Search tips
Search criteria 


Logo of cviPermissionsJournals.ASM.orgJournalAEM ArticleJournal InfoAuthorsReviewers
Clin Vaccine Immunol. Jul 2012; 19(7): 999–1004.
PMCID: PMC3393373
Immunogenicity and Safety of a Meningococcal Quadrivalent Conjugate Vaccine in Saudi Arabian Adolescents Previously Vaccinated with One Dose of Bivalent and Quadrivalent Meningococcal Polysaccharide Vaccines: a Phase III, Controlled, Randomized, and Modified Blind-Observer Study
Yagob Al-Mazrou,a Mohamed Khalil,a Helen Findlow,corresponding authorb Helen Chadha,b Valerie Bosch Castells,c David R. Johnson,d and Ray Borrowbe
aMinistry of Health, Riyadh, Saudi Arabia
bVaccine Evaluation Unit, Health Protection Agency, Manchester Royal Infirmary, Manchester, United Kingdom
cSanofi Pasteur, Marcy l'Etoile, France
dSanofi Pasteur, Lyon, France
eUniversity of Manchester, Inflammation Sciences Research Group, School of Translational Medicine, Stopford Building, Manchester, United Kingdom
corresponding authorCorresponding author.
Address correspondence to Helen Findlow, helen.findlow/at/
Received February 6, 2012; Revisions requested March 2, 2012; Accepted April 26, 2012.
Reduced immune responses to repeated polysaccharide vaccination have been previously reported, but there are limited immunogenicity data on the use of meningococcal polysaccharide vaccine (PSV) followed by meningococcal conjugate vaccine. Saudi Arabian adolescents (aged 16 to 19 years) who had previously been vaccinated with ≥1 dose of bivalent meningococcal polysaccharide vaccine and 1 dose of quadrivalent meningococcal polysaccharide (MPSV4) were enrolled in a controlled, randomized, and modified observer-blind study (collectively termed the PSV-exposed group). The PSV-exposed group was randomized to receive either quadrivalent meningococcal conjugate vaccine (MCV4) (n = 145 PSV-exposed/MCV4 group) or MPSV4 (n = 142 PSV-exposed/MPSV4 group), and a PSV-naïve group received MCV4 (n = 163). Serum samples collected prevaccination and 28 days postvaccination were measured by baby rabbit serum bactericidal antibody (rSBA) assay, and vaccine tolerability and safety were also evaluated. For each serogroup, the postvaccination geometric mean titers (GMTs) were significantly higher in the PSV-naïve group than in either group comprised of the PSV-exposed participants. The postvaccination serogroup C rSBA GMT was significantly higher in the PSV-MCV4 group than in the PSV-MPSV4 group after adjusting for prevaccination GMTs. Although not statistically significant, similar differences were observed for serogroups A, Y, and W-135. No worrisome safety signals were detected. This study demonstrated MCV4 to be safe and immunogenic in those who had previously received polysaccharide vaccination, and it suggests that conjugate vaccine can partially compensate for the hyporesponsiveness seen with repeated doses of polysaccharide vaccine.
Meningococcal disease remains a serious public health issue in the Kingdom of Saudi Arabia (KSA), with epidemics historically occurring due to serogroup A. KSA uniquely experiences a yearly influx of international visitors to perform Hajj and Umra, and meningococcal disease epidemics have occurred during the Hajj (6). Many pilgrims and visitors originate from areas where invasive meningococcal disease is endemic, increasing the risk of disease in the KSA. Prior to 2000, Saudi Arabian authorities required pilgrims attending either Hajj or Umra to be vaccinated with the bivalent meningococcal A/C polysaccharide vaccine (6). Between 2000 and 2002, a shift from epidemics arising from serogroup A to serogroup W-135 was observed (1, 5). In response, the Saudi Ministry of Health recommended the use of a quadrivalent meningococcal A/C/Y/W-135 polysaccharide vaccine (MPSV4) to provide coverage against serogroup W-135 disease for pilgrims and Saudi school children (15). An MPSV4 campaign in 2003 targeted children aged 6 months to 5 years old, with those <2 years of age receiving two doses 2 months apart and children aged ≥2 years receiving one dose. A significant increase in the proportion of participants with antibody titers at a level thought to correlate with disease protection (≥8) as measured by baby rabbit serum bactericidal antibody (rSBA) assay was observed only in those aged ≥2 years (9).
A quadrivalent meningococcal diphtheria toxoid conjugate vaccine, MCV4, containing conjugated polysaccharides from serogroups A, C, Y, and W-135, was licensed for use in 2005 by the U.S. FDA (3),d in 2006 in Canada (13), and more recently in other countries and the Gulf Cooperation Council ( Compared to quadrivalent polysaccharide vaccines, quadrivalent conjugate vaccines have potential immunologic superiority and retain broad serogroup coverage, which makes them an attractive option for the prevention of meningococcal disease in KSA. There is concern, however, over potential immunological hyporesponsiveness due to the prior administration of multiple doses of meningococcal polysaccharide vaccine in children. We report here on the immune response to MCV4 compared to the response to MPSV4 in adolescents who have previously received one dose of MPSV4 and at least one dose of a bivalent A/C polysaccharide.
(This work was presented in part at the 10th European Monitoring Group for Meningococci Meeting, Manchester, United Kingdom, 17 to 19 June 2009.)
Trial design.
This phase III, controlled, randomized, and modified blind-observer study was conducted at seven sites located in two regions within the KSA. Study conduct was according to the Edinburgh revision of the International Conference on Harmonization (ICH) guidelines and the European Directive 2001/20/EC. The final study protocol was approved by the Medical Research Ethics Committee of the Ministry of Health of the KSA (an independent ethics committee) before study initiation. Good clinical practice and all applicable national and local regulations were observed throughout the study. Written informed consent was obtained from the participant's parent or legal representative if the participants were <18 years of age or from the participant if he or she was ≥18 years of age.
A single 0.5-ml dose of MCV4 vaccine containing 4 μg of each serogroup A, C, Y, and W-135 purified polysaccharide conjugated to 48 μg of diphtheria toxoid carrier protein (Menactra; Sanofi Pasteur, Swiftwater, PA) was administered intramuscularly, or a single 0.5-ml dose of MPSV4 containing 50 μg of each serogroup A, C, Y, and W-135 polysaccharide (Mencevax ACWY; GlaxoSmithKline, Belgium) was administered subcutaneously. Although both study vaccines were injected in the deltoid region, the administration routes for MCV4 and MPSV4 are not identical, with MCV4 administered intramuscularly and MSPV4 subcutaneously, and the study was not strictly double blinded. To reduce bias, the person assessing safety was different from the person administering the vaccine. Approximately 5 ml of blood was collected from participants at two time points, one immediately before vaccination and one 28 days (window, 21 to 35 days) postvaccination.
Study population.
Participants were healthy adolescents who were 16 to 19 years of age at study inclusion. Two cohorts were enrolled. In one cohort were those who had received ≥1 dose of bivalent (serogroups A and C) meningococcal polysaccharide vaccine and 1 dose of MPSV4 but had received no additional meningococcal vaccine less than 2 years prior to enrolment. For convenience, we omit the “M” that ordinarily would denote a meningococcal vaccine; we refer to these participants as simply the polysaccharide vaccine (PSV)-exposed participants. A second cohort consisted of individuals completely naïve to meningococcal vaccination (PSV-naïve participants). The PSV-exposed group was given study-related vaccination of either MCV4 (PSV-exposed/MCV4 group) or MPSV4 (PSV-exposed/MPSV4 group) according to a blinded, scratchable randomization list provided by the study sponsor. Adolescents who never received any meningococcal vaccinations received 1 dose of MCV4 (PSV-naïve/MCV4 group).
Key exclusion criteria included serious chronic disease, known or suspected impairment of immunologic function, acute illness with or without fever ≤72 h before or an oral temperature ≥37.5°C at study entry, or the use of antibiotics ≤72 h before vaccination. All other exclusion criteria are available at the website (14).
Serogroup-specific antibody responses were determined by rSBA assay using baby rabbit complement (Pel-Freez Incorporated, Rodgerson, AZ) as described elsewhere (12) as an exogenous complement source. SBA assays were performed at the Vaccine Evaluation Unit, Health Protection Agency, Manchester, United Kingdom. The following strains were used (name, strain, standardized strain characterization in parentheses): MenA, F8238 (A:4:P1.20,9); MenC, C11 (C:16:P1.7-1,1); MenW135, M,01.240070 (W135:NT:P1.18-1,3); and MenY, M,03.241125 (also known as S1975) (Y:2a:P1.5,2). rSBA titers were expressed as the reciprocal of the final serum dilution yielding ≥50% killing at 60 min. For computational purposes, titers of <4 were assigned a value of 2.
Investigators observed participants for 30 min after vaccination to record and treat any immediate adverse events (AEs). On the day of vaccination and for the next 7 days, participants recorded reactogenicity data, including oral temperature, in a diary. Local and systemic adverse reactions were recorded and graded as follows. Injection-site swelling and erythema were recorded as grade 1, <2.5 cm; grade 2, ≥2.5 and <5 cm; and grade 3, ≥5 cm. Injection-site pain was recorded as the following: grade 1, easily tolerated; grade 2, discomfort interferes with normal activities; and grade 3, unable to perform normal activities. Fever was recorded as grade 1 (≥37.5 and ≤38.0°C), grade 2 (>38.0 and ≤39.0°C), and grade 3 (>39.0). Headache, malaise, and myalgia were recorded as the following: grade 1, noticeable discomfort but does not interfere with daily activities; grade 2, interferes with daily activities; grade 3, prevents daily activities. For each reaction, the percentage of participants with solicited adverse reactions and their respective two-sided 95% confidence intervals (CIs) were calculated by each reaction and study group. Adverse events and serious adverse events were evaluated and recorded for the entire study period.
Statistical analysis.
For the analysis of immunogenicity data, two study populations were defined. The per-protocol set contained participants who satisfied all protocol-defined inclusion/exclusion criteria, were randomized correctly, received a dose of study vaccine, had blood drawn during the proper time window, and did not receive prohibited therapy while in the study. The full analysis set contained participants who received a single dose of study vaccine, but if the participant was incorrectly randomized, her/his data were analyzed according to the randomization scheme. The safety analysis set contained participants who received a single dose of study vaccine, but if the participant was incorrectly randomized, her/his data were analyzed according to the dose received.
The sample size was determined on the assumption there would be a dropout rate of 30%; therefore, 429 participants would be required (143 participants per group) to have ≥100 evaluable participants per group. Statistical analyses were performed by using SAS version 8.2 or higher (SAS Institute, Cary, NC). Immunogenicity outcomes were determined for each serogroup and each study group pre- and postvaccination as previously defined. At each time point, rSBA geometric mean titers (GMTs) with 95% CIs and the percentage of participants achieving a ≥4-fold increase in rSBA titers from pre- to postvaccination were calculated (with 95% CIs). Comparisons between the proportion of participants with a ≥4-fold rise in rSBA titers from prevaccination to 28 days postvaccination in the three groups were performed using Fisher's exact test. Comparisons between the pre- and postvaccination GMTs for each serogroup and each study group were performed using an analysis of variance (ANOVA) on the log2 result of the titer. For each ANOVA comparison, the significance level was 0.05, and when a statistical difference was noted, a multiple comparison was performed. The hypothesis that the PSV-MCV4 group's postvaccination serogroup C GMT was superior to that of the PSV-MPSV4 group was supported if the lower limit of the 95% CI of the ratio of these GMTs was >1.
The study (recruitment through postvaccination follow-up visits) was conducted between 12 February 2007 and 28 July 2007. The flow of participants through the study and the rationale for excluding some participant data from subsequent analysis are presented in Table 1. All immunogenicity data presented herein are based on the full analysis set; the per-protocol set analysis yielded similar results. A total of 450 participants were enrolled and were allocated to 1 of the 3 study groups at the following levels: PSV-exposed/MCV4 group, 145 participants; PSV-exposed/MPSV4 group, 142; and PSV-naive group, 163 (Table 2). The mean age of the participants was 17.84 ± 1.16 years (range, 16 to 19.9 years). Approximately 45% of the PSV-exposed/MCV4 and PSV-exposed/MPSV4 groups were male, but in the PS-naive group 68% were male.
Table 1
Table 1
Participant disposition
Table 2
Table 2
Participant demographicsa
Comparing the PSV-exposed to the PSV-naïve participants, a significant difference was observed between the prevaccination rSBA GMTs for serogroups A (P = 0.002), Y (P < 0.001), and W-135 (P = 0.001), with higher GMTs in the PSV-exposed groups. For serogroup C, the prevaccination GMTs were similar regardless of meningococcal vaccination history (Table 3).
Table 3
Table 3
Serogroup A, C, Y, and W-135 rSBA GMTs and percentages of participants with a ≥4-fold rise in rSBA titer prior to and 28 days after vaccinationa
The postvaccination rSBA GMTs increased for all 4 serogroups in all study groups (Table 3). The rSBA GMTs of the PSV-naïve group were significantly higher than those of the PSV-exposed/MCV4 group (serogroups A, C, and W-135, P < 0.001; serogroup Y, P = 0.007) and the PSV-exposed/MPSV4 group (serogroups A, C, Y, and W-135, P < 0.001). Although the PSV-exposed/MCV4 participants consistently had higher postvaccination GMTs across all 4 serogroups than the PSV-exposed/MPSV4 group, these differences were not statistically significant (P = 0.538, 0.061, 0.068, and 0.065 for serogroups A, C, Y, and W-135, respectively).
The percentage of participants with a ≥4-fold increase in rSBA titer from pre- to postvaccination was significantly lower (P < 0.001) for those who had been PSV exposed versus those who were naïve for each of the serogroups. The number of participants with a 4-fold rise in rate was greater in the PSV-exposed group who received MCV4 than in those who received MPSV4 for each of the serogroups (Table 3).
Greater than 90.5% of participants had serogroup A prevaccination rSBA titers of ≥128, rising to 100% postvaccination (Fig. 1). For serogroup C, the percentage of participants prevaccination with the titer categories of <8, 8 to 16, and ≥128 were similar between study groups. After vaccination, 94.4% of participants in the PSV-naïve group had serogroup C rSBA titers of ≥8, whereas in the PSV-exposed participants the percentages with SBA titers of ≥8 were lower, 77.9% (PSV-exposed/MPSV4 group) and 85.7% (PSV-exposed/MCV4 group). For serogroups Y and W-135, the percentages of participants in the 2 PSV-exposed groups with prevaccination rSBA titers of <8, 8 to 64, and ≥128 were similar, while a lower percentage of PS-naïve participants had rSBA titers of ≥8. After vaccination, ≥91.8% of all participants had serogroup W-135 rSBA titers of ≥8, and ≥96.3% had serogroup Y titers of ≥8.
Fig 1
Fig 1
Percentage of participants achieving baby rabbit serum bactericidal antibody (rSBA) titer thresholds of <8 (black histogram segments), 8 to 64 (gray segments), and ≥128 (white segments) by study arm. rSBA activity was assayed before receiving (more ...)
For serogroup C, the superiority of MCV4 over MPSV4 in those who were PSV exposed was assessed postvaccination. The ratio of the PSV-exposed/MCV4 to PSV-exposed/MPSV4 rSBA GMTs was 1.82 (95% CI, 0.98 to 3.39). Since the lower bound of this ratio's CI was <1, superiority was not demonstrated. However, when the postvaccination serogroup C GMTs were adjusted for prevaccination titers, the GMT ratio was 1.93 (95% CI, 1.10 to 3.37) and the superiority standard was achieved.
Safety and reactogenicity.
No participants given MCV4 experienced an immediate reaction after vaccination, but 1 in the PSV-MPSV4 group did experience an immediate reaction. This participant presented with muscle cramps of moderate intensity; the episode persisted for 3 days, resolved spontaneously, and was considered by the investigator to be unrelated to vaccine administration.
Injection-site reactions occurred between days 0 and 3 in a majority of participants. Most participants reporting these reactions rated them as grade 1, with most resolving by the third day after vaccination. The most commonly reported postvaccination systemic events were headache, malaise, and myalgia (Table 4). These systemic reactions were also most often rated as grade 1, and the majority were resolved by the third day after vaccination. Within each vaccine group there were participants for whom systemic reactions lasted more than 7 days. The frequencies of reports of each reaction type for each study group were similar and had overlapping 95% CIs (Table 4).
Table 4
Table 4
Participants with ≥1 solicited injection-site and systemic reactions 7 days after MCV4 or MPSV4 vaccinationa
Unsolicited AEs were reported at low and approximately equal rates across the study groups. The most frequently reported unsolicited AEs were infections. Unsolicited AEs were of mild or moderate intensity, with the exception of one serious AE in the PSV-exposed/MCV4 group. This participant developed a perianal abscess 19 days after vaccination, requiring hospital admission and treatment. The investigator judged the incident to be unrelated to study participation. No other serious AEs, no AE-related premature study terminations, and no deaths occurred.
To our knowledge, this is the first report on the immune response to a quadrivalent A, C, Y, and W-135 conjugate vaccination in adolescents who had previously been vaccinated with bivalent A/C polysaccharide vaccine and an MPSV4 vaccine. This study has demonstrated that MCV4 induced robust immune responses in those who had received prior meningococcal polysaccharide vaccines and yielded no unusual safety signals.
Prior to vaccination, serogroup A rSBA GMTs in all 3 study groups were high compared to those of the other serogroups, which could be due to natural priming from the prior circulation of serogroup A meningococci in KSA. Similarly to serogroup A, there were no differences in the serogroup C rSBA GMTs of the three groups prevaccination. Higher serogroup C rSBA titers may have been expected in the PSV-exposed groups, but serogroup C rSBA titers have been shown to decline rapidly following polysaccharide vaccination (4). For serogroups W-135 and Y, the prevaccination rSBA GMTs were lower in the PSV-naïve group, as expected.
Repeat doses of meningococcal polysaccharide vaccine can induce hyporesponsiveness, whereby persons previously vaccinated with meningococcal polysaccharide vaccine mount an immune response to revaccination with polysaccharide or conjugate vaccine that is lower in magnitude than the immune response of persons of a similar age being immunized for the first time (2, 7, 10). Although the clinical significance of such hyporesponsiveness is not known, most experts would prefer to avoid it if is possible to do so with relative ease. Hyporesponsiveness was evident in this study for all four serogroups, as the rSBA GMTs after MCV4 vaccination in those who had received prior meningococcal polysaccharide vaccination were significantly lower than those of the PSV-naïve group. These results are consistent with previously reported data for serogroup C (8, 11). Further, Keyserling et al. (8) reported lower SBA GMTs after MCV4 in U.S. adolescents who had received MPSV4 vaccination 3 years earlier, which is in agreement with the observations in this study.
This study demonstrated reduced rSBA responses to serogroup A after MCV4 administration in PS-exposed participants. Previous reports examining the response of multiple doses of serogroup A polysaccharide vaccine are conflicting. Several studies have demonstrated a boosting effect to repeated doses of serogroup A polysaccharide vaccine (4, 7, 9), whereas reduced responses have also been reported for this serogroup as well (2, 10). Lakshman et al. (10) assessed immune responses to bivalent A/C polysaccharide or conjugate vaccine in young adults, and serogroup A hyporesponsiveness was seen after two doses of polysaccharide vaccine, but rSBA response to conjugate vaccine after a prior polysaccharide vaccine was not reduced compared to responses in naïve participants. The converse was seen in this study. A key difference between our study and that by Lakshman et al. (10) is the mean time between MPSV4 vaccination, 38 months as opposed to 52 to 60 weeks, respectively.
In the PSV-exposed participants after either MCV4 or MPSV4 vaccination, no statistically significant differences in the postvaccination rSBA GMTs were observed for any of the 4 serogroups. However, there were consistently higher rSBA GMTs in those vaccinated with MCV4. The serogroup C rSBA response after MCV4 vaccination compared to that after MPSV4 vaccination in the PSV-exposed participants, without adjusting for prevaccination titers, were not significantly different. After the adjustment of the day 28 GMTs for the baseline titers using ANOVA, the serogroup C rSBA GMT was significantly higher after MCV4 than after MPSV4 vaccination.
The nature of local and systemic reactions did not materially differ between the study vaccines or for the administration of MCV4 as a booster or primary dose; the majority of solicited reactions were mild.
In conclusion, the findings from this study suggest that immunological hyporesponsiveness can happen with repeated doses of meningococcal polysaccharide vaccine for all 4 serogroups (A, C, Y, and W-135), and that quadrivalent meningococcal conjugate vaccination in those with a history of repeated polysaccharide vaccine receipt is safe and induces robust immune responses that may partially compensate for hyporesponsiveness.
This study was funded by Sanofi Pasteur SA.
We gratefully acknowledge the dedicated effort of the Saudi field team: Mohamed Hamdy, Mohamed Zaki, Magdi Shahin, Taher Hajaj, and Fahmy Imam. We also thank Sanofi Pasteur employees Florence Coux for operational support, Catherine Bravo for her study coordination efforts, and Robert Lersch for writing and editorial support.
We declare the following potential conflicts of interest. M.K. and Y.A.M. have received assistance from Sanofi Pasteur to attend scientific meetings. R.B., H.F., and H.C. have performed contract research on behalf of the Health Protection Agency (funded by Pfizer, Novartis Vaccines, Baxter Bioscience, GlaxoSmithKline, Sanofi Pasteur, Alexion Pharmaceuticals Inc., and Merck). V.B.C. and D.R.J. are Sanofi Pasteur employees.
Published ahead of print 2 May 2012
1. Al-Mazrou YY, Al-Jeffri MH, Abdalla MN, Elgizouli SA, Mishskas AA. 2004. Changes in epidemiological pattern of Meningococcal disease in Saudi Arabia. Does it constitute a new challenge for prevention and control? Saudi Med. J. 25:1410–1413. [PubMed]
2. Borrow R, et al. 2000. Reduced antibody response to revaccination with meningococcal serogroup A polysaccharide vaccine in adults. Vaccine 19:1129–1132. [PubMed]
3. Centers for Disease Control and Prevention 2005. Prevention and control of meningococcal disease. Recommendations of the Advisory Committee on Immunization Practices. MMWR Recomm. Rep. 54:1–21. [PubMed]
4. Gold R, Lepow ML, Goldschneider I, Draper TL, Gotschlich EC. 1975. Clinical evaluation of group A and group C meningococcal polysaccharide vaccines in infants. J. Clin. Investig. 56:1536–1547. [PMC free article] [PubMed]
5. Hajjeh R, Lingappa JR. 2000. Meningococcal disease in the Kingdom of Saudi Arabia: an evalutation of disease surveillance and control after an outbreak of serogroup W-135. CDC Rep. 23:2000.
6. Health Protection Agency 1992. Pilgrimage to Mecca (Hajj) 1992. Commun. Dis. Rep. CDR Wkly. 2:43. [PubMed]
7. Jokhdar H, et al. 2004. Immunologic hyporesponsiveness to serogroup C but not serogroup A following repeated meningococcal A/C polysaccharide vaccination in Saudi Arabia. Clin. Diagn. Lab. Immunol. 11:83–88. [PMC free article] [PubMed]
8. Keyserling H, et al. 2005. Safety, immunogenicity, and immune memory of a novel meningococcal (groups A, C, Y, and W-135) polysaccharide diphtheria toxoid conjugate vaccine (MCV-4) in healthy adolescents. Arch. Pediatr. Adolesc. Med. 159:907–913. [PubMed]
9. Khalil M, et al. 2005. Immunogenicity of meningococcal ACYW135 polysaccharide vaccine in Saudi children 5 to 9 years of age. Clin. Diagn. Lab. Immunol. 12:1251–1253. [PMC free article] [PubMed]
10. Lakshman R, Burkinshaw R, Choo S, Finn A. 2002. Prior meningococcal A/C polysaccharide vaccine does not reduce immune responses to conjugate vaccine in young adults. Vaccine 20:3778–3782. [PubMed]
11. Leach A, et al. 1997. Induction of immunologic memory in Gambian children by vaccination in infancy with a group A plus group C meningococcal polysaccharide-protein conjugate vaccine. J. Infect. Dis. 175:200–204. [PubMed]
12. Maslanka SE, et al. 1997. Standardization and a multilaboratory comparison of Neisseria meningitidis serogroup A and C serum bactericidal assays. The Multilaboratory Study Group. Clin. Diagn. Lab. Immunol. 4:156–167. [PMC free article] [PubMed]
13. National Advisory Committee on Immunisation 2007. Statement on conjugate meningococcal vaccine for serogroups A, C, Y and W135. An Advisory Committee Statement (ACS). Can. Commun. Dis. Rep. 33:1–23. [PubMed]
14. U. S. National Institutes of Health 2012. January 6, posting date Immunogenicity and safety of Menactra vaccine in adolescents in Saudi Arabia.
15. World Health Organization 2002. Meningococcal vaccines: polysaccharide and polysaccharide conjugate vaccines. Wkly. Epidemiol. Rec. 77:331–339. [PubMed]
Articles from Clinical and Vaccine Immunology : CVI are provided here courtesy of
American Society for Microbiology (ASM)