**2. Whooping cough: current situation**

This same incidence is observed nowadays in many developing countries and also in some high-income countries among children under 1 year of age. The vaccine age begins in the 1940s with the whole-cell pertussis vaccines (wP vaccines) and in 1992 with acellular pertussis vaccines (aP vaccines) in developed countries, with a marked decrease in the number of sick individuals as well as in the number of hospitalizations. Despite this, there has been an increase in the incidence and deaths due to pertussis in infants fewer than 6 months of age between 1980 and 2010 in the USA, in Europe, and in many other countries [1–5]. Whooping cough is a highly infectious disease caused by *Bordetella pertussis* and, more rarely, by *Bordetella parapertussis*, *B. bronchiseptica*, or *B. holmesii*. It is the most ill-controlled vaccine-preventable bacterial disease in countries with high vaccination coverage, in which disease peaks occur every 3–5 years. Although routine childhood vaccination has produced a substantial reduction in the number of cases, it continues to cause high morbidity and mortality in children in countries across the globe [6–8]. In developed countries with pertussis vaccination coverage above 90–95%, such as the USA, the UK, several European countries, and Australia, pertussis has manifested in children under 6 months of age when they have not yet completed their primary series and in adolescents and adults who lost their immunity induced by the vaccine (the last booster is given at the age of 5 years). Young infants present atypical and potentially serious conditions, with about 50% of the cases leading to hospitalizations and often even to death, while adolescents and adults also present atypical but mild symptomatology, and as a result, the individual is often mistakenly diagnosed with other infections of the upper respiratory tract [2, 3, 9, 10]. The causes of the decreasing immunity to pertussis are varied: from the primary vaccine failure due to bacterial adaptations to the failure of the vaccine to eliminate the bacteria from the carriers' organism and thereby prevent transmission to the dropping of protective antibodies. The duration of protection of the acellular vaccines is approximately 3 years, with 85% efficacy, and the risk of contracting the disease increases by 1.33 times each year after the last dose of the vaccine. Therefore, the vaccine protects against the disease, but not against bacterial colonization and its consequent transmission. Loss of vaccine-derived protection over time and increased circulation of *B. pertussis* lead to increased susceptibility of adolescents and adults. As a result, whooping cough is often reported as a cause of persistent cough in adolescents and adults [6, 11–14]. The variation in the notification of the age group affected by pertussis can be explained in part by a growing recognition of the less typical manifestations of the disease in adolescents and adults and by severe cases in young infants. It can also be explained by the development of more sensitive laboratory tests and by a more sensitive and extended healthcare surveillance to cover all life periods [15–17]. Outbreaks in areas of high vaccination coverage demand a review of vaccination strategies. It is necessary to take into account adolescent and adult transmitters, as well as health professionals and pregnant women. In order to better assess changes in epidemiology over time and to optimize disease control, it is important to improve whooping cough surveillance, from clinical recognition of the disease to laboratory diagnosis [18]. In 2013, according to the WHO estimates, pertussis caused about 63,000 deaths in children under 5 years of age, although there is considerable uncertainty about these estimates in view of the scarcity of reliable

26 Pertussis - Disease, Control and Challenges

There is no consensus as to why the number of pertussis cases has increased in countries with high vaccination coverage. The reasons range from improvements in diagnosis, earlier diagnosis, and more accurate surveillance. These changes have led to an increase in the number of reported cases, but there is also evidence of increased circulation of the bacteria in the population. There are several other explanations for increased epidemics: changes in circulating pertussis virulence, vaccine failure against new bacteria, vaccine failure to block transmission of infection, decreased adherence to vaccination, rapid loss of immunity in adolescents and adults due to the vaccine or due to the disease itself over time, making the vaccinated individuals susceptible, and also the increase of susceptible individuals in the population [10, 17, 23–25].
