Haemophilus influenzae (Hi) is a Gram negative bacterium that can be capsulated or non-capsulated. The structure of the capsule polysaccharides allows dividing the capsulated isolates into six distinct serotypes (Hi a to f) while non-capsulated isolates correspond to non-typeable isolates (NTHi). Hi is a commensal resident of the respiratory and the genital mucosa. Asymptomatic carriage of Hi in healthy children under 5 years is common (27.7%) varying between 73.2% in winter and 26.8% in summer [
1]. Hi isolates can be responsible for non-invasive local infections (most frequently respiratory infections) and invasive systemic infections that are defined by the detection of Hi by culture and / or by detection of Hi DNA in a normally sterile site. These invasive infections are mainly septicaemia and meningitis but also arthritis and epiglottitis. The incidence of confirmed cases of invasive infections in Hi in Europe was 0.7 cases per 100,000 in 2016 (0.5 in 2012) with an incidence that varies between 0.0 and 3.6 per 100,000 people in 2016 according to the country (France 0.9 per 100,000 people) [
2]. NTHi isolates caused the majority of cases in all age groups (78% of all cases for which typing results were available). Hif caused 11% of all cases with known type. Hib ranked second among these isolates with 6%. However, Hib were the most frequent typeable isolates among children under the age of 5 years [
2]. Prior to routine immunization against Hib in the early 1990s, Hib was the most prevalent cause of invasive Hi disease (IHiD) among children. Studies suggested that a serum anti-PRP antibody concentration of at least 0.15 μg/ml and 1.0 μg/ml might correlate with short-and long-term protection respectively from invasive Hib disease [
3]. In France, the Hib conjugate vaccine was introduced into the routine childhood immunization program in early 1992 as a 3 + 1 scheme at 2, 3, and 4 months (primary vaccination) and a booster at the age of 16–18 months. The vaccination scheme was simplified in 2013 to a 2 + 1 scheme at 2 and 4 months (primary vaccination) and a booster at the age of 11 months using a hexavalent vaccine (against diphtheria, tetanus, pertussis (acellular vaccine), poliomyelitis,
Haemophilus influenzae b, and hepatitis B). In 2017, the vaccine coverage was estimated at 95.4% at the age of 24 months (
https://www.santepubliquefrance.fr/determinants-de-sante/vaccination/donnees. Accessed 14/03/2020).
In France, in 2017, NTHi represented 75% of invasive Hi isolates while Hif and Hib represented 10 and 7% respectively. However, 50% of the Hib cases were among children < 5 years old and represented 13% of Hi cases among children < 5 years [
4]. The proportion of Hib cases among children < 5 years prompted enhancing surveillance of Hib cases and conducting a seroprevalence study to measure Hib antibodies in population before and after changing the vaccination schedule. Enhancing surveillance was also warranted as waning of immune response was reported after vaccination schemes in infants < 1 year with conjugate polysaccharide vaccines against
Neisseria meningitidis serogroup C as protective titres fell to 36% 18 months after vaccination [
5]. Similar observation was reported for Hib [
6] and for several serotypes of
Streptococcus pneumoniae [
7]
. For example, after primary vaccination at 2, 3, and 4 months of age, antibody levels that were measured at 8, and 11 months declined for serotype 4 below the seroprotection level of 0.35 μg/ml [
7].