PEDIATRICS Vol. 111 No. 5 May 2003, pp. 1042-1045
Immunogenicity of a Three-Component Acellular Pertussis Vaccine Administered at Birth
* Division of Neonatology and Neonatal Intensive Care
Biometric Unit
Research Laboratories for Pediatric Oncohematology IRCCS Policlinico San Matteo Pavia
¶ Department of Pediatrics, University of Pavia
|| Division of Neonatology and Neonatal Intensive Care, Spedali Civili, Brescia, Italy
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ABSTRACT |
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Objective. To evaluate within the first 6 months of birth the immunogenicity of a 3-component acellular pertussis (aP) vaccine containing filamentous hemagglutinin (FHA), pertactine (PRN), and genetically detoxified pertussis toxin (PT) in infants who received a dose of vaccine at birth, in addition to the recommended schedule administered at 3, 5, and 11 months. Furthermore, we investigated the influence of maternal antibodies on aP vaccine response.
Methods. We used enzyme-linked immunosorbent assay to evaluate immunoglobulin G antibody levels in 45 infants immunized at birth and at 3, 5, and 11 months (group 1) and in 46 infants immunized at the ages of 3, 5, and 11 months (group 2). All mothers were also tested at delivery.
Results. At the age of 5 months the geometric mean titer of anti-PT, anti-FHA, and anti-PRN was significantly greater in group 1 (who had received 2 doses) than in group 2 (1 dose). At 6 months geometric mean titers were significantly higher in group 1 than in group 2 for anti-PRN and anti-FHA, whereas no significant differences were observed for anti-PT.
Conclusions. Immunization at birth may be important for an earlier prevention of the pertussis disease in infants under 6 months, especially in Italy, where the recommended ages for aP vaccine administration are 3, 5, and 11 months.
Key Words: immunogenicity • acellular anti-pertussis vaccine • immunologic priming
Abbreviations: aP, acellular pertussis • FHA, filamentous hemagglutinin • PRN, pertactine • PT, pertussis toxin • ELISA, enzyme-linked immunosorbent assay • GMT, geometric mean titer • EU/ml, ELISA units per milliliter • IgG, immunoglobulin G
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INTRODUCTION |
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Complications of pertussis are common, and mortality is highest in infants under 6 months of age.1 In industrialized countries the reported incidence of pertussis changed dramatically after the introduction of the whole-cell vaccine.2 In addition, the age distribution of pertussis cases was modified, although its incidence remained high in infants. For example, 50% of all cases reported in the United States from 1980–1989 occurred in children under 12 months of age, and the highest crude annual incidence rate was among infants between 1 and 2 months of age.3 Regional data regarding Emilia Romagna (Northern Italy) showed that the incidence of pertussis in infants under 6 months was
1 per 1000 live births in the period 1996–1999 (3% of all registered pertussis cases). In the same area, the median age at onset of infection shifted from 4.8 years in 1992–1995 to 7.2 years in 1996–1999.4 In France, the classical childhood form was fairly exceptional compared with early cases in infants and later cases in adolescents and adults.5 In Canada, 78.9% of the 1082 cases of pertussis reported between 1991 and 1997 involved children under 6 months of age. Ten deaths occurred, all of which were among infants under 6 months. Furthermore, intensive care was required more frequently and the hospitalization period was longer for infants than for older children.6 Vaccinating adults with the acellular pertussis (aP) vaccines might be considered to reduce the incidence of pertussis in infants, because they could act as a reservoir for pertussis.7 Early immunization against pertussis could also prevent the severe damage of infection in infants under 6 months of age.8,9 Previous experience on the administration of the anti-pertussis whole-cell vaccine in the neonatal period gave poor results.10 However, it is now possible to use acellular DNA-recombinant vaccines containing purified antigens11 that have induced equivalent or higher antibody titers than the whole-cell vaccine in adults, preschool children, and infants.12–14 The risk of severe disease in infants under 6 months of age prompted us to consider the feasibility of an anti-pertussis vaccination at birth, particularly in Italy, where the recommended ages for the routine administration of currently licensed childhood vaccines are at 3, 5, and 11 months.
The present study evaluated the immunogenicity in the first 6 months of life of a 3-component aP vaccine containing filamentous hemagglutinin (FHA), pertactine (PRN), and genetically detoxified pertussis toxin (PT) in infants immunized at birth, and at 3, 5, and 11 months in comparison with infants immunized at the ages of 3, 5, and 11 months. We also evaluated the influence of maternal antibodies on aP vaccine response.
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METHODS |
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Study Population
Ninety-one full-term healthy newborns (48 males and 43 females) admitted to the neonatal unit of the Policlinico San Matteo, Pavia between January and August 1999 were enrolled in the study within 48 hours of birth, along with their mothers. Inclusion criteria were gestational age between 37 and 42 weeks and an appropriate weight for their gestational age. Perinatal brain damage, major congenital abnormalities, severe illness or infections, or mother known to be positive for the human immunodeficiency virus precluded entry into the study. Forty-five infants were immunized at birth and at 3, 5, and 11 months (group 1), whereas 46 infants were immunized at 3, 5, and 11 months (group 2).
Infant mean gestational age was 39.3 ± 1.4 weeks, and mean birth weight was 3293 ± 440 g.
The mean age of the mothers was 29.8 ± 4.3 years (range: 17–37).
Informed written consent was obtained from at least 1 parent, by the neonatologist who administered the vaccine. The study protocol was approved by the Ethical Committee of IRCCS Policlinico San Matteo, Pavia.
Study Design
This was a randomized, controlled, observer-blind study. Each infant was randomly allocated to 1 of the 2 groups. The randomization list was completed before beginning subject enrollment. Group 1 infants received 0.5 mL of Acelluvax vaccine (Biocine, Emeryville, CA) in thigh muscle on the fourth day of life, whereas group 2 infants were not inoculated. The neonatologist who vaccinated the children at birth was not the same one who followed the infants over the next 24 months. All enrolled infants were then vaccinated according to the Italian schedule with the Acelluvax DTPa (Biocine) vaccine at 3, 5, and 11 months, together with other compulsory vaccinations (anti-hepatitis B virus, oral polio virus/intramuscular polio virus) and anti-Haemophilus influenzae type b. The Acelluvax vaccine contains 3 antigens: 5 µg of PT, 2.5 µg of FHA, and 2.5 µg of PRN. The Acelluvax DTPa also contains 30 IU of diphtheria toxoid and 60 IU of tetanus toxoid.
To measure serum anti-PT, anti-FHA, and anti-PRN antibody concentrations, we drew blood samples from all mothers at delivery, and from all infants at birth (on day 3 at the same time as routine blood investigation) and at 12 months (1 month after the last dose); in addition, there were 3 intermediate blood samplings: at 3 months (before the first dose in group 2 or the second dose in group 1), at 5 months (before the second dose in group 2 or the third in group 1), and at 6 months (1 month after the second dose in group 2 or third dose in group 1). Each infant was randomly assigned to 2 of the blood collections to reduce the number of phlebotomies. Serum samples were stored at -20°C until tested.
Safety
Using diary cards, parents recorded solicited local effects such as pain, redness, and swelling for 3 days following vaccination and solicited general adverse events (fever and rash/exanthema) for 14 days following vaccination. Any redness or swelling was measured, a diameter of >20 mm being defined as severe.
Serologic Testing
Anti-PT, anti-FHA, and anti-PRN immunoglobulin G (IgG) antibody levels were detected by enzyme-linked immunosorbent assay (ELISA) as previously described,15,16 and the reference serum was calibrated against reference serum samples provided by the US Food and Drug Administration (serum lot 3 or 4; Bethesda, MD). The reference-line method17 was used to calculate ELISA units per milliliter (EU/ml). The estimated minimum detection level of the method was 1.5 EU/ml for IgG antibodies to PT, 1 EU/ml for IgG antibodies to FHA and 3 EU/ml for IgG antibodies to PRN. For each antigen, samples from the same subject were run simultaneously on the same plate to avoid assay variability.
Statistical Method
The geometric mean titer (GMT) of anti-PT, anti-FHA and anti-PRN antibodies was calculated using the log-transformation of titers and taking the anti-log of the mean of these transformed values. Comparisons between antibody levels and their changes over time in the 2 groups of infants were made by means of analysis of variance for repeated measures, after log transformation of the original data. Comparisons at each time point (birth, 3, 5, 6, 12 months) were made by means of t test with the Scheffè correction.
Response was defined as a 4-fold increment in prevaccination antibody levels.18 Comparisons between the frequency of infants with at least a 4-fold increment in birth levels in the 2 groups were made by means of 
2.
All the analyses were performed with STATA, P values < .05 were considered statistically significant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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We collected 91 blood samples from infants at birth, 44 samples at 3 months, 42 at 5 months, 44 at 6 months, 83 at 12 months, and 91 blood samples from mothers at delivery. At birth, all infants had anti-PT, anti-FHA, and anti-PRN antibody levels passively acquired from their mothers. No statistically significant differences in anti-PT, anti-FHA, and anti-PRN levels were observed between the mothers at delivery and newborns at birth. The analysis of variance for repeated measurements shows a significant increase in antibody levels for all 3 antigens over time and as the number of doses increases (P = .025 for anti-PRN, P = .006 for anti-FHA, and P = .044 for anti-PT).
At 3 months of age, 10% of the 42 infants vaccinated at birth reached a 4-fold increment in prevaccination antibody levels against the 3 antigens. At 5 months of age the percentage of infants with at least 4 times the prevaccination levels of anti-PT, anti-FHA, and anti-PRN antibodies was significantly higher (P = .0001, P = .04, and P = .01, respectively) in group 1 (who had received 2 doses) than in group 2 (1 dose; Table 1). No statistically significant differences in the percentage of infants with at least a 4-fold increment were observed in the 2 groups at 6 or 12 months of age.
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At 3 months of age no statistically significant differences in the GMT of antibodies between the 2 groups were observed for the 3 antigens. At 5 months the GMTs of anti-PT were significantly higher in group 1 than in group 2 (P = .0006), whereas no difference was observed at 6 months of age (Table 2). On the contrary, at 12 months the anti-PT GMT was significantly higher in group 2 (who had not received the first dose of vaccine at birth) than in group 1 (P < .0001). At 5, 6, and 12 months, GMTs of anti-FHA (Table 2) were significantly higher in group 1 than in group 2 (P < .0001, P < .0001, and P = .002, respectively). At 5 and 6 months of age, GMTs of anti-PRN (Table 2) were significantly higher in group 1 than in group 2 (P = .006 and .009, respectively), although no difference was observed at 12 months of age. We compared the GMT of antibody levels between group 1 at 6 months of age and group 2 at 12 months after an equivalent number of doses: they were similar for anti-FHA and anti-PRN, whereas the anti-PT level was higher in group 2.
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No correlation was observed between maternal antibody levels at delivery and infant antibody levels at 3, 5, and 6 months in both infant groups (r2 = .001 and .22 for anti-PRN, .18 and .01 for anti-FHA, .02 and .002 for PT in group 1 and 2, respectively, at 6 months of age).
No side effects were observed in children of either group.
In a further year of follow-up, no pertussis cases were recorded among all the vaccinated infants.
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DISCUSSION |
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It is well-known that the immune system is partially immature at birth, resulting in a deficiency of cellular- and humoral-mediated immunity: T-cell function is diminished, including T cell-mediated cytotoxicity and T cell help for B cell differentiation.19–22 Therefore, neonatal immunization does not generally lead to rapid antibody responses, but it may result in an efficient immunologic priming which can act as a basis for future responses.23 In a previous study, we demonstrated that in infants immunized at birth with the hepatitis B vaccine, the percentage of seroconversion after the first dose was <10%; however, 1 month after the booster dose, 98% of all infants had anti-hepatitis B surface antigen serum levels >10 mIU/mL.24
Likewise, in the present study we demonstrated that only 10% of infants immunized at birth with the aP vaccine reached a 4-fold increment in prevaccination antibody levels against the 3 components after the first dose. However, at the age of 5 months, after the second dose of vaccine, a 4-fold increase in prevaccination antibody levels against all 3 components was observed in a significantly higher number of infants, whereas only 14% of infants who were not vaccinated at birth showed a 4-fold increase in prevaccination antibody levels. In actual fact, antibody levels after the second dose (administered at 3 months) in infants immunized at birth were similar to the levels achieved at 6 months by infants who had not been immunized at birth, as well as after the third dose (at the age of 6 months in group 1 and 12 months in group 2).
These observations establish that it is possible to induce early protection against pertussis disease by immunization at birth. The ability to develop antibodies earlier than infants vaccinated according to the routine schedule (3–5-11 months) is important in improving disease control in infants under 6 months old.
Controversial data25–27 have been reported regarding the role of maternal antibodies that may potentially transfer a transient protection to the infants, but the same antibodies may also interfere with active immunization. In the present study, we found detectable anti-PT, anti-FHA, and anti-PRN antibody levels in all mothers at delivery, but the rapid induction of antibody levels after the second dose of the aP vaccine in infants immunized at birth suggests that passively acquired maternal antibodies do not interfere with vaccine response. Maternal antibody levels may have been too low to have any suppressive and/or protective effect in our study.
At 12 months of age we observed significantly lower levels of anti-PT antibodies in group 1 compared with group 2. We can speculate that, at 6 months, PT (that could be the most immunogenic antigen) elicited higher affinity antibodies in group 1 (who had received 3 doses) than in group 2 (2 doses). These antibodies could have interfered with the fourth dose of vaccine in group 1. Therefore, we suggest that the fourth dose could be administered later than at 11 months of age (ie, at 15–18 months), as recommended by the immunization schedule approved by the Advisory Committee on Immunization Practices, the American Academy of Pediatrics, and the American Academy of Family Physicians.28
Infants who develop high levels of anti-aP antibodies earlier are less likely to develop clinical disease. However, even if none of the children evaluated in our study contracted pertussis, these data do not establish a correlation between antibody levels against a single vaccine component and the protective efficacy.
We have previously reported on the long-term efficacy of neonatal immunization against hepatitis B in infants born to hepatitis B surface antigen-negative mothers,29 but the long-term effectiveness of an early anti-pertussis vaccination and the necessity for a booster dose require further investigation.
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ACKNOWLEDGMENTS |
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We thank Elisa Lenta and Attilio Ascione for their technical assistance and the professional nurses of the Division of Neonatology: Donatella Rovida, Anna Grasso, and Carla Bardone.
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FOOTNOTES |
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Received for publication May 23, 2002; Accepted Oct 18, 2002.
Reprint requests to (C.B.) Divisione di Neonatologia e Patologia Neonatale-Padiglione Ostetricia, IRCCS Policlinico S. Matteo, Viale Golgi 19, 27100 Pavia, Italy. E-mail cbelloni{at}smatteo.pv.it
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PEDIATRICS (ISSN 1098-4275). ©2003 by the American Academy of Pediatrics
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