Evolution of invasive H. influenzae cases
The total number (n = 501) of invasive isolates of Hi is depicted in Fig. 1 that showed an increase of the total number between 2017 and 2019. The isolates were of serotype a (n = 20; 4%), serotype b (n = 56; 11.1%), serotype c (n = 2; 0.4%); serotype d (n = 1; 0.2%), serotype e (n = 8; 1.6%) and serotype f (n = 49; 9.7%). The majority of the isolates (n = 365; 72.6) were non-typeable. The number of cases due to serotype b isolates increased to reach 25 cases in 2019 with a total number of 56 Hib invasive isolates during this 3-years period. Most of these Hib isolates (n = 37; 66.1%) were among children under 5 years of age (median age of 0.8 that ranged between 0.2 years and 4.7 years. This was also observed for Hia invasive isolates with 13 out of 20 isolates (65.0%) among children under 5 years old during the 2017–2019 period. However, Hif and non-typeable invasive isolates were more frequently observed among subjects of 5 years of age and older n = 29 out of 49 (59.2%) and n = 302 out of 365 (82.7%) respectively. Whole genome sequence (WGS) data were available for 55 of the 56 Hib isolates (98.2%) and showed that most of these isolates belonged to the ST-6 (n = 32) and 21 other isolates differed by 1, 2 or three loci from ST-6 (Fig. 2). Indeed, 53 isolates (96% of the sequenced Hib isolates) can then be grouped in one clonal complex (clonal complex ST-6). Only two isolates belonged to two distinct ST (ST-280 and ST-836) that both were detected in adult cases of Hib invasive disease.
Vaccine failure among Hib cases
The high number (n = 37) and proportion (66.1%) of Hib cases among children under 5 years of age during the period 2017–2019 was unexpected. We checked the vaccination status of Hib cases. There were 24 children who were vaccinated completely (n = 9) or partially (n = 15 with one or two doses) while the remaining 13 did not receive any dose. There was no geographical clustering of vaccine failure cases. The clinical presentations were mainly meningitis (n = 16) followed by bacteraemia (n = 7) and one case of epiglottitis. All the 24 vaccinated cases were vaccinated according to the new schedule 2 + 1 that was implemented in 2013 in France. They were non premature with no known immunodeficiency. These cases were distributed during this period as 2 cases in 2017 and 11 cases in each 2018 and 2019. We next explored the levels of anti-PRP IgG among the completely vaccinated children and obtained sera at the admission from the 6 corresponding vaccine failures with a complete 2 + 1 scheme.
The levels of anti-PRP IgG were lower than 1 μg/ml among all the 6 vaccine failure cases (complete 2 + 1 scheme) with median and geometric mean of anti-PRP IgG concentration of 0.43 μg/ml and 0.40 μg/ml (95% CI 0.16–0.98) Respectively (Fig. 3). The median age of these 6 children at the onset of the disease was 2.5 years old and ranged between 1.2 years and 4.1. The median delay between the third dose and the onset of the disease was 1.6 years and ranged between 0.3 and 3.2 years. Sera after 1 month of infection were available from four of these 6 patients with a significantly higher median (6.85 μg/mL) and higher geometric mean of anti-PRP IgG concentration 6.59 μg/ml (95% CI 2.85–15.22; P = 0.01), indicated a significant secondary immune response 1 month post infection. All these data suggest that the vaccine failures are most likely due to decline of IgG titres after the third dose at the age of 11 months.
Sero-prevalence of anti-PRP IgG
The observation that all vaccine failures were among children who were immunised according to the 2 + 1 schedule that was introduced since 2013 prompted the exploration of the seroprevalence of anti-PRP IgG among children under the age of 5 years. We used a collection of residual 232 sera that were classified into two groups according to the age: the first group (n = 130) was composed of children borne before 2013 and named “3 + 1 group” as this schedule was used before 2013. The second group (n = 102) contained the children borne since 2013 and corresponded to the “2 + 1 group”. Each group was further divided into 6 subgroups according to age at the time of sampling: < 6 months, 6–11 months, 1 year, 2 years, 3 years and 4 years of age (Fig. 4).
The median values of anti-PRP IgG were lower in the 2 + 1 group compared to those of the 3 + 1 group in all the 6 subgroups. However, these values did not differ significantly between the corresponding sub-groups except for the two subgroups of children of 2 years of age. The median values of anti-PRP concentrations for children of 2 year of age were 2.9 and 0.58 for the 3 + 1 and 2 + 1 groups respectively (P = 0.047) (Fig. 4).
The proportion of children who achieved the threshold of 0.15 μg/ml of anti PRP IgG (the putative threshold for short-term protection from invasive Hib disease) were also lower for the 2 + 1 group compared to those of the 3 + 1 group. This was also observed when the threshold was 1 μg/ml of anti-PRP IgG (the putative threshold for long-term protection from invasive Hib disease) and particularly for children of 2 years of age (56% versus 25%) (Fig. 5). These data did not reach significant levels due to small numbers in each subgroup. However, they still suggest that the 3 + 1 group showed higher and more persistent titres of anti-PRP IgG.