Introduction

In Italy, as well as in most European countries, hexavalent vaccines, protecting against six diseases (diphtheria, tetanus, pertussis [DTaP], poliovirus, hepatitis B virus [HBV], and Haemophilus influenzae type b [Hib]), are routinely the standard of care [1,2,3,4,5,6]. Combined vaccines allow the reduction of the number of injections and number of side effects, the reduction of costs and the increase in adherence of the family to the vaccination schedule both in terms of the number of doses and timing [7, 8]. As a consequence, in countries which have adopted a program based on hexavalent vaccines, the coverage rates in the first year of life are high (from 90 to 99%) and higher than those obtained in countries that use other multivalent vaccines as the only option or as an alternative to hexavalent vaccination (70–90%) [8]. According to the Italian 2017–2019 National Vaccine Prevention Plan (PNPV) schedule (Fig. 1), the primary immunization cycle with hexavalent vaccines includes 3 doses, to be administered at 3–5-11 months of age, followed by booster doses in preschool age (at 6 years) with DTPa and inactivated poliovirus vaccines (DTPa-IPV) and with dTaP-IPV in adolescents (12–18 years) and adults, to be repeated every 10 years [9]. This strategy has been associated with a dramatic reduction in the number of cases of infectious diseases targeted by the hexavalent vaccines in Italy [10]. Currently, three hexavalent vaccines are marketed in Italy, Infanrix Hexa®, Hexyon® and Vaxelis®, which can be administered in preterm infants. The European Medicines Agency (EMA) authorizes the use of the three hexavalent vaccines even in the most severe preterm infants ( [11,12,13] Fig. 2).

Fig. 1
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Vaccination schedule in Italy.). According to the Italian 2017–2019 National Vaccine Prevention Plan (PNPV)

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Fig. 2
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Infanrix Hexa®, Hexyon® e Vaxelis®. Summary of product characteristics as reported by EMA (European Medicine Agency). Paragraph concerning safety in preterm infants, including those born with ≤28 weeks of gestational age and/or recent history of respiratory distress

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In term infants the three hexavalent vaccines have been proved to be not inferior to each other for immunogenicity and safety profile, even in co-administration with anti-pneumococcal and anti-rotavirus vaccination [11,12,13,14,15]. Moreover, large effectiveness studies have been performed in several European countries (including Sweden, Denmark, and Germany) concerning diseases such as pertussis and invasive Hib [16,17,18]. The safety data collected in the clinical studies on hexavalent vaccines have also demonstrated good tolerability of these vaccines, confirmed by phase IV post-marketing surveillance studies, reporting a higher percentage, but not statistically significant, of fever and mild, transitory, local symptoms compared to vaccines with fewer components [11,12,13, 19, 20]. It was estimated that in 2018 approximately 500,000 preterm infants have been vaccinated with Hexyon®, while a similar number of preterm infants in the same year were vaccinated with Infanrix Hexa® and Vaxelis® [8]. A recent publication by a panel of Italian experts from the Italian Society of Hygiene and Preventive Medicine, The Italian Society of Pediatrics and the Italian Federation of Pediatrics confirmed that all the three hexavalent vaccines can be used in preterm infants (Tables 1 and 2), [7]. The panel underlined that no delay in undertaking the national vaccination schedule can be justified in preterm infants, as well as reluctance in the co-administration of other vaccines, including anti-pneumococcal vaccine and anti-rotavirus vaccines. Once the absolute contraindications are excluded, vaccination should be executed basing on the chronological age of the child [6, 21,22,23,24]. This is particularly important considering that high prevalence of sepsis [25,26,27,28], pertussis [29], and other diseases [30, 31] have been reported in preterm infants.

Table 1 Sub-categories of preterm birth, based on gestational age. From WHO. Preterm birth. Fact sheet. 2016. www.who.int/mediacentre/factsheets/fs363/en/. (modified)
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Table 2 Comparison of indications and use of the three hexavalent vaccines (from Orsi et al., 2018 [7], modified)
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Despite the national and international recommendations, however, several studies have shown low vaccination rates and delays in the majority preterm infants [32,33,34,35]. A post-marketing surveillance study on vaccination with Infanrix Hexa® and Hexyon® vaccines found that only 57.6% of preterm infants of the Puglia in 2017 had been vaccinated by their 90th day of life [36]. The ACTION follow-up project (Access to Intensive Care and Neonatal Obstetrics, Access to Obstetrical and Neonatological Intensive Care follow-up project) evaluated preterm infants at 22–31 weeks of EG in 5 Italian regions in 2003–2005 (Friuli-Venezia-Giulia, Tuscany, Marche, Lazio, Calabria) and showed that the delay in starting vaccination was correlated to a lower weight and EG at birth, to a second hospitalization after discharge from the neonatal intensive care unit (NICU), to maternal / paternal unemployment, the number of children within the family, lower socio-economic status, positive anamnesis for cerebral palsy and ethnicity other than Caucasian [33]. Some possible reasons of the vaccination delay in preterm infants concern safety and efficacy of hexavalent vaccines in these children [37,38,39,40]. Hereby, we reviewed and summarized the recent literature regarding safety, efficacy and effectiveness of hexavalent vaccines in preterm infants, which further support the national and international recommendations.

Methods

A systematic search of the literature published from 1 January 2008 to 30 June 2019 was performed on PubMed MEDLINE and Cochrane Library databases, using the following Boolean expression (“infant, preterm” [MeSH Terms]) AND “vaccination” [MeSH terms] AND “hevavalent vaccines” [MeSH terms]) and selecting only articles published in English. The reference of articles retrieved by this search strategy were also examined in order to recover any further relevant publications. A first screening of the selected articles was carried out on the basis of the title and the abstract. We included all prospective or retrospective observational studies and clinical trials regarding efficacy, effectiveness or safety profile of hexavalent vaccines in preterm infants. Studies that carried out a comparison between the hexavalent vaccination in full-term and preterm infants and studies in which hexavalent vaccines was co-administered with other vaccination types (e.g. anti-pneumococcal, anti-rotavirus, anti-VRS, anti-influenza) were included. The non-pertinent articles, commentaries, letters, case series concerning < 10 children cases, reviews, duplicates and articles not written in English were excluded. For each study the following data were evaluated and summarized in the tables: year of publication, study design, gestationale age (GA) of the population, number of study included, type of vaccination administered, outcomes (e.g. laboratory parameters for immunogenicity, clinical indicators for efficacy and reported adverse events for safety), follow-up period and any study bias / limits.

Results

Initially, 101 articles were retrieved by the search strategy. Sixteen articles regarding hexavalent vaccination in preterm infants have been identified, of which 13 and 5 concerned safety ([39, 40, 42]; Table 3) and efficacy/effectiveness (Table 4), respectively. In all these studies hexavalent vaccines were administered according to the primary 3-dose immunization schedule, as recommended by the Centers for Disease Control and Prevention [43]. Among the 13 safety studies, 11 had hexavalent vaccination [39, 40, 42], as main objective, while 2 studies had, as their main objective, other types of vaccines (anti-pneumococcal and anti-rotavirus) in co-administration with hexavalent vaccines. In 9 out of 11 safety studies on hexavalent vaccines, other vaccines were co-administered [39, 40, 42] in the same vaccination session. Regarding the safety of hexavalent vaccines in preterm infants, several studies showed a similar or lower incidence of both local and systemic adverse events in preterm compared to full-term infants, probably due to the lower ability to induce an inflammatory response in preterm infants, even when co-administered with pneumococcal vaccine None of the serious adverse events observed in these studies were considered causally related to the vaccination. Apnoea and alterations in reactivity in preterm infants were the most frequently reported adverse events. Numerous studies have shown an increase in the incidence of apnoea episodes in preterm infants after hexavalent vaccination [40,41,42]. Preterm infants who have experienced apneoa episodes after hexavalent vaccination are generally those in more critical clinical conditions (e.g. previous late-onset sepsis), who required greater support through continous positive airway pressure, and who have a positive history for similar episodes, particularly in the 24 h prior to the administration of the vaccination [40,41,42]. A lower birth weight (< 2 kg), a lower GA (≤ 31 weeks) and chronological age (< 67 days), a positive history for similar episodes and hospitalization for complications related to prematurity are risk factors of recurrence of post-vaccination apnoea [40] also at the second dose of the vaccination schedule with variable percentage in the datasets (4.4–18%) [39, 40]. However, a causal relationship between the vaccine and the appearance of cardio-respiratory events continues to be, widely debated. The available studies are often retrospective, with no control group and with numbers of included children too limited to demonstrate a statistically significant difference in the incidence of these events in preterm infants. Furthermore, it is difficult to distinguish apnoea related to vaccination from those due to the clinical instability of preterm infants due to other associated comorbidities (e.g. periventricular hemorrhage, bronchopulmonary dysplasia or late onset sepsis). Indeed, the only available prospective randomized controlled trial by Carbone et al. suggests that the hexavalent vaccine administration is not associated with cardio-respiratory events, showing no difference in the frequency and severity of apnoea / bradycardia episodes in those who had received vaccination compared to controls. Given the potential risk of apnoea reported in several studies, however, preterm infants still hospitalized at 2 months of life should be vaccinated before discharge from the NICU with a clinical and respiratory monitoring for the 48–72 h following vaccination, particularly with regard to the very low birth weight (VLBW) preterm with a positive medical history for cardio-respiratory events, especially in the 24 h prior to the administration of the vaccine. If a cardio-respiratory episode has occurred after the first vaccine dose, the second dose should be administered in a protected environment in those individuals at high risk of recurrence.

Table 3 Studies concerning the safety of hexavalent vaccine co-administered with other types vaccine in pre-term infants
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Table 4 Studies concerning the immunogenicity of hexavalent vaccination in preterm
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Focusing on the immunogenicity of hexavalent vaccines, high seroprotection rates have been reported in preterm infants. However, a lower GA seems to be associated with lower antibody titres against some vaccine antigens (eg HBV, Hib, poliovirus serotype 1, and pertussis), regardless of the type of hexavalent vaccine used. These data are in agreement with previous studies in which the seroprotection rates reached 98.7–100% in preterm infants [43, 44]. Lower seroprotection rates, although still high, have been reported with respect to pertussis and Hib (92.4 and 92.5% respectively) [44]. Although lower GA and birth weight seem to be associated with lower antibody titres against Hib, a booster dose of hexavalent vaccine was able to induce a protective serological response (evaluated as anti-PRP [anti-polyribosyl ribitol phosphate] antibody concentration > 1 μg/mL) in 98.2% of preterm infants, suggesting an adequate immunological memory in this population [45]. On the other hand, the ability to induce an adequate humoral response to HBV in preterm infants is still debated. In large studies the seroprotection rates for HBV were similar in very low birth weight (VLBW) and low birth weight (LBW) (93.7% vs. 94.9% respectively), However, seroprotection rates were reinforced (> 98%) by booster vaccination for all antigens except for HBs in VLBW children: only 88.7% of those had anti-HBs antibody concentrations > or = 10 mIU/mL, compared with 96.5% of LBW children (the difference was not statistically significant) [43]. In contrast, Omenaca et al. did not observe no difference in the humoral response to HBV vaccination in preterm infants of lower GA and birth weight neither after primary cycle nor after a booster dose [46]. Only one study investigated the T cell-mediated response in preterm infants, showing comparable levels of memory T lymphocyte response for poliovirus antigens in full-term and preterm infants after hexavalent vaccination, but the latter demonstrate several nonspecific and poliovirus-specific functional T cell limitations. Moreover, although all infants developed seroprotective poliovirus antibody titers, serotype 1 titers were lower among preterm infants (P = 0.03).

The immunogenicity of the hexavalent vaccines in co-administration with other types of vaccines was evaluated in several studies [47]. Effectiveness studies regarding the use of hexavalent in preterm infants are very scarce at the moment. Furthermore, in the available studies, a 3 + 1 schedule was used characterized, as recommended by the Centers for Disease Control and Prevention [48], while in some European countries, including Italy, a reduced (2 + 1) vaccination schedule is employed. Further studies are needed in order to define in greater detail the immunogenicity and effectiveness of hexavalent vaccines with reduced schedule in preterm infants, focusing also on the potential differences in the subgroup of severe preterm infants (< 29 weeks of EG), in order to assess the need for additional vaccine doses or for early administration of booster doses.

Conclusions

Hexavalent vaccines administered to preterm infants display a good safety and immunogenicity profile even when co-administrated with other vaccines included in the Italian schedule. Summarizing the results of the present review, it should be bared in mind that:. the three hexavalent vaccines available in Italy have the same indications and can be administered in preterm infants; • although further studies are needed regarding their effectiveness, no delay in vaccination of clinically stable preterm infants is justifiable; • preterm infants must be immunized according to their chronological age, in the same time and manner as those born at term; • severe preterm infants who are still hospitalized at the time when they should be vaccinated should receive the first dose during the hospitalization, particularly in the case of preterm infants ≤31 weeks, with a birth weight < 2.0 kg, with previous episodes of apnoea / bradycardia before vaccination and / or with severe bronchopulmonary dysplasia, in order to provide monitoring for cardiorespiratory events for the 48–72 h following vaccination; • if a cardio-respiratory episode has occurred after the first vaccination dose, the second dose should be administered in a protected environment; • the specific recommendation for the administration of hexavalent vaccines in the most severe preterm infants (≤ 28 weeks) or with a recent history of respiratory distress, as reported by EMA, includes an additional respiratory monitoring for 48 to 72 h after the vaccination.