Bordetella pertussis Isolates Circulating in China Where Whole Cell Vaccines Have Been Used for 50 Years.

TO THE EDITOR—Despite extensive vaccinations, whooping cough has resurged in industrial countries. The major explanations include wanning immunity associated with the acellular pertussis vaccines (ACVs) and Bordetella pertussis adaptation to vaccine-induced immunity. ACVs contain purified antigens in different combinations: pertussis toxin (Ptx), filamentous hemagglutin, pertactin (Prn), and fimbriae (Fim) 2 and Fim3. However, divergence in vaccine antigens have been found between vaccine strains and circulating isolates [1–3]. Moreover, contiunous changes occur in B. pertussis genomes as determined by multilocus variable number of tandem repeat analysis (MLVA) and pulsed-field gel electrophoresis (PFGE) [1–3]. Today the common allele profile and MLVA type found in the industrial countries are ptxA1/prn2/ptxP3/fim3-2 and MT27. The vaccine strains used for ACVs or whole cell vaccines (WCVs) are mostly with ptxA2 or ptxA3/prn1/ptxP1/ fim3-1. Our current knowledge of B. pertussis adaptation is obtained from studies conducted in industrial countries where ACVs were introduced in the 1990s. To better understand bacterial adaptation to ACVs induced selection pressurewe need to compare B. pertussis in countries where WCVs have been continuously used. In China, vaccination with WCVs was introduced in the early 1960s. In 1982, a booster dose given at 18–24 months was added [4]. The vaccination schedule has remainedunchanged,althoughbothWCVs and ACVs have been used since 2007. The number of notified cases has been decreasing, and a total of 2183 and 1712 cases were reported in 2012 and 2013 [5]. Most reported cases were infants. We analyzed 16 B. pertussis isolates collected from 2012 to 2013 during a prospective study in Xi’an, China (Table 1) [6]. They were isolated throughout the study period and not from local small outbreaks. The analyses included sequencing of ptxA, prn, ptxP, and fim3 genes, MLVA and PFGE using XbaI as the restriction enzyme [1]. PFGE profiles were analyzed at National Pertussis Reference Laboratory, Turku, Finland. The nomenclature was based on the profiles already defined for Finland (BpFINR)