Epidemiology

From January, 01 to May, 31 2024, a total of 1,242 respiratory samples from 1,151 children and adolescents were screened for respiratory pathogens including B.pertussis at the Bambino Gesù Children's Hospital in Rome, Italy. During this period, we observed a marked increase in the detections of B.pertussis, the maximum frequency being recorded in May. Specifically, a diagnosis of B.pertussis, confirmed by positive molecular assays, was made in 66 patients (Fig. S1); 39.4% (N = 26) of patients were from the outpatient department and thus had suspected pertussis, while 51.5% (N = 34) were from the emergency department with respiratory symptoms of varying intensity (cough, apnea or dyspnea). The remaining 9.1% (N = 6) of patients came from the hospital wards.

General population

The majority of patients (N = 37, 56.1%) at diagnosis had less than one year (28.8% had < 3 months, 18.2% had 3- < 6 months, and 9.1% had 6–12 months), while 29 (43.9%) had > 1 year.

More than half of the patients (N = 36, 54.5%) were hospitalized for a median (IQR) duration of hospitalization of 8 (4–10) days. Of these, 6 (9.1%) required admission to the intensive care unit (ICU). Overall, 44 (66.8%) paediatric patients did not require ventilatory support, while the remaining 22 needed respiratory support. Specifically, 8 (12.1%) patients required low flow oxygen, 9 (13.6%) needed high flow nasal cannula, 2 (3.0%) and 3 (4.5%) patients required Helmet continuous positive airway pressure and mechanical ventilation, respectively. Regarding vaccination status, which was available for 45 patients, 23 (51.1%) patients were too young to be vaccinated, while only 17 (37.8%) had received total or partial number of pertussis vaccine doses; 5 (11.1%) patients had not been vaccinated by parental decision. For 21 patients, vaccination status was unknown. Only 4 of the patients’ mothers had received Tdap vaccine before or during the current pregnancy.

Demographic and clinical characteristics are reported in Table 1.

Table 1 Demographic and clinical characteristics of patients according to respiratory failurePopulation stratified according to respiratory failure

Looking at the severity of infection, 14 (21.2%) patients had respiratory failure, while 52 (78.8%) patients had a mild infection without respiratory failure.

By comparing the demographic and clinical characteristics in these two groups, we observed that patients with respiratory failure were significantly younger than those without respiratory failure (median [IQR] age: 0.21 [0.12–0.41] vs 2.2 [0.28–5.55] years, p-value < 0.001). In contrast, patients aged > 12 months belonged more to the group of patients without respiratory failure (53.8% no respiratory failure vs. 7.0% respiratory failure, p-value: 0.002).

As expected, patients with respiratory failure were all hospitalized (N = 14, 100%) while only 22 (42.3%) patients without respiratory failure required hospitalization (Table 1). Of note, the majority of the latter patients had less than 1-year-old (N = 18/22).

Regarding B.pertussis load, similar PCR Ct/Cp values were observed between two groups (median [IQR] 28.0 [20.4–32.5] Ct/Cp in respiratory failure compared to 25.8 [20.9–30.0] Ct/Cp in non-respiratory failure, p-value: 0.535) (Table 1).

Nine (64.3%) patients with respiratory failure required High Flow Nasal Cannula (HFNC), and 2 (14.3%) patients required Helmet continuous positive airway pressure (hCPAP). Mechanical ventilation was required in 3 (21.4%) patients.

In addition, patients with respiratory failure had a higher white blood cell count and a higher rate of leukocytosis than those without respiratory failure (white blood cell count, median [IQR]: 21760 [15288–30340] vs. 16190 [9970–15630] cells/mm3, p-value 0,064; leucocytosis, n [%]: 13 [92.9%] vs. 15 [65.2%], p-value: 0.062).

Co-infection in general population

Of the 66 patients with pertussis infection enrolled, 43 had concomitantly required evaluation of other respiratory microorganism. Of these, 10 (23.3%) patients (median [IQR] age of 0.29 [0.09–2.11] years) had B.pertussis alone, while 33 (76.7%) patients (median [IQR] age: 0.28 [0.20–0.87] years, p value: 0.857) were characterized by respiratory co-infections (Fig. 1, panel A).

Fig. 1

Distribution of microorganism detections in overall population (panel A) and according to respiratory failure (panel B)

By characterizing the type of co-infection, the majority involved viruses (N = 30, 90.9%), while bacterial co-infections (Haemophilus influenzae + Staphylococcus aureus and Mycoplasma pneumoniae) were observed in 2 (6.1%) patients. Only one patient, in addition to B.pertussis, had a co-infection characterized by both bacteria and virus (Haemophilus influenzae + Moraxella catarrhalis + Human Rhinovirus/Enterovirus).

Among the 33 patients with co-infection, 19 had co-infection with only one other pathogen (Human Rhinovirus/Enterovirus [N = 7], Human Rhinovirus [N = 5], Metapneumovirus [N = 3], Parainfluenza virus type 3 (PIV3) [N = 1], Coronavirus OC43 [N = 1], Respiratory Syncytial Virus [N = 1] and Mycoplasma pneumoniae [N = 1]), while 14 patients had multiple co-infections. Of interest, no demographic and clinical differences were observed between B.pertussis mono- and co-infections. A trend toward a lower Ct/Cp values was observed in mono-infection with respect to co-infection (median [IQR]: 20.7 [18.0–25.0] vs 25.4 [19.1–30.5] Ct/Cp, p-value:0.071). Similarly, no difference in term of co-infection was found between vaccinated- (or their mothers) and unvaccinated children.

Co-infection in population stratified according to respiratory failure

Looking at the distribution of mono- and co-infections according to respiratory failure, a higher prevalence of co-infections was found in patients with respiratory failure than in those without respiratory failure (92.9% vs. 69.0%, p-value: 0.041) (Fig. 1, panel B). In both groups, co-infection with Rhinovirus alone or associated with other pathogens was most prevalent (6/13 [46.1%] in respiratory failure and 9/20 [45.0%] in non-respiratory failure) followed by Parainfluenza virus type 3 (3/13 [23.1%] in respiratory failure and 3/20 [15.0%] in non-respiratory failure). Of note, Human bocavirus co-infections were observed exclusively in patients with respiratory failure (3/13, 23.1%).

Patients admitted to ICU

Concerning the 6 patients who required intensive care admission, 5/6 patients were less than 3 months old and thus had not received the vaccine; also their pregnant mothers had not received the vaccine. The last patient, 10 months old and with a diabetic ketoacidosis, had received 2 doses of vaccine. Overall, only one of these patients had presented to the hospital with suspected pertussis.

Of note, the bacterial load was higher in these patients than in other patients with respiratory failure who did not need ICU admission (Cp/Ct: 21.6 [15.6–28.0] vs. 30.8 [26.6–35.3], p-value:0.061).

Of interest, among these paediatric patients requiring admission to ICU, 5 patients were characterized by co-infections (Haemophilus influenzae + Staphylococcus aureus, Metapneumovirus, Human Rhinovirus + Respiratory Syncytial Virus, Human Rhinovirus/Enterovirus and Adenovirus + Human Rhinovirus/Enterovirus).