Although the global epidemic of pertussis has been controlled through the expanded Programme on Immunization (EPI), the incidence of pertussis has increased significantly in recent years, with a "resurgence" of pertussis occurring in developed countries with high immunization coverage. The incidence of pertussis in Shenzhen, was about 2.02/100,000, far exceeding that of the whole province and the whole country (both < 1/100,000). At the same time, more and more studies have shown that there is antigenic drift in Bortella pertussis, which may be associated with the increased incidence. 50 strains of Bordetella pertussis isolated from 387 suspected cases were collected in Shenzhen in 2018 for genotype distributions and molecular epidemiological characteristics analysis.
There were 387 suspected cases of pertussis enrolled at surveillance sites in Shenzhen from June to August 2018. Nasopharyngeal swabs of suspicious cases were collected for separation and culture, and the positive strains were identified by real-time PCR. The immunization histories of patients were analyzed to investigate the relationship between pertussis vaccination and infection. The major antigen genes of the isolated positive strains, including ptxA, ptxC, ptxP, prn, fim2, and fim3, were analyzed by second-generation sequencing. The homology and phylogenetic analysis of these genes was performed using the public genome sequence downloaded from GenBank.
50 strains of Bordetella pertussis were successfully isolated from nasopharyngeal swabs of 387 suspected cases, with a positive rate of 12.9%, including 28 males and 22 females, accounting for 56.0% and 44.0% respectively. It is worth noting that 38 were under one-year-old among the positive patients, accounting for 76.0%. Among the cases with a history of vaccination, 71.4% of positive patients did not complete the basic vaccination process of the DTaP at the time of onset. Three major antigen genotypes different from CS and Tohama I vaccine strains were identified, and they had distant genetic relationships and 62.0% of which was prn2/ptxC2/ptxP3/ptxA1/fim3-1/fim2-1.
The positive rate of cases under one-year-old was significantly higher than that of other age groups and should be monitored. The major antigenic genes of the Bordetella pertussis strains isolated in Shenzhen were different from those of common vaccine strains. This study explained the resurgence of whooping cough from certain angles, including immunization strategy, vaccination time and genome variation of strains, which is beneficial to prevent pertussis infections.