Serotype 3 Antibody Response and Antibody Functionality Compared to Serotype 19A Following 13-Valent Pneumococcal Conjugate Immunization in Children : The Pediatric Infectious Disease Journal

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Serotype 3 Antibody Response and Antibody Functionality Compared to Serotype 19A Following 13-Valent Pneumococcal Conjugate Immunization in Children

Fuji, Naoko PhD*; Pham, Minh PhD†; Kaur, Ravinder PhD*; E. Pichichero, Michael MD*

Author Information

*From the Center for Infectious Diseases and Immunology, Rochester General Hospital Research Institute, 1425 Portland Ave, Rochester, New York

†Department of Decision Sciences, Lam Family College of Business, San Francisco State University, 1600 Holloway Ave, San Francisco, California.

Accepted for publication October 29, 2023

The authors have no conflicts of interest to disclose.

NIH NIDCD (R01 o8671 and CDC contract # 75D30119C06842,75D30121C12195) to MEP, funded collection of clinical samples, Merck MISP 61385 to R.K, funded the analysis.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website (www.pidj.com).

Address for correspondence: Michael Pichichero, MD, Center for Infectious Diseases and Immunology, Rochester General Hospital Research Institute, 1425 Portland Avenue, Rochester, NY 14621. E-mail: [email protected].

The Pediatric Infectious Disease Journal 43(3):p 294-300, March 2024. | DOI: 10.1097/INF.0000000000004192

Abstract

Background: 

Prevention of infections in children vaccinated with 13-valent pneumococcal conjugate vaccine (PCV13) may be less effective against serotype 3 than 19A.

Objective: 

The aim of this study was to to determine differences in IgG and functional antibody for serotype 3 versus 19A following PCV13 immunization, in IgG antibody levels induced by PCV13 compared to naturally-induced immunity, and assess effectiveness of PCV13 against serotype 3 and 19A in prevention of acute otitis media (AOM) and colonization among 6–36-month-old children.

Methods: 

Samples were from a prospective, longitudinal, observational cohort study conducted in Rochester, NY. Pneumococcal detection was by culture. 713 serum were tested for antibody levels by enzyme-linked immunosorbent assay, 68 for functional antibody by opsonophagocytosis and 47 for antibody avidity by thiocyanate bond disruption. PCV13 effectiveness in preventing AOM and colonization was determined by comparison of pre-PCV13 detection of serotypes 3 and 19A to post-PCV13.

Results: 

The proportion of children who reached the antibody threshold of ≧0.35 µg/mL after PCV13 was higher for serotype 19A than serotype 3. Only serotype 19A showed significant increase in PCV13-induced opsonophagocytosis assay titers and antibody avidity. Serotype 3 naturally-induced immune children showed a positive trend of increase in antibody level as children got older, but not PCV13-immunized children. PCV13 effectiveness was not identified in preventing AOM or colonization for serotype 3 but effectiveness of 19A was confirmed.

Conclusions: 

PCV13 elicits lower antibody levels and lower effectiveness to serotype 3 versus serotype 19A. Post-PCV13-induced antibody levels for serotype 3 are likely insufficient to prevent AOM and colonization in most young children.

Pneumococcal infections are major causes of morbidity and mortality worldwide.1,2 The polysaccharide capsule is an important pneumococcal virulence factor.3–6 More than 100 known pneumococcal capsular serotypes have been identified.7,8 Serotype 3 isolates are an important cause of pneumococcal infections in children and adults.9 Current licensed pneumococcal conjugate vaccines (PCVs) are generally effective among infants and young children against invasive pneumococcal disease (IPD),10–12 non-IPD11,12 and nasopharyngeal carriage.11–14 There is global evidence that PCV13 has reduced rates of IPD and acute otitis media (AOM) compared with PCV7.9,15 Prior studies reported that PCV13 immunization produced limited effectiveness against serotype 3 but higher effectiveness against serotype 19A infections.16–23 Our group showed additional serotypes included in PCV13 were effective in preventing AOM and colonization comparing the PCV7 era (2007–2009) with the early PCV13 era (2010–2013).15 However, we found no difference in serotype 3 AOM cases or colonization.15

Anti-capsular polysaccharide (CPS) antibodies following PCV immunization are protective against IPD when the serum antibody level is sufficiently high.24,25 A serotype-specific enzyme-linked immunosorbent assay (ELISA) level of 0.35 μg/mL, proposed as a protective threshold by the World Health Organization (WHO),25,26 is used by regulatory authorities for licensure of PCVs against IPD. Anti-CPS antibodies protect the host by opsonization and killing of pneumococci.27 The ability of antibodies to promote phagocytosis and killing of pneumococci can be measured by opsonophagocytosis assay (OPA). OPA has been used to assess vaccine-mediated protection.28 OPA titers ≥8 have been used as putative correlates of immunity against IPD.29,30 Antibody avidity has been used as a measure of antibody quality in studies of pneumococcal vaccines.31,32 In vitro, higher avidity antibody is associated with greater opsonophagocytic capacity and protection in mouse models.33–35 Pediatric IPD cases show low opsonic activity and low avidity for the serotypes causing invasive diseases,36 consistent with susceptibility to infection.

The objective of this study was to determine differences in IgG antibody measured by ELISA, functional antibody by OPA and antibody avidity, for serotype 3 compared to serotype 19A following PCV13 immunization in children. We also determined differences in IgG antibody levels induced by PCV13 compared to naturally-induced immunity for serotypes 3 and 19A. Finally, we reassessed effectiveness of PCV13 against serotype 3 and 19A in prevention of AOM and colonization among young children.

METHODS

Study Population and Samples

Samples were secured during a prospective, longitudinal, observational cohort study conducted in Rochester, NY during years 2006–2021 of AOM, asymptomatic nasopharyngeal colonization and child host immune response to bacterial and viral respiratory pathogens as described previously in detail including parent consent and Institutional Review Board approval.37 The study design called for blood samples from children at the time of well-child visits collected at 6, 9, 12, 15, 18, 24 and 30–36 months of age and at the time of AOM. The sera tested were sequential samples from the same children, although not all children provided 7 samples. Child subjects were from diverse socioeconomic status families living in urban, suburban and rural areas. All children received PCV7 or PCV13 vaccine exclusively at 2, 4 and 6 months of age and a booster at 15 months of age. Standard microbiology processing and identification techniques were used in detecting pneumococci in the nasopharynx and middle ear fluid (MEF) samples collected by tympanocentesis, as previously described.38,39

Anti-CPS Antibody ELISA

We adapted the US FDA-approved ELISA method as previously described40 to determine anti-IgG levels of CPS 3 and 19A in sera. Antibody concentrations were determined with the human reference serum 007sp, obtained from US FDA Center for Biologics Evaluation and Research IgG data from a previous study was used in the analysis.40 ELISA testing was done on 713 sera (338 sera from 122 children who were PCV7 immunized and 375 sera from 150 children who were PCV13 immunized).

Opsonophagocytic Assay

The quantity of neutralizing antibodies was measured using a multiplexed opsonophagocytic killing assay (MOPA). The protocol followed the method of the National Institutes of Health pneumococcal serology reference lab (www.vaccine.uab.edu/uab-mopa.pdf). MOPA target strains were secured from BEI Resources (www.beiresources.org). MOPA reagents were validated before testing sera from children. Opsonization titer/index (OI) was defined as the serum dilution that killed 50% of bacteria.41 OPA was used to test 68 sera from 32 children (serotype 3: 42 samples from 22 children; serotype 19A: 54 samples from 29 children) who were PCV13 immunized.

Antibody Avidity

Avidity of serotype-specific IgG in sera was evaluated by ELISA using our previously published method with a minor modification.42,43 Briefly, the sera were preadsorbed to C-polysaccharide and 22F CPS, then added to CPS 3- or 19A-coated microtiter plates with a fixed concentration to give an optical density of 1.0 (determined by ELISA), and incubated for 2 hours for serotype 3 and overnight for serotype 19A at 37 °C. After washing, ammonium thiocyanate, at concentrations 0–1.6 M, was added to each well and incubated for 15 minutes at room temperature. After washing, diluted goat antihuman IgG HRP-conjugate was added and incubated for 2 hours at room temperature. Substrate solution was added, followed by incubation for 2 hours at room temperature. Plates were read at the optical density of 405 nm. Avidity of serotype-specific IgG was expressed as an avidity index corresponding to the molar concentration of ammonium thiocyanate required to produce a 50% reduction in absorbance. Antibody avidity was tested on 47 sera from 29 children (serotype 3: 34 samples from 21 children; serotype 19A: 44 samples from 26 children) who were PCV13 immunized.

PCV13 Effectiveness Analysis

For this analysis, from July 2006 to 2021, 211 children who had birthdays before September 2009 were included in the PCV7 cohort and 570 children who had birthdays after October 2010 were included in the PCV13 cohort to accommodate timing of introduction of PCV13. In the PCV7 cohort, MEF samples were obtained from 321 AOM cases from 188 children (mean 1.7 per child). Ninety-eight AOM cases were positive with pneumococci (30.5% of AOM cases). In the PCV13 cohort, MEF samples were obtained from 657 AOM cases from 339 children (mean 1.9 per child). One hundred seventy-five AOM cases were positive with pneumococci (26.6% of AOM cases).

Statistical Analysis

The proportions exceeding IPD protective antibody level were analyzed by Fisher exact test. Log2 transformed serotype-specific antibody level, OPA and antibody avidity comparisons were analyzed by analysis of variance or Kruskal-Wallis with multiple comparisons, as appropriate. Age effects on antibody levels were modeled by generalized estimating equations (GEE) as previously described, with a modification.44 A GEE model was formulated to establish the relationship among antibody level, age (age when the sera was collected), colonization history and vaccination history (only for PCV13 immunized). The formula applied was: log Ab = log (age) × pre.col (× vaccination), where age was the age of the child when the antibody measurement was made, pre.col an indicator variable for whether a child was having a colonization event detected with a particular serotype at the time point and vaccination an indicator variable for whether a child had received a booster immunization at the time point. PCV13 effectiveness analysis employed methods previously described.15

RESULTS

Frequency of Exceeding IPD Protective Antibody Level Thresholds

Serotype 3- and 19A-specific antibody levels in sera following PCV13 were compared in 7 age groups (Table 1). At 6 of 7 age time points, the proportion of children who reached the WHO standardized protective threshold against IPD25,45,46 (≧0.35 µg/mL) was higher for serotype 19A compared to serotype 3 levels. The proportion of children who had antibody levels in the range of ≥1µg/mL (the serotype 19A-specific correlate of protection against IPD18) and the proportion in the range of ≥3 µg/mL (the serotype 3-specific correlate of protection against IPD18) was consistently higher for serotype 19A compared to serotype 3. Figure, Supplemental Digital Content 1, https://links.lww.com/INF/F315 shows reverse cumulative distribution curves of serotype-specific antibody levels following PCV13 to show the proportion of children achieving varying antibody levels at 4 time points.

TABLE 1. - Serotype-specific Antibody Levels for Serotype 3 and 19A Following PCV13 Immunization at Different Ages in Children
PCV13 2 months After 2 Doses 3 months After 3 Doses 6 months After 3 Doses Before Booster 3 months After Booster 8 months After Booster ≥15 months After Booster
6 months 9 months 12 months 15 months 18 months 24 months 30–36 months Total Number
Antibody Level (µg/mL) 3 19A 3 19A 3 19A 3 19A 3 19A 3 19A 3 19A 3 19A
≥0.35 27 (47%) 47 (78%)* 23 (39%) 50 (77%)† 4 (21%) 12 (92%)† 17 (24%) 49 (67%)† 35 (48%) 77 (97%)† 6 (25%) 10 (91%)* 2 (100%) 4 (80%) 114 (37%) 249 (81%)
≥1 10 (18%) 29 (48%)* 7 (12%) 18 (28%) 1 (5%) 3 (23%) 7 (10%) 24 (33%)* 10 (14%) 69 (87%)† 2 (8%) 8 (73%)† - - 38 (12%) 154 (50%)
≥3 3 (5%) 11 (18%) 2 (3%) 4 (6%) - 1 (8%) 1 (1%) 6 (8%) 3 (4%) 47 (59%)† - 4 (36%)‡ 1 (50%) 3 (60%) 10 (3%) 76 (25%)
Number of children 57 60 59 65 19 13 72 73 73 79 24 11 2 5 306 306
±1 month were included in each age group. Fisher exact test. P < 0.01 was used to define significance. 306 serum samples from 133 children were included for serotype 3. 306 serum samples from 129 children were included for serotype 19A. Booster dose was given at 15 months old. Serum samples were collected before the booster dose was given.
*P < 0.001.
†P < 0.0001.
‡P < 0.01.

Quantitative Comparison of Antibody Levels

Significantly higher serotype 19A antibody levels compared to serotype 3 antibody levels were observed at child age 6, 9, 12, 15 and 18 months (Fig. 1). PCV13-induced antibody levels to serotype 3 were geometric mean of 1.4-fold lower at 6 months old, 1.4-fold lower at 9 months old, 2.1-fold lower at 12 months old, 1.5-fold lower at 15 months old and 3.4-fold lower at 18 months old than serotype 19A antibody levels. Of note, the largest fold change difference between serotype 3 and 19A was observed at 18 months old, which is 3 months after the booster dose of PCV13.

F1
FIGURE 1.:
Serotype-specific antibody level differences for serotype 3 and serotype 19A following PCV13. Antibody levels against serotype 3 (○) and serotype 19A (∆) among PCV13-immunized children. Data are divided in each age group. Mean and 95% confidence interval are shown. Statistical comparison of antibody levels are done in each age group between serotype 3 and serotype 19A by analysis of variance with Bonferroni correction. ***P < 0.001. ****P < 0.0001. Serotype 3: 6 months old (after 2 doses), n = 57; 9 months old (after 3 doses), n = 59; 12 months old, n = 19; 15 months old (at the time of booster), n = 72; 18 months old (after booster), n = 73. Serotype 19A: 6 months old (after 2 doses), n = 60; 9 months old (after 3 doses), n = 65; 12 months old, n = 13; 15 months old (at the time of booster), n = 73; 18 months old (after booster) n = 79.

Functionality and Quality of Serotype 3- and 19A-specific Antibody Following PCV13.

No significant increase in PCV13-induced OPA titers for serotype 3 was found in comparisons across 4 age time points (Fig. 2A). In contrast, for serotype 19A, higher OPA titers were measured at 18 months old (after booster) compared to 6 months old (P < 0.01, Fig. 2A). The median OPA titer for serotype 19A at 18 months old was above the putative protection level (≥8).

F2
FIGURE 2.:
Serotype-specific antibody profiles following PCV13. A: Opsonophagocytic killing responses against serotype 3 (○) and serotype 19A (∆) among PCV13-immunized children. Opsonophagocytic killing responses in serum are described by OI where diluted serum kills 50% of bacteria of the specific serotype. Median and 95% confidence interval are shown. A line describes OI = 8. Kruskal-Wallis with Bonferroni correction test, *P< 0.01. Serotype 3: 6 months old (after 2 doses), n = 13; 9 months old (after 3 doses), n = 13; 15 months old (at the time of booster), n = 9; 18 months old (after booster), n = 7. Serotype 19A: 6 months old (after 2 doses), n = 15; 9 months old (after 3 doses), n = 19; 15 months old (at the time of booster), n = 8; 18 months old (after booster), n = 12. B: Antibody avidity against serotype 3 (○) and serotype 19A (∆) among PCV13-immunized children. Antibody avidity measuring IgG avidity is displayed as avidity index given in molar concentration of ammonium thiocyanate required to produce a 50% reduction in absorbance. Median and 95% confidence interval are shown. Kruskal-Wallis with Bonferroni correction. *P < 0.05. Serotype 3: 6 months old (after 2 doses), n =10; 9 months old (after 3 doses), n = 9; 15 months old (at the time of booster), n = 8; 18 months old (after booster), n = 6. Serotype 19A: 6 months old (after 2 doses), n = 12; 9 months old (after 3 doses), n = 15; 15 months old (at the time of booster), n = 8; 18 months old (after booster), n = 9.

No significant increase in PCV13-induced avidity of antibody for serotype 3 was found in comparisons across 4 age time points (Fig. 2B). In contrast, for serotype 19A, significantly higher avidity of antibody was measured at 18 months old (after booster) compared to 6 months old (P < 0.05, Fig. 2B). Figure, Supplemental Digital Contents 2 and 3, https://links.lww.com/INF/F316, https://links.lww.com/INF/F317 show the detailed comparisons of OPA titers and antibody avidity titers between serotypes 3 and 19A, respectively.

Age Gradients of Serotype-specific Antibody Levels Between Naturally-induced Immunity and PCV13-immunized Children

Age gradients of antibody measurements were plotted to compare naturally-induced and PCV13-induced levels (Fig. 3). Serotype 3 naturally-induced antibody levels showed a positive regression, with a coefficient value of 2.69 (6.45 µg/mL) over age (P < 0.0001) reflecting natural exposure to serotype 3. In contrast, PCV13-induced antibody levels showed a negative regression, with a coefficient value −0.22 (1.02 µg/mL) over age (P < 0.0001) (Fig. 3A). The intercept was higher for PCV13-immunized children −1.69 (0.31 µg/mL) compared to naturally-induced immune children −5.94 (0.02 µg/mL) reflecting the beneficial effect of immunizations at child age 2 and 4 months old. But the absence of an increase in antibody levels over time despite a booster dose at age 15 months is notable (Fig. 1). Serotype 19A showed positive regression, with a coefficient value of 5.58 (47.8 µg/mL) over time (naturally-induced immune children) and 3.12 (8.6 µg/mL) over time (PCV13-immunized children) (Fig. 3B). Serotype 19A for PCV13-immunized children also showed higher intercept −3.25 (0.11 µg/mL) compared to naturally-induced immune children −7.18 (0.007 µg/mL). PCV13-immunized children showed higher serotype 19A antibody level up to 24 months old compared to naturally-induced immune children. Each data point is shown in Figure, Supplemental Digital Content 4, https://links.lww.com/INF/F318.

F3
FIGURE 3.:
Age gradients of serotype-specific antibody levels among children who were naturally-induced immune and PCV13 immunized. GEE model was used to show the effect of age and PCV13 doses among naturally-induced immune and PCV13-immunized children. The fitted regression was statistically significant. A: Serotype 3: naturally-induced immune, n = 101 from 45 children, coefficient = 2.69, P < 0.0001; PCV13 immunized, n = 312 from 132 children, coefficient = −0.22, P < 0.0001. B: Serotype 19A: naturally-induced immune, n = 255 from 98 children, coefficient = 5.58, P < 0.0001; PCV13 immunized, n = 318 from 129 children, coefficient = 3.12, P < 0.0001.

Update the Effectiveness of PCV13 for Protection Against AOM and Colonization Caused by Serotypes 3 and 19A

Our group previously reported on effectiveness of PCV13 for protection against AOM and nasopharyngeal colonization based on data from 2010 to 2013, immediately following the introduction of the vaccine in the United States.15 Here we update the analysis to include 2014–2021. Demographic information of children in the 2 cohorts were similar (Table, Supplemental Digital Content 5, https://links.lww.com/INF/F319), except the PCV13 cohort had higher daycare attendance (20.7%) at the time of enrollment compared to the PCV7 cohort (14.7%) (P < 0.01). Serotype 3 pneumococcal isolations from the PCV13 cohort were not significantly lower compared to the PCV7 cohort. In contrast, serotype 19A pneumococcal isolations were lower from the PCV13 cohort than from the PCV7 cohort (Table 2, P < 0.0001).

TABLE 2. - Proportion of Children with Streptococcus pneumoniae Serotype 3 or 19A Isolated in the MEF or Nasopharynx in PCV13 or PCV7 Immunization
PCV13 PCV7 Estimated Percent Relative Effectiveness (95% CI) P
MEF at onset of AOM
 Number of isolates 175 98
 Serotype 3 4 (2.3%) 3 (3.1%) 16.7% (−128.1% to 54.1%) 0.7
 Serotype 19A 5 (2.9%) 38 (38.8%) 70.5% (62.1%–76.9%) <0.0001
 All 6 additional serotypes 9 (5.1%) 46 (46.9%) 71.5% (78.1%–62.8%) <0.0001
Nasopharyngeal samples at onset of AOM
 Number of isolates 474 181
 Serotype 3 8 (1.7%) 2 (1.1%) −38.8% (−391.9% to 46.3%) 0.73
 Serotype 19A 7 (1.5%) 53 (29.3%) 68.1% (62.7%–72.2%) <0.0001
 All 6 additional serotypes 15 (3.2%) 65 (35.9%) 75.2% (69.7%–79.5%) <0.0001
Nasopharyngeal samples during well-child visits
 Number of isolates 1255 443
 Serotype 3 9 (0.7%) 0 (0%) ∞ 0.12
 Serotype 19A 34 (2.7%) 84 (19.0%) 67.5% (62.04%–71.7%) <0.0001
 All 6 additional serotypes 48 (3.8%) 102 (23.0%) 67.6% (62.3%–71.8%) <0.0001
Estimated percent relative effectiveness was calculated as (1 − relative risk × 100%).

Serotype 3 pneumococcal isolations from the nasopharynx at onset of AOM did not show a significant reduction in the PCV13 cohort compared to the PCV7 cohort (Table 2). Detection of serotype 3 from the nasopharynx (at the onset of AOM and well-child visit combined) did not show a significant reduction comparing the PCV13 and PCV7 cohorts (Table, Supplemental Digital Content 6, https://links.lww.com/INF/F320). In contrast, serotype 19A pneumococci were isolated less frequently at the onset of AOM from the PCV13 cohort compared to the PCV7 cohort (P < 0.0001) and at well-child visits (P < 0.0001) (Table 2). While comparing the isolation of pneumococci expressing any of the 6 additional serotypes in PCV13 versus the serotypes in PCV7, significant reductions in MEF (P < 0.0001) and nasopharyngeal at the onset of AOM (P < 0.0001) and nasopharyngeal during well-child visits (P < 0.0001) occurred (Table 2).

DISCUSSION

In this study, we show that serotype 3 antibody levels induced by PCV13 are lower and there is no clear effect of a booster dose in OPA or antibody avidity for serotype 3 compared to serotype 19A in children during the first 2 years of life despite vaccination with a 3 + 1 schedule. The measured levels for serotype 3 were below the WHO standardized protective levels for IPD. Our group previously showed that a correlate of protection value for serotype 19A for colonization was 4-fold higher and AOM 2-fold higher among naturally-induced immune children44 compared to the one Andrews et al18 proposed for IPD. Thus, it might be expected that protection for colonization and AOM would likewise not be achieved for serotype 3. Our effectiveness analysis is consistent with that conclusion.

PCV13-induced antibody levels were always higher (1.4- to 3.3-fold) for serotype 19A compared to serotype 3 starting at an age of 6 months, measured 2 months after 2 doses of PCV13. Larger differences were found for serotype 19A compared to serotype 3 at an age of 18 months, measured 3 months after a booster dose. The waning of antibody titers after each PCV13 dose is different for each serotype and serotype 3 antibody levels may wane faster than serotype 19A. Long-term immunogenicity studies show that frequently carried pneumococcal strains are associated with maintenance of higher antibody levels over time.31,47 Serotype 3 is detected less as a nasopharyngeal colonizer strain compared to serotype 19A15,48,49 and that may affect the antibody levels we measured.

A higher antibody level is needed to protect from serotype 3 infections due to the unique features of serotype 3 polysaccharide.50,51 Andrew et al18 proposed a level of 2.8 µg/mL as the serum antibody level necessary to prevent IPD. Only 4% of children in our cohort after receiving 4 doses of PCV13 exceeded 3 µg/mL serotype 3-specific IgG.

We did not identify differences in OPA titers between serotypes 3 and 19A but the sample size was a limitation. Our group previously reported that serotype 3 was the second lowest in elicitation of OPA titers after serotype 1.41 Significantly higher antibody avidity with serotype 19A compared to serotype 3 was measured at 6 months old and at 15 months old. Since there is no correlate of protection of avidity, the direct comparisons between serotypes 3 and 19A focus on the differential responses that may explain the observed clinical differences in carriage and disease. Antibody avidity after PCV vaccinations can give information on the antibody protective quality of antibodies34 and the development of B-cell memory.52 Avidity maturation following vaccinations varies for individual serotypes.53,54 Ekstrom et al54 showed that low antibody avidity maturation after PCVs was associated with poor protection against AOM. In assessing optimal primary PCV immunization dosing schedules, Spijkerman et al55 found that serotype 3 was the second lowest in elicitation of high avidity antibody after primary vaccination and lowest after the booster dose among PCV13 serotypes tested in any dosing schedule.

PCV13 induced higher antibody levels after 2 primary doses compared to naturally-induced immune children for both serotypes 3 and 19A. However, there was no age gradient between 6 and 36 months old for serotype 3-specific antibody levels, consistent with a poor booster response to vaccination. A prior cross-sectional immunogenicity study showed superiority in PCV13-vaccinated children for antibody levels to serotype 3 and 19A compared to nonvaccinated children at 2, 4 and 6 months56 and after a booster dose at 11 months.32 Prior immunogenicity studies focused on measurement of antibody levels about a month after PCV13 doses, whereas we measured levels 2–3 months after vaccinations and used mathematical models to show longitudinal age gradients of antibody levels to understand different kinetics and longevity of antibody responses in comparisons between serotypes 3 and 19A.

Post booster, PCV13 induced higher IgG antibody, higher functional OPA responses and higher avidity levels for serotype 19A compared to the level after 2 doses but not for serotype 3. A booster dose of PCV13 had a significant effect on serotype 19A antibody avidity maturation, reflecting good B-cell memory development, evidenced by increased antibody levels and killing ability at 18 months old. A similar effect of a booster dose was not observed for serotype 3. The failure to increase avidity after the booster immunization, as shown for serotype 3, likely explains in part the failure to increase OPA activity. Several prior randomized trials reported that PCV13 induces relatively lower responses after a booster dose in antibody level and OPA titer for serotype 3 compared to other serotypes included in PCV13.48,57,58

In updating our data on effectiveness of PCV13, serotype 3 vaccination could not be shown to prevent AOM or nasopharyngeal colonization whereas serotype 19A vaccination was highly effective. Limited effectiveness of PCV13 against pediatric IPD cases caused by serotype 3 has been previously reported from Massachusetts, USA for 2002–201759 and CDC ABC surveillance for 2004–2013.9

Our study has limitations. Due to the availability of samples, longitudinal data over age was not from multiple sequential samples of the same children. Serotype 3 detection was uncommon in our cohort, even during the PCV7 era. Therefore, our study may be underpowered to detect differences between the 2 time periods. The OPA and avidity assay results involved a subset of sera due to the sample availability, thereby limiting statistical power. Antibody avidity assays are not standardized as ELISA and OPA.

In summary, we found that PCV13 elicits lower antibody levels and lower efficacy of booster doses to serotype 3 compared to serotype 19A in young children. PCV13-induced antibody levels for serotype 3 are likely insufficient to prevent colonization or AOM.

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Keywords:

Streptococcus pneumoniae; serotype 3; pneumococcal conjugate vaccine; acute otitis media; vaccine effectiveness

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