Why pertussis is still a concern? Reasons for resurgence and improving strategies

Pertussis is a highly contagious, vaccine-preventable, bacterial infectious disease, caused by B. pertussis. It affects children less than 6 months of age with severe clinical symptoms especially in newborns. Pertussis continues as a public health concern threat given its re-emergence despite high vaccination coverage; re-emergence could be caused by the waning of vaccine immunity consequent to introduction of the acellular (aP) vaccine, by the adaptation of circulation of variants B. pertussis strains and by the improvement of diagnostic methods and active surveillance. We conducted a longitudinal case-controls double center study on 141 infants younger than 6 months, hospitalized with typical symptoms of pertussis and 235 healthy controls. Inclusion criteria: age lower than 180 days, with cough lasting more than 5 days, paroxysmal cough, apnea or cyanosis and post-cough vomit. Exclusion criteria: chronic diseases and genetic syndromes. A total of 157 breastfeeding mothers from cases and controls infants were included in the immunological study and requested to undergo a blood and breast milk sample collection. A total of 167 parents from 145 infants were enrolled in the study about the serum-epidemiology of pertussis. Our project leads to identify a prevalence of more than 50% of pertussis cases among the population studied. We demonstrated that breastfeeding does not exert a protective role against pertussis infection in infants, showing a low immunologic activity of breast milk against B. pertussis, compared with other pathogens, and that the presence of at least one sibling doubled the risk to contract pertussis. In addition, comparing the specific immune response of mother of cases and mothers of controls, we showed that both mothers groups have pre-existing pertussis-specific antibodies and memory B cells and react against the infection with a recall response increasing the levels of specific serum IgG and the frequency of all isotypes of memory B cells. Finally, we found that 40% of parents of infants hospitalized with pertussis had a serological evidence of recent infection, and only 30% showed respiratory symptoms in the previous 3 weeks; interestingly, 30% of parents who not reported symptoms had a serological evidence of recent infection. The combination of these results allows to suppose a source of contagion in almost 90% of them. In conclusion, the prevalence and severity of the disease among infants should increase the attention on the disease, leading to better strategies for its prevention and care. It is mandatory to advise parents about pertussis transmission to the newborns and to be aware of respiratory symptoms in the household. Breastfeeding remains a milestone of prevention for several infectious diseases, but in our study, we showed no protective role in the prevention of B. pertussis infection. Vaccination remains the major strategy for the prevention of pertussis but, this project supports an improving in pertussis surveillance, the detection of escape mutants, and the development of more effective vaccines.