolates were detected. As the number of PRN deficient isolates has significantly increased with the time, the amount of PRN produced by the positive isolates has also started to decrease, especially after the use of aPv containing PRN. During this period, fluctuations in PT expression were observed.

The study demonstrated that in response to aPv-induced selection pressure, different types of selection of B. pertussis has occurred. For FHA and ACT, a steady increase in their production is observed, whereas the frequency of PRN deficient isolates is increased with time.