Association between season of vaccination and antibody levels against infectious diseases.

Vaccination has reduced the disease burden of vaccine-preventable diseases. However, the extent to which seasonal cycles of immunity could influence vaccine-induced immunity is not well understood. A national cross-sectional serosurveillance study performed in the Netherlands (Pienter-2) yielded data to investigate whether season of vaccination was associated with antibody responses induced by DT-IPV (diphtheria, tetanus and poliomyelitis), MMR (measles, mumps and rubella) and meningococcus C (MenC) vaccines in children. In total, 434 children met the inclusion criteria to study DT-IPV immunity, 811 for MMR and 311 for MenC. Differences in log(antibody levels) by season of vaccination were investigated with linear multivariable regression analyses. Seroconversion rates varied according to season of vaccination for rubella (90% of autumn-vaccinated children vs. 99% of winter-vaccinated had concentrations above cut-off levels). Summer-vaccinated boys showed a slower decline of tetanus antibodies (6% per month), in comparison with winter-vaccinated boys. In conclusion, season of vaccination showed little association with immunological protection. However, a number of associations were seen with a P-value of about 0.03; and adding data from a just-completed nationwide serological study might add more power to the current study. Further immunological and longitudinal investigations could help understand the mechanisms of seasonal influence in vaccine-induced responses.

Antibody responses to Bordetella pertussis and other childhood vaccines in infants born to mothers who received pertussis vaccine in pregnancy - a prospective, observational cohort study from the United Kingdom.

The maternal Tdap (tetanus, diphtheria and acellular pertussis) vaccination programme in the United Kingdom has successfully reduced cases of pertussis in young infants. In addition to prevention of pertussis cases, it is also important to investigate the persistence of maternal antibodies during infancy and the possible interference of maternal antibodies with infant responses to vaccines. We recruited mother-infant pairs from vaccinated and unvaccinated pregnancies and measured concentrations of immunoglobulin (Ig)G against pertussis toxin (PTx), filamentous haemagglutinin (FHA), pertactin (Prn), diphtheria toxin (DTx), tetanus toxoid (TTx) Haemophilus influenzae type b (Hib) and Streptococcus pneumoniae in mothers and infants at birth, and in infants at 7 weeks and at 5 months. Thirty-one mother-infant pairs were tested. Tdap-vaccinated women had significantly higher antibody against Tdap antigens, compared to unvaccinated women (DTx, P = 0·01; PTx, FHA, Prn and TTx, P < 0·001). All antibodies were actively transferred to the infants (transfer ratio  > 1) with higher transfer of DTx (P = 0·04) and TTx (P = 0·02) antibody in Tdap-vaccinated pregnancies compared to unvaccinated pregnancies. Infants from Tdap-vaccinated pregnancies had significantly elevated antibodies to all antigens at birth (P < 0.001) and at 7 weeks (FHA, Prn, TTx, P < 0·001; DTx, P = 0.01; PTx, P = 0·004) compared to infants from unvaccinated pregnancies. Infants from Tdap-vaccinated and -unvaccinated pregnancies had comparable antibody concentrations following primary pertussis immunization (PTx, P = 0·77; FHA, P = 0·58; Prn, P = 0·60; DTx, P = 0·09; TTx, P = 0·88). These results support maternal immunization as a method of protecting vulnerable infants during their first weeks of life.

Long-Term Protection against Diphtheria in the Netherlands after 50 Years of Vaccination: Results from a Seroepidemiological Study.

BACKGROUND AND AIMS: To evaluate the National Immunisation Programme (NIP) a population-based cross-sectional seroepidemiological study was performed in the Netherlands. We assessed diphtheria antitoxin levels in the general Dutch population and in low vaccination coverage (LVC) areas where a relatively high proportion of orthodox Protestants live who decline vaccination based on religious grounds. Results were compared with a nationwide seroepidemiological study performed 11 years earlier. METHODS: In 2006/2007 a national serum bank was established. Blood samples were tested for diphtheria antitoxin IgG concentrations using a multiplex immunoassay for 6383 participants from the national sample (NS) and 1518 participants from LVC municipalities. A cut-off above 0.01 international units per ml (IU/ml) was used as minimum protective level. RESULTS: In the NS 91% of the population had antibody levels above 0.01 IU/ml compared to 88% in the 1995/1996 serosurvey (p<0.05). On average, 82% (vs. 78% in the 1995/1996 serosurvey, p<0.05) of individuals from the NS born before introduction of diphtheria vaccination in the NIP and 46% (vs. 37% in the 1995/1996 serosurvey, p = 0.11) of orthodox Protestants living in LVC areas had antibody levels above 0.01 IU/ml. Linear regression analysis among fully immunized individuals (six vaccinations) without evidence of revaccination indicated a continuous decline in antibodies in both serosurveys, but geometric mean antibodies remained well above 0.01 IU/ml in all age groups. CONCLUSIONS: The NIP provides long-term protection against diphtheria, although antibody levels decline after vaccination. As a result of natural waning immunity, a substantial proportion of individuals born before introduction of diphtheria vaccination in the NIP lack adequate levels of diphtheria antibodies. Susceptibility due to lack of vaccination is highest among strictly orthodox Protestants. The potential risk of spread of diphtheria within the geographically clustered orthodox Protestant community after introduction in the Netherlands has not disappeared, despite national long-term high vaccination coverage.

Immunity against poliomyelitis in the Netherlands, assessed in 2006 to 2007: the importance of completing a vaccination series.

Europe has been declared polio-free since 2002. Here we describe the seroprotection against poliomyelitis in the Dutch population using banked serum samples. Samples from 1,581 inhabitants of eight municipalities with low vaccination coverage (LVC) and an additional 6,386 samples from a nationwide (NS) group (clinical trial number: ISRCTN20164309; collected in 2006­07) were tested for neutralising antibodies (log² reciprocal titres (GMT); non-protection <3) against all three poliomyelitis serotypes. Demographic and epidemiological data were used for statistical regression analysis. Seroprevalence in the NS was 94.6% (type 1), 91.8% (type 2) and 84.0% (type 3). Infants (0­7 months-old) had ≥80% seroprevalence for all serotypes. The highest seroprevalence was found in children, with type 1 and type 2 in five year-olds and type 3 in nine to 10 year-olds. In the LVC group, orthodox protestants, many of whom refuse vaccination, showed seroprevalence rates of 64.9% (type 1), 61.0% (type 2) and 62.1% (type 3). In the NS group, non-Western immigrants and travellers to non-European continents had higher seroprevalences compared to Western immigrants and travellers within Europe, respectively. The Dutch National Immunisation Programme against poliomyelitis has provided good seroprotection, with high and long-lasting GMTs against all serotypes upon completion. The unvaccinated population remains at risk.

High risk of a large measles outbreak despite 30 years of measles vaccination in The Netherlands.

Our aim was to assess progress towards measles elimination from The Netherlands by studying humoral measles immunity in the Dutch population. A population-based seroepidemiological study was conducted in 2006-2007 (N = 7900). Serum samples were analysed by a bead-based multiplex immunoassay. IgG levels ⩾0·2 IU/ml were considered protective. The overall seroprevalence in the Dutch population was 96%. However, 51% of socio-geographically clustered orthodox Protestant individuals aged <10 years were susceptible. Infants might be susceptible to measles between ages 4 months and 14 months, the age at which maternal antibodies have disappeared and the first measles, mumps, rubella (MMR) vaccination is administered, respectively. Waning of antibody concentrations was slower after the second MMR vaccination than after the first. The Netherlands is at an imminent risk of a measles outbreak in the orthodox Protestant minority. To prevent subsequent transmission to the general population, efforts to protect susceptible age groups are needed.

A novel method for evaluating natural and vaccine induced serological responses to Bordetella pertussis antigens.

We studied the time course of serum IgG antibodies against 3 different pertussis vaccine antigens: PT (pertussis toxin), FHA (filamentous hemagglutinin), Prn (pertactin) in sera from individuals vaccinated with four different pertussis vaccines at 4 years of age: (N=44, 44, 23 and 23, respectively,) and compared the responses to/after natural infection with Bordetella pertussis (N=44, age 1-8 years). These longitudinal data were analyzed with a novel method, using a mathematical model to describe the observed responses, and their variation among subjects. This allowed us to estimate biologically meaningful characteristics of the serum antibody response, like peak level and decay rate, and to compare these among natural infections and vaccine responses. Compared to natural infection, responses to PT after vaccination with the tested vaccines are smaller in magnitude and tend to decay slightly faster. When present in vaccines, FHA and Prn tend to produce high peak levels, higher than those in naturally infected patients, but these decay faster. As expected, the Dutch whole cell vaccine produced lower antibody responses than the acellular vaccines. This model allows a better comparison of the kinetics of vaccine induced antibody responses and after natural infection over a long follow up period.

Impaired antibody response to conjugated meningococcal serogroup C vaccine in asplenic patients.

The purpose of this study was to determine the quantity and quality of antibodies against the meningococcal serogroup C (MenC) conjugated vaccine in asplenic patients. In 116 asplenic patients, antibody concentrations (IgG) were measured against meningococcal serogroup C before and after immunisation. Of MenC-specific IgG, both antibody avidity and subclasses of IgG1 and IgG2 were determined. The mean MenC IgG concentration rose from 0.16 µg/mL prior to vaccination to 3.69 µg/mL 3 weeks post-vaccination, with 67% of patients reaching the threshold of ≥ 2.0 µg/mL. The mean IgG concentration at 35 weeks post-vaccination was 3.10 µg/mL. IgG2 concentrations increased more than IgG1. Marginal avidity maturation was seen. Hypo-responders to the first MenC vaccine (IgG anti-MenC ≤ 2.0 µg/mL) were offered a booster dose. After revaccination, 59% reached the chosen IgG threshold. The IgG concentration rose from 0.29 to 1.12 µg/mL, with an increase in the IgG1/IgG2 ratio. Avidity indices remained below 33%. In asplenic patients, the quantity and quality of antibodies produced after one dose of conjugated MenC vaccination is lower than that observed in previous studies in healthy adults. Booster vaccination does, indeed, lead to a rise in IgG geometric mean concentrations (GMCs), but does not lead to higher avidity of antibodies.

Transplacental transport of IgG antibodies to preterm infants: a review of the literature.

Newborn infants, especially preterm infants, have an immature immune system, which is not capable to actively protect against vaccine-preventable infections. Therefore, the newborn is dependent on transplacental transport of Immunoglobulin G (IgG), an active, FcRn receptor mediated process. Fetal IgG rises from approximately 10% of the maternal concentration at 17-22weeks of gestation to 50% at 28-32weeks of gestation. If transplacental acquired IgG is lower in preterm than in term infants, preterm infants are especially at risk for these vaccine-preventable diseases. The aim of this study was to review the transplacental transfer of IgG against vaccine-preventable diseases (measles, rubella, varicella-zoster, mumps, Haemophilus influenza type B, diphtheria, tetanus, pertussis and polio) to (pre)term infants and to identify factors that influence the transplacental transfer of these antigens. After selection, 18 studies on transplacental transport to preterm infants were included. In general, these studies showed for all antibodies that preterm infants have lower antibody concentrations compared with term infants. Maternal and infants antibody concentrations showed a strong correlation in 7 of the included studies. Infant antibody concentration was not associated with parity, maternal age, height or weight. Infants of vaccinated mothers had lower anti-measles antibody titers than infants of natural immunized mothers. IgG titers of preterm infants decrease earlier in life below protective antibody titers than term infants. Combined with their immature immune system, this puts preterm infants at increased risk for vaccine-preventable diseases.

Seroprevalence of IgG antibodies against 13 vaccine Streptococcus pneumoniae serotypes in the Netherlands.

In this study the seroprevalence of IgG antibodies against 13 vaccine serotypes of the pneumococcus was assessed in the Netherlands. Sera from 7904 persons obtained in a cross-sectional population-based study were analysed. The 13 serotype specific IgG concentrations were assessed simultaneously using a fluorescent bead-based multiplex immuno assay (MIA). Overall, the geometric mean IgG concentrations (GMCs) against the 13 serotypes in unvaccinated individuals increased with age up to 5 years and remained at a plateau thereafter. The data also show that individuals develop antibodies against an increasing number of different serotypes with increasing age. The highest GMCs were found for antibodies directed against serotype 14 and 19F, whereas antibodies against serotypes 4 and 5 had the lowest GMCs. There was no uniform relationship between the occurrence of serotypes causing invasive pneumococcal disease (IPD) and the GMCs against these serotypes. Increased IPD incidence in the elderly did not seem to be the result of a decline in the concentration of IgG antibodies.

High tetanus antitoxin antibody concentrations in the Netherlands: a seroepidemiological study.

We assessed the level and determinants of tetanus-antitoxin (TT)-antibodies in the Dutch population. Additionally, we evaluated the national guidelines for post-exposure prophylaxis. Serum samples and questionnaire data from a cross-sectional, population-based study were obtained from 7903 individuals. Serum antitoxin antibodies were assessed with a multiplex immunoassay. Multivariable linear regression was used to explore factors associated with antibody concentration. The overall seroprevalence was 94% with a geometric mean concentration (GMC) of 0.91 IU/ml. The TT-GMC increased with age in the age-cohorts of 13-23 years, which coincides with the meningococcal C conjugate mass-vaccination in 2002. Lower seroprevalences were found in individuals born before introduction of routine vaccination, first-generation migrants from non-Western countries born before 1984, and conservative Protestants living in the Dutch 'Bible belt'. Only 10% of those eligible for post-exposure prophylaxis were not sufficiently protected against tetanus.

Second national serum bank for population-based seroprevalence studies in the Netherlands.

In 2006/2007 a large serum bank was established by means of a cross-sectional population-based study. This serum bank will be used to evaluate the Dutch national immunisation programme (NIP) by serosurveillance and additional immunological and epidemiological research. In this paper we describe the design of this population-based cross-sectional serosurvey and report the participation rates as well as general characteristics of the study population. A similar serum bank was collected in 1995/1996. Dutch inhabitants (aged 0-79 years, men and women) were invited from 40 municipalities throughout the country and also from eight additional municipalities known with low vaccination coverage (LVC). An oversampling of the migrant population was performed. Blood samples were obtained from all participants accompanied with extensive information on demographic and epidemiological data, such as vaccination history, risk factors and travelling. In addition, sociodemographic data are available from individuals who declined to participate (non-response survey). Overall 33% of all invitees were included in this study. The serum bank comprises 6386 sera in the nationwide sample including the extra sample of immigrants (n=646) and 1518 sera from the LVC municipalities. The sera will be analysed for antibodies against all NI P antigens but will also be used for other infectious diseases research. Results of this second serosurveillance study will contribute to the discussion whether it is needed to reconsider the schedule and/or the vaccine components of the current National Immunisation Programme.

[Vaccination against pneumococci and hepatitis B in the Dutch National Immunisation Programme]. / Pneumokokken- en hepatitis B-vaccinatie in het Rijksvaccinatieprogramma.

All infants in the Netherlands, which are born after March 2006, receive additional vaccinations at the age of 2, 3, 4 and 11 months to protect them against pneumococcal infections. During the same visit to a consultation bureau, the children also receive a combination vaccine against diphtheria, pertussis, tetanus, poliomyelitis and Haemophilus influenzae (DTPa-IPV-Hib). Children of which at least one parent was born in a country where hepatitis B occurs relatively often are also vaccinated in the Netherlands against hepatitis B. This currently pertains to about 15% of all newborns. These children now receive a new combination vaccine in which a hepatitis B component has been added to the DTPa-IPV-Hib components. They will receive this combination vaccine 4 times. This combination vaccine is given during the same visit as the pneumococcal vaccination. Although pneumococcal vaccination may have a somewhat negative effect on the immune response to hepatitis B, it is expected that the new 4-fold vaccination schedule will induce good and long-lasting protection against hepatitis B in the vast majority of the children. About 700 children are born out of mothers infected with hepatitis B each year in the Netherlands. In the new vaccination schedule, they now receive 5 active vaccinations against hepatitis B and are examined serologically on an individual basis in order to detect breakthrough infections. This will also generate greater insight into the efficacy of the different vaccination schemes and intervention programmes to prevent vertical transmission of the virus.

Evaluation of diphtheria convalescent patients to serve as donors for the production of anti-diphtheria immunoglobulin preparations.

AIMS: The study was conducted to evaluate the possibility of selecting convalescent diphtheria patients to serve in emergency situations as donors for the production of anti-diphtheria immunoglobulin. To select suitable donors, the criterion of an antitoxin titer >/=3.0 IU/ml was used. In addition, the effects of treatment and the effect of immunization with diphtheria toxoid on the level of anti-diphtheria toxin antibodies were evaluated. SCOPE: Three groups of diphtheria patients were included in the study. The first group (n = 23) consisted of patients who had a basic antibiotic treatment, with or without serotherapy using horse antitoxin and/or human immunoglobulin. The second group (n = 12) comprised patients examined immediately after the onset of disease. The immunological history of this group was not known. The third group (n = 20) included patients with a known immunization history, treated only with antibiotics but having received a booster immunization with diphtheria toxoid. Antitoxin titers were measured using the toxin binding inhibition (ToBI) assay. CONCLUSIONS: In the first group, 47.8% (11/23) of the patients had a diphtheria antibody titer >/=3.0 IU/ml. For most of them, however, the antibody titers could have resulted from treatment with exogenous antibodies from horse antitoxin or human immunoglobulin (18/23). Only two of the 11 high-titer subjects had received antibiotics only. Among the second group, only two (16.76%) of the patients had an antibody titer of >/=3.0 IU/ml. In the third group 50% (10/20) of the patients showed an antibody titer of >/=3.0 IU/ml prior to vaccination, and therefore could be directly considered as donors. Three weeks after booster vaccination, 70% (14/20) had an antibody titer of >/=3.0 IU/ml and 1 year after booster vaccination, 28.6% (2/7) of the subjects still had titers of >/=3.0 IU/ml. In 40% of these patients, a decrease was observed 3-4 weeks after the booster dose. It was concluded that convalescent diphtheria patients could be considered as donors in an emergency situation, since approximately half of them showed antitoxin titers of >/=3.0 IU/ml.

Immunodominant domains of the Measles virus hemagglutinin protein eliciting a neutralizing human B cell response.

The most important neutralizing and protective antibodies against Measles virus (MeV) are directed against the hemagglutinin protein (MeV-H). To define the MeV binding domains recognized by human antibodies a set of 10 non-redundant MeV-H-specific monoclonal antibodies (mabs) was used to block their binding in a competition ELISA. Sera from both naturally infected and vaccinated individuals showed similar competition patterns. Two distinct domains were identified as the main target of human antibodies. One domain corresponded to the region of the previously described hemagglutinin noose epitope (HNE, aa 380-400) [35], which is recognized by hemagglutination-inhibiting, neutralizing and protective mabs. The second region is defined by a mab with strong neutralizing but weak hemagglutination-inhibiting activity. Mabs with a strong neutralizing capacity with respect to wild-type viruses seemed to displace more human antibodies than those with a weaker neutralizing activity. Human antibodies seem to react more weakly with the hemagglutinin regions that bind the CD46 and the fusion protein and more strongly with the putative CD150 binding site and the top loops of beta-sheet 2 and 3 of the hemagglutinin.

Antibodies against mumps in The Netherlands as assessed by indirect ELISA and virus neutralization assay.

To obtain insight into mumps immunity 9 years after introduction of vaccination in The Netherlands, antibodies were measured in a national sample (n = 8298) and in clustered religious groups with low vaccine acceptance (n = 254). All sera were tested by indirect ELISA, and agreement with neutralization assay was assessed in a subsample (n = 623). Overall seroprevalence in the adult age groups in the national sample was 96.2% (95% confidence interval 95.4-97.0%). Seroprevalence was somewhat lower in the vaccinated age groups, but still sufficient to maintain herd immunity. After the first dose of vaccine, an increase up to age three years to 93.2% (89.8-96.6%) and a subsequent decline in prevalence to 88.9% (81.7-96.0%) at age 7 years was seen. Seroprevalence in those eligible for two vaccinations was 94.4% (91.3-97.4%). In the religious groups, seroprevalence was generally lower in the age group 1-4 years (30% (18-95%)) than in the national sample, but similar in the older age groups. Seroprevalence as estimated by neutralization assay was only slightly lower for all age groups > or = 1 year. Therefore, the indirect ELISA is a reliable method for measuring mumps virus-specific antibodies in population-based studies. However, to allow for inter-laboratory comparison, international unitage should be developed.

Evaluation of the national immunisation programme in the Netherlands: immunity to diphtheria, tetanus, poliomyelitis, measles, mumps, rubella and Haemophilus influenzae type b.

The immunity to vaccine-preventable diseases included in the Dutch immunisation programme in the general population and among orthodox reformed individuals who refuse vaccination was assessed. The programme induces good protection. However, a large proportion of adults lacks diphtheria and tetanus immunity. Measles, mumps and rubella seroprevalence was somewhat lower among vaccinated compared to unvaccinated cohorts. The prevalence of HibPS antibodies declined during 2.5 years after the fourth vaccination. However, protection occurs also by memory immunity. Herd immunity is sufficient among the general population, but not among orthodox reformed individuals. Immunosurveillance is an efficient way to evaluate the effects of immunisation programmes and identify risk groups for infection.

Prevention of meningococcal serogroup B infections in children: a protein-based vaccine induces immunologic memory.

Immunologic memory against meningococci was studied in 177 children (100 children were 10-11 years old and 77 were 5-6 years old) 2.5 years after vaccination with hexavalent meningococcal outer membrane vesicle (OMV) vaccine or hepatitis B (HepB) vaccine. Children were revaccinated with monovalent P1.7(h),4 meningococcal OMV vaccine. Serum bactericidal antibodies (SBAs) were measured before revaccination and after 4-6 weeks. A minimum 4-fold increase in SBAs against serosubtype P1.7(h),4 was detected in 48.5% of the children after hexavalent meningococcal vaccine and in 8.9% after HepB vaccine. Of the initial responders given hexavalent meningococcal vaccine, 78% had > or =4-fold increase in SBAs against strain P1.4. Thus, immunologic memory is present in toddlers and school-aged children previously given 3 hexavalent meningococcal vaccinations. Booster vaccination with monovalent P1.7(h),4 meningococcal OMV vaccine induces a significant increase in SBAs against serosubtype P1.7(h),4 and cross-reactivity against other serosubtypes in the hexavalent vaccine.

Antibodies to Haemophilus influenzae serotype b in the Netherlands a few years after the introduction of routine vaccination.

We assessed antibodies to the capsular polysaccharide of Haemophilus influenzae type b (HibPS) in the Dutch population a few years after a mass vaccination against H. influenzae (Hib) was begun. We observed sharp declines in the geometric mean titer (GMT) and the prevalence of HibPS antibodies at levels of < or =0.15 microg/mL in children who had received 4 doses of vaccine: from 8.65 microg/mL (prevalence, 99.4%) after 0-2 months to 0.8 microg/mL (prevalence, 83.3%) after 27-29 months. In adult groups, both the prevalence of HibPS antibodies and the GMT declined significantly with increasing age but remained high (prevalence, > or =83.7%; GMT, 0.73 > or = microg/mL). We conclude that the overall immunity in the Dutch population seems satisfactory. We draw our conclusions from the current serosurveillance study and from the sharp decline in invasive Hib disease noted after the introduction of vaccination. The key questions for the future are (1) whether Hib and cross-reacting organisms will circulate sufficiently to provide natural reexposure, and (2) how long memory immunity will persist after vaccination without reexposure.

Specificity and sensitivity of high levels of immunoglobulin G antibodies against pertussis toxin in a single serum sample for diagnosis of infection with Bordetella pertussis.

Laboratory confirmation of pertussis by culture, PCR, or detection of antibody increase in paired sera is hampered by low sensitivity in the later stages of the disease. Therefore, we investigated whether, and at which level, concentrations of immunoglobulin G (IgG) antibodies against pertussis toxin (PT), IgG-PT, in a single serum sample are indicative of active or recent pertussis. IgG-PT, measured by enzyme-linked immunosorbent assay in units per milliliter, was analyzed in 7,756 sera collected in a population-based study in The Netherlands, in the sera of 3,491 patients with at least a fourfold increase of IgG-PT, in paired sera of 89 patients with positive cultures and/or PCR results, and in the sera of 57 patients with clinically documented pertussis with a median follow-up of 1.4 years. We conclude that, independently of age, IgG-PT levels of at least 100 U/ml are diagnostic of recent or active infection with Bordetella pertussis. Such levels are present in less than 1% of the population and are reached in most pertussis patients within 4 weeks after disease onset and persist only temporarily.

Immunogenicity and safety of monovalent p1.7(h),4 meningococcal outer membrane vesicle vaccine in toddlers: comparison of two vaccination schedules and two vaccine formulations.

The safety and immunogenicity of two PorA-based meningococcal outer membrane vesicle (OMV) vaccines against the P1.4 serosubtype adsorbed with AlPO(4) or Al(OH)(3) were studied in 134 toddlers. Vaccinations were given three times with an interval of 3-6 weeks or twice with an interval of 6-10 weeks. A vaccination was repeated after 20-40 weeks. Pre- and post-immunization sera were tested for bactericidal activity against an isogenic strain expressing P1.7(h), 4 PorA. Both meningococcal OMV vaccines were well tolerated. The percentage of children with a fourfold increase in bactericidal activity was 96% (AlPO(4)-adjuvated vaccine/2+1 schedule), 100% (AlPO(4)-adjuvated vaccine/3+1 schedule), 93% (Al(OH)(3)-adjuvated vaccine/2+1 schedule) and 97% (Al(OH)(3)-adjuvated vaccine/3+1 schedule). Adsorption with AlPO(4) makes the OMV vaccine more immunogenic than adsorption with Al(OH)(3). Bactericidal activity was highest after the 3+1 schedule, although the response shortly after the primary series was higher in the two-dose priming group.