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Application of a Pneumococcal Serotype-specific Urinary Antigen Detection Test for Identification of Pediatric Pneumonia in Burkina Faso

Bountogo, Mamadou MD, MSc, MPH*,†,‡; Sanogo, Bintou MD§; Pride, Michael W. PhD; Jiang, Qin MS; Nikièma, Zakari MD§; Njanpop-Lafourcade, Berthe-Marie PhD, MSc*; Ouédraogo, Abdou Salam PharmD, PhD§; van der Linden, Mark P. G. PhD; Moisi, Jennifer PhD, MSc*; Tall, Haoua PharmD, MSc*; Essoh, Alima MD, PhD*; Betsem, Edouard MD,MSc, PhD*; Gessner, Bradford D. MD, MPH*,¶; Meda, Nicolas MD, PhD, HDR‡,**

Author Information
The Pediatric Infectious Disease Journal: May 2021 - Volume 40 - Issue 5 - p 418-425
doi: 10.1097/INF.0000000000003065
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Abstract

Nonbacteremic pneumonia is the most common severe pneumococcal syndrome.1 In adults, a serotype-specific urine antigen detection (UAD) assay with high specificity and sensitivity against a gold standard of bacteremic pneumonia was developed and validated by Pfizer (UAD-1).2 Initially, this test was developed to support evaluation of 13-valent pneumococcal conjugate vaccine (PCV13) efficacy for nonbacteremic pneumonia in a randomized controlled trial among older Dutch adults.3 Subsequently, it has been used more widely to assess serotype distribution for adult community-acquired pneumonia (CAP).4 More recently, a UAD-2 assay was developed to detect 11 additional serotypes (2, 8, 9N, 10A, 11A, 12F, 15B, 17F, 20, 22F and 33F) in individuals with radiographically confirmed CAP.

A serotype-specific assay for use in pediatric nonbacteremic pneumonia would greatly advance understanding of pneumococcal epidemiology and, hence, vaccine decision-making. In children, however, diagnostic methods to identify Streptococcus pneumoniae and pneumococcal serotypes for nonbacteremic pneumonia remain elusive. For example, a previous evaluation of the BinaxNOW S. pneumoniae UAD assay (Abbott Laboratories, Abbott Park, IL), which identifies pneumococcal pneumonia based on the detection of C-polysaccharide in the urine, found that almost 50% of community-based children without pneumonia had a positive assay, presumably due to high pneumococcal carriage prevalence.5 We describe here a first study to formally evaluate whether Pfizer’s serotype-specific UAD assays can distinguish children with pneumonia from healthy control children in a population from Western Burkina Faso.

METHODS

Study Setting

The current study was conducted in western Burkina Faso, centered around the city of Bobo-Dioulasso, at 3 hospitals: District Hospitals of Do and Dafra and Centre Hospitalier Universitaire Sourou Sanou in Bobo-Dioulasso. Children age <5 years were enrolled from October 2015 through February 2017. PCV13 was introduced during 2013 in an 8-, 12- and 16-week infant immunization schedule with no booster and no catch-up among older age cohorts; other vaccines given at 8, 12, and 16 weeks included rotavirus and pentavalent diphtheria-tetanus-whole cell pertussis-Haemophilusinfluenzae type b (Hib)-hepatitis B (Hib vaccine was introduced during 2006). Official PCV13 coverage during 2017 among children age 12–23 months was 91% although this was based on administrative data and no coverage survey had been conducted through 2017 (https://www.who.int/immunization/monitoring_surveillance/data/bfa.pdf). The HIV prevalence among persons age 15–49 years of age was 0.8% during 2017 (https://www.unaids.org/en/20190402_country_focus_BurkinaFaso).

Study Population

Two cohorts were enrolled. First, we enrolled 526 children with a physician diagnosis of clinical pneumonia (Table 1). At the Centre Hospitalier Universitaire Sourou Sanou, all subjects were hospitalized while in Do and Dafra both hospitalized patients and outpatients were enrolled. The UAD analysis population was further restricted to subjects who had urine collected, were hospitalized for <48 hours or were not hospitalized at enrollment and had no documentation of receiving PCV13 within 30 days before urine collection. Of the 526 subjects assessed for eligibility, 493 were included in the UAD analysis population.

TABLE 1. - Characteristics of 526 Children With Clinically Defined Pneumonia and 401 Community Controls
Pneumonia (N = 526); Number (%) Control (N = 401); Number (%) P*
Age <0.001
 0–5 mo 71 (13%) 53 (13%)
 6–11 mo 135 (26%) 49 (12%)
 12–17 mo 100 (19%) 45 (11%)
 18–23 mo 61 (12%) 53 (13%)
 ≥24 mo 159 (30%) 201 (50%)
Age median in months (interquartile range) 14.5 (8, 26) 24 (11, 41)
Maternal education level <0.001
 None 381 (73%) 161 (40%)
 Primary 73 (14%) 121 (30%)
 Secondary or greater 71 (14%) 119 (30%)
Female sex 204 (39%) 201 (50%) <0.001
<3 rooms in residence compound 275 (52%) 212 (53%) 0.91
<4 persons age <15 y in household 331 (63%) 277 (69%) 0.059
<3 persons age <15 y sleeping in the same room with subject 333 (63%) 335 (84%) <0.001
*Fisher exact except χ2 for age and maternal education.

Second, to establish UAD cutoff values for determining case positivity, and consistent with methodology for adult serotype-specific UAD studies,2 401 healthy children were enrolled who had experienced no hospitalization during the previous 2 weeks, were not febrile or otherwise acutely ill and who had no substantial underlying illness. Subjects were selected by mapping Bobo-Dioulasso and then randomly selecting households to visit as previously described.6 Of the 401 controls, 379 met the UAD analysis criteria described earlier.

Study Procedures

Urine specimens collected for UAD analysis were treated with 0.5 M PIPES to a final concentration of 25 mM and subsequently stored at −80 °C until testing at the Pfizer Laboratory in Pearl River, New York. Patients with suspected pneumonia had an anteroposterior chest radiograph (CXR) that was interpreted according to World Health Organization (WHO) criteria for pediatric pneumonia, with a positive CXR defined as alveolar consolidation or pleural effusion as agreed upon by 2 of 3 independent readers.7 We did not assess CXRs for other findings such as interstitial infiltrate, ground-glass opacity or hilar enlargement as no other findings have been shown to predict pneumonias caused by pneumococcus. No blood cultures were collected from children with pneumonia as these were not standard of care at study healthcare centers. Even had we implemented systematic blood culture collection, positivity for pneumococcus in young children is low in West Africa, due to difficulty with adequate blood volume, contamination, and pretreatment with antibiotics, as documented in a study in neighboring Togo where of 251 children age <5 years hospitalized with pneumonia, 2 had a blood culture positive for pneumococcus.8

For pneumococcal carriage determination, study subjects had a nasopharyngeal specimen collected via flocked nylon swab that was placed in a transport tube with 1 mL of skim milk, tryptone, glucose and glycerin media, followed by culture and microbiologic identification of pneumococci using methods previously implemented in the study area6 based on WHO standards.9 Semiquantitative estimate of pneumococcal carriage density was based on visual inspection of culture plates with categorical values assigned as follows ordered from most to least dense: category 4, >10 pneumococcal colonies counted in the fourth quadrant; category 3, <10 pneumococcal colonies present in the fourth quadrant and >10 in the third quadrant; category 2, <10 pneumococcal colonies present in the third quadrant and >10 in the second quadrant; and category 1, <10 pneumococcal colonies counted in the second quadrant and any colony present in the first quadrant. The original design of this study included serotype determination of carried pneumococci. However, when results from the laboratories in Burkina Faso were compared with results from the German National Pneumococcal Reference Laboratory in Aachen, Germany, substantial discrepancies were identified. This may have occurred because of contamination of specimens in Burkina Faso, or overgrowth during storage, as Burkina Faso experiences periodic power outages, or other issues. Regardless, because we could not determine a cause for these errors, these data were omitted from our analysis.

Serotype-specific UAD Assays

The current study was not designed to compare results from the UAD to a reference standard for the diagnosis of nonbacteremic serotype-specific pneumococcal pneumonia because for this outcome no such reference standard exists. In theory, one could perform lung punctures; however, for uncomplicated pediatric pneumonia in the Burkina Faso context, this would be both unethical and impractical. Moreover, even if implemented, it would provide information only on consolidated pneumonias located in the peripheral lung fields. Instead, the current study employed clinically validated limit assays for which statistical rules were developed to derive positivity cutoff levels that would yield high specificity and sensitivity against a gold standard of bacteremic pneumonia.2,10 As described below, these rules then were applied to urines from healthy pediatric controls in Burkina Faso to derive positivity cutoffs in the current population.

In brief, the UAD assays are multiplex assays based on the Luminex platform,2,10–12 using in total 24 unique serotype-specific monoclonal antibodies and 24 different rabbit polyclonal antibodies and purified polysaccharides. Pfizer’s serotype-specific UAD-1 and UAD-2 assay methodologies and establishment of positivity cutoff limits have been described previously.2,10 The UAD-1 detects the 13 serotypes in PCV13 including 1; 3; 4; 5; 6A/C; 6B/D; 7F/A; 9V/A; 14; 18C/A, B, F; 19A; 19F and 23F (serotypes after the slash in each case are cross-reacting non-PCV13 types).2 The UAD-2 assay detects all 11 serotypes in 23-valent pneumococcal polysaccharide vaccine not included in PCV13 (serotypes 2; 8; 9N; 10A/39; 11A/D,F; 12F; 15B/C; 17F/A; 20A/B; 22F/A and 33F/A).9 The UAD-1 and UAD-2 assays have been clinically validated among adults by comparison to a gold standard of bacteremic pneumonia, including patients with pneumonia and blood culture results in one of the following categories: (1) positive for UAD-1 or UAD-2 serotype pneumococci; (2) positive for other pneumococci; (3) positive for nonpneumococcal pathogens; and (4) negative for pathogens.

To classify a test sample as positive in the UAD assay requires a fluorescent signal that is above a predetermined level, that is, the positivity cutoff limit. As such, the UAD assay is categorized as a limit assay, according to the published nomenclature for bioanalytical validation.13,14 Positivity cutoff limits, based on antigen concentrations read off a standard curve, were established for each serotype using 400 control urine specimens obtained from subjects without clinical suspicion of CAP.2,10 Nonparametric tolerance intervals were computed from these concentrations, giving a range predicted to contain 98% of healthy control urine samples with 99% confidence level, thus achieving at least 97% assay specificity for each serotype among adult populations. The assay acceptance criteria for specificity dictates that among the control population the positivity rate for any serotype cannot differ at the 95% confidence level from a 0.25% positive detection rate. For the target sample size of 400 control urines, ≥4 urines above the positivity cutoff value for a specific serotype would differ significantly from 0.25% and thus mandate that the cutoff value be reset to a higher value. While up to 3 urines per serotype in theory could be positive, the combination of rules means that practically speaking 1%–2% of control urines for all serotypes combined in each of UAD-1 and UAD-2 will be above the cutoff. Applying these rules to the original population in which the UAD-1 and UAD-2 were validated yielded sensitivities of 97.1% and 92.2% and specificities of 100% and 95.9%, respectively, among adults2,10 when compared with a gold standard of bacteremic pneumonia.

To ensure that this confidence level is maintained when evaluating urine samples from novel populations, such as pediatric populations, these same established rules are applied to a target of 400 urines from a study matched population of control subjects without pneumonia. As with other populations in which the UAD assays are employed, the goal in the current study was not to revalidate the assays. While in theory this could be done, it would be impractical to achieve as it would require the collection of hundreds of blood cultures with large numbers that were positive for assay serotype pneumococci, other pneumococci and other pathogens. Because blood cultures often are not part of standard of care in Burkina Faso, this would take years to achieve. Instead, our goal was to employ the statistical approach described earlier in a pediatric population to determine if cutoff levels could be identified that would distinguish children with pneumonia from healthy controls. In the current study, this was achieved by including as controls 401 healthy children with similar age distribution as cases, of whom 379 made analysis inclusion criteria as noted earlier.

Ethics

The parent or guardian of all study subjects provided written informed consent for children to participate. The study was reviewed and approved by the Institutional Review Board of Centre Muraz and le Comité Ethique pour la Recherche en Santé (the Burkina Faso National Ethics Committee).

RESULTS

Subject Characteristics

Among the entire study cohort of 526 subjects with clinically defined pneumonia and 401 community controls, subjects with pneumonia differed from controls with respect to several selected variables, likely reflecting risk factors for pneumonia (Table 1). Subjects enrolled as community controls were equally divided among females and males as well as those age <2 and 2+ years.

UAD Positivity Cutoff Determination

For all serotypes, application of the accepted statistical methodology led to an upward adjustment in positivity cutoffs for children compared with adults (Table 2). All revised positivity cutoffs were within the established assay limits. Of the 379 controls included in analysis, a signal above the revised cutoff was seen in 8 (2.1%) subjects for UAD-1 and 7 (1.8%) subjects for UAD-2; no individual serotype had >1 subject with a signal above the revised cutoff. By contrast, use of standard positivity cutoffs for adults would have yielded signals above the cutoffs for 64 (17%) subjects for UAD-1 and 46 (12%) subjects for UAD-2. This illustrates the value of applying a standard statistical methodology within a population to determine positivity cutoff values rather than assuming that cutoffs are the same across populations. The great majority of signals within each serotype were logarithmically below the positivity cutoff, with serotype 19A shown as an example (Fig. 1).

TABLE 2. - Urinary Antigen Detection Assay Serotype-specific Cutoffs Organized by PCV13 (UAD-1) and Remaining Serotypes in Pfizer’s In-development PCV20 as well as PPSV23 (UAD-2) Serotypes
Serotype Group Serotype Adult Cutoff Pediatric Cutoff Upper Limit of Assay Range
UAD-1 (PCV13 serotypes) 1 17.8 44.2 477.2
3 330.5 516.5 997.2
4 27.3 97.4 192.5
5 5.0 8.8 45.4
6A/C 106.7 735.5 978.0
6B/D 79.7 496.7 732.6
7F/A 6.6 10 165.4
9V/A 49.3 191.8 1480.5
14 4.7 21.3 44.9
18C/A,B,F 13.7 20.3 58.3
19A 9.8 53.2 174.3
19F 149.9 172.5 428.6
23F 44.4 61 99.0
UAD-2 (additional PCV20 and PPSV23 serotypes) 2 90.4 336.3 1541.7
8 48.9 64.4 138.9
9N 63.3 191.7 203.7
10A/39 275.3 797.5 1083.2
11A/D,F 74.5 353.1 500.0
12F 67.7 176.5 631.9
15B/C 86.6 163.1 167.9
17F/A 115.7 473 551.8
20A/B 33.7 192.5 777.2
22F/A 34.2 34.2 369.6
33F/A 57.7 57.7 625.0
All results in PnPs U/mL.
*Serotypes after the slash in each case are cross-reacting non-PCV13/PPSV23 types.

FIGURE 1.
FIGURE 1.:
Urinary antigen detection assay 1 signal results for serotype 19A among 379 healthy control children age <5 years from Burkina Faso.

UAD Results

Of the 493 subjects with clinically defined pneumonia who met the inclusion criteria for UAD analysis, 472 (96%) had a CXR result of whom 108 (23%) met the WHO consolidation endpoint criteria; the remaining 364 subjects had some other finding including a consolidation identified by only one reader, other infiltrates or no infiltrates.

Among the 493 children with clinically defined pneumonia, 66 (13%) exceeded the positivity cutoff for one of the PCV13 serotypes assessed by UAD-1 or one of the additional 11 serotypes assessed by UAD-2 (Table 3). Among these 66 children, 77 serotypes were identified, including 6 children with a serotype on both UAD-1 and UAD-2, 4 with 2 UAD-1 serotypes, and 1 with 2 UAD-2 serotypes. There were 50 children (10%) with a positive UAD-1, including 4 children with 2 serotypes identified, and 22 children (4.5%) with a positive UAD-2, including 1 child with 2 serotypes identified.

TABLE 3. - Percent of Children Positive on Serotype-specific UAD-1 or UAD-2 Assay* Stratified by WHO CXR Positivity and Age Group
UAD Test and Age WHO CXR+ CAP Other CXR Result Total Clinically Suspected CAP‡
UAD-1 positive* 23.3% (25/108) 6.6% (24/364) 10.1% (50/493)
 Age <2 y 15.5% (13/84) 5.9% (14/239) 8.3% (28/338)
 Age 2–4 y 50.0% (12/24) 8.1% (10/123) 14.4% (22/153)
UAD-2 positive* 8.3% (9/108) 3.6% (13/364) 4.5% (22/493)
 Age <2 y 9.5% (8/84) 5.0% (12/239) 5.9% (20/338)
 Age 2–4 y 4.2% (1/24) 0.8% (1/123) 1.3% (2/153)
UAD-1 or UAD-2 positive* 28.7% (31/108) 9.3% (34/364) 13.4% (66/493)
 Age <2 y 22.6% (19/84) 10.0% (24/239) 13.0% (44/338)
 Age 2–4 y 50.0% (12/24) 8.1% (10/123) 14.4% (22/153)
*UAD-1 tests for serotypes 1; 3; 4; 5; 6A/C; 6B/D; 7F/A; 9V/A; 14; 18C/A, B, F; 19A; 19F and 23F (the 13 serotypes in 13-valent pneumococcal conjugate vaccine); UAD-2 tests for serotypes 2; 8; 9N; 10A/39; 11A/D,F; 12F; 15B/C; 17F/A; 20A/B; 22F/A and 33F/A (the remaining 11 serotypes in 23-valent pneumococcal polysaccharide vaccine). Serotypes after the slash are cross-reacting serotypes not included in the 2 vaccines.
†WHO CXR positivity was defined as an alveolar consolidation or pleural effusion as judged by 2 of 3 independent readers.
‡Not all children received a CXR so for some rows the values of WHO CXR+ and other CXR result do not sum to total clinically suspected CAP.

Among children with and without WHO CXR+ pneumonia, respectively, 23.3% and 6.6% were UAD-1 positive (Table 3) (risk ratio 3.5; 95% confidence interval [CI], 2.1–5.9). Nevertheless, 24 of 49 (49%) UAD-1 positive children with a CXR did not have WHO CXR+ pneumonia. For UAD-2, 8.3% of children with a WHO CXR+ pneumonia tested positive compared with 3.6% of other children with clinical pneumonia (risk ratio, 2.3; 95% CI, 1.0–5.3); 13 of 22 (59%) UAD-2 positive children who had a CXR did not have WHO CXR+ pneumonia. Compared with children 2–4 years of age, children <2 years were less likely to have a positive UAD-1 (8.3% vs. 14%) but more likely to have a positive UAD-2 (5.9% vs. 1.3%). One of the subjects with no CXR result had a positive UAD (serotype 3 on UAD-1).

The most common serotypes identified by UAD were 5 (n = 9), 9V (n = 7), 19A (n = 7) and 18C (n = 6) (Fig. 2). Other conclusions for specific serotypes related to age group or the presence or absence of a WHO+ CXR were difficult to make given the small sample sizes for these stratified analyses.

FIGURE 2.
FIGURE 2.:
Pneumococcal serotype distribution detected by UAD-1 assay (containing serotypes in PCV13) or UAD-2 assay (containing additional serotypes in Pfizer’s in-development PCV20 as well as 23-valent pneumococcal polysaccharide vaccine [listed as “other”]).

Pneumococcal Carriage

Among the 493 included children, pneumococcal carriage prevalence did not differ among community controls versus children with pneumonia regardless of CXR findings. Pneumococcal carriage was identified in 145 of 493 (29%) total children, including 37 of 108 (34%) children with WHO CXR+ pneumonia, 103 of 387 (27%) of the remaining children with a CXR result, and 5 of 21 (24%) with no CXR result. Among community controls, 126 of 379 (33%) children with a result available had pneumococcal carriage.

Among the 145 children with pneumococcal carriage, 26 (18%) were positive on either UAD-1 or UAD-2 including 20 (14%) who were UAD-1 positive and 7 (5%) who were UAD-2 positive (one child was positive for both assays); 119 children (82%) were UAD negative.

Among children with pneumococcal carriage, semiquantitative carriage density did not differ among community controls (124 of whom had a result available) versus children with pneumonia regardless of CXR findings (Fig. 3A) or UAD results (Fig. 3B).

FIGURE 3.
FIGURE 3.:
A: Among children with pneumococcal carriage, the semiquantitative visual carriage density among children with clinically suspected pneumonia, by CXR status, and community controls. B: Among children with pneumococcal carriage, the semiquantitative visual carriage density among children with clinically suspected pneumonia, by UAD assay result, and community controls.

DISCUSSION

Using UAD assays validated or clinically validated to identify 24 pneumococcal serotypes among adult pneumonia patients, our study has documented that use of the same assays and statistical methods to determine positivity cutoffs can distinguish healthy children from children with pneumonia among a cohort age <5 years from Burkina Faso. This was particularly true for children who had pneumonia with consolidation or pleural effusion (ie, WHO endpoint pneumonia). Our results suggest that over one quarter of WHO endpoint pneumonia was attributable to pneumococcal serotypes included in the UAD-1 and UAD-2 assays. These data are consistent with the Gambian pediatric PCV9 randomized controlled trial, which found that the vaccine efficacy for WHO endpoint pneumonia was much higher than for other pneumonias, with 37% of this outcome preventable by PCV9.15 Nevertheless, many of the children with UAD-positive urine samples in our study did not have WHO endpoint pneumonia. This result also is consistent with studies from Israel16 and Finland,17 which reported reductions in pediatric acute respiratory infection without lobar consolidation associated with PCV use. The distribution of serotype groups by age category was consistent with an impact of PCV13, which was introduced in Burkina Faso among infants with no booster dose or catch-up: notably, we found a higher percentage of CAP due to PCV13 serotypes among older children and a higher percentage due to non-PCV13 serotypes among younger children.

In our study, pneumococcal carriage and semiquantitative visual carriage density were not associated with the presence of pneumonia. However, we did not use a quantitative measure of carriage density and did not have serotype-specific carriage data available to assess correlation with UAD results. Consequently, it remains possible that the UAD was positive because of the well-documented increase in upper airway pneumococcal carriage density associated with respiratory viral infection,18–21 and that this higher density carriage led to the presence of antigenuria.5 Assigning causality is further complicated because increased pneumococcal carriage density itself may be associated with an increased risk of pneumococcal pneumonia.20

Fortuitously, a pneumococcal meningitis study was conducted in Burkina Faso during 2016–2017, contemporaneous with our study.22 The percent of cases positive for individual serotypes cannot be compared across studies as the denominator for our study was all pneumonias and for the meningitis study the denominator was pneumococcal meningitis. Nevertheless, relative distributions provide some insight (Fig. 4). As for pneumonia in our study, for meningitis, UAD-1 serotypes were substantially more common than UAD-2 serotypes, for meningitis by factors of 2.3 (age < 2 years) and 3.9 (age 2–4 years). Also similar to pneumonia, children <2 compared with those age 2–4 years of age with pneumococcal meningitis were less likely to have a UAD-1 serotype (43% vs. 49%) and more likely to have a UAD-2 serotype (18% vs. 13%). The most striking differences between pneumonia and meningitis were seen at the individual serotype level. For example, serotype 1 was by far the most common meningitis serotype identified accounting for 24% and 38% of cases among children age <2 and 2–4 years, respectively, while it was a more modest contributor to pneumonia. This suggests that like serogroup A meningococcus, serotype 1 pneumococcus has a predilection for meningitis in the African meningitis belt. By contrast, serotype 5 was the most common pneumonia serotype identified, but a more modest cause of meningitis, like the results seen among adults in the United Kingdom where serotype 5 was commonly identified for nonbacteremic pneumonia but infrequently for IPD.23

FIGURE 4.
FIGURE 4.:
Serotype distribution for pneumococcal meningitis cases identified among children age <5 years in Burkina Faso, 2016–2017. All data derived from Ref. 22.

More generally, our data provide important insights into the serotype distribution of nonbacteremic pediatric pneumonia. Perhaps somewhat surprisingly, PCV13 serotypes remained a common cause of pediatric pneumonia despite a reported 91% pediatric PCV13 coverage. This may have occurred for several reasons. It is possible PCV13 coverage was substantially less than that reported or that problems with vaccine delivery occurred, such as cold chain breaks. It also is possible that the accelerated infant PCV13 schedule of 8, 12 and 16 weeks with no booster dose did not provide sufficient direct protection against common PCV13 serotypes through early childhood (the age group most associated with transmission) or led to insufficient indirect protection, either of which could have resulted in continued transmission pressure from 3 to 5 year old children to children in our study age group. Regardless, the UAD results illustrate the great potential utility of a serotype-specific test for understanding pediatric nonbacteremic pneumonia epidemiology.

The present study had several limitations, in addition to lack of serotype-specific carriage data. The sample size was relatively small, limiting interpretation of data for individual serotypes or robust conclusions related to carriage and UAD positivity. While information was collected on vaccination for some children, definitive immunization history could not be obtained for everyone, precluding assessment of PCV13 effectiveness in preventing pneumonia associated with a positive UAD test. Additionally, viral testing was not performed on children with clinical pneumonia, preventing assessment of a potential role for viruses as a cofactor in UAD positivity. Carriage prevalence among control children and children with pneumonia was somewhat low. This could reflect reliance on microbiologic rather than molecular techniques, outpatient antibiotic use, the relatively recent roll-out of pediatric PCV (ie, before full serotype replacement in carriage) or laboratory error. Regardless, no association was apparent between carriage and either case vs. control status or UAD positivity. Last, there is no gold standard for determining UAD sensitivity for the outcome of nonbacteremic pneumonia, including in adults. UAD sensitivity for nonbacteremic pneumonia may be substantially less than the 92%–97% reported for adult bacteremic pneumonia,2,9 and consequently, we may have underestimated the contribution of UAD serotypes to pediatric pneumonia in Burkina Faso. Whether this is true or not will require assessment of differences in PCV-associated rate reductions for all clinical pneumonia versus UAD-positive pneumonia, as was done for adults in a randomized controlled trial in The Netherlands where the former value was 2.8-fold greater than the latter value.24

Our results suggest that Pfizer’s serotype-specific UAD assays can distinguish children with clinically suspected pneumonia, and particularly those with radiologically confirmed lobar consolidation or pleural effusion, from healthy controls. Future studies will be needed to assess several issues such as if the UAD is identifying true pneumococcal pneumonias or some aspect of pneumococcal carriage such as virally-amplified pneumococcal density; to determine the link between serotype-specific carriage and UAD results; and to determine the relationship between viral infection and UAD positivity. Additionally, our results will need replication in other populations because the relative ratio of carriage prevalence or carriage density to pneumococcal pneumonia size or density could vary by population, for example, based on crowding, access to antibiotics before hospital presentation or rapidity of care-seeking for pneumonia. This in turn in may influence UAD results, at least in theory. The standard procedure of collecting 400 controls to assign UAD cutoffs for each studied population helps to mitigate this concern. If our results can be replicated across sites, and if additional studies can more definitively separate the high yield among pneumonia cases from potentially confounding factors, it would prove a great asset to public health. Potential uses might include monitoring the success of pediatric immunization programs, tracking the impact of programmatic changes such as reduced dosing schedules, informing individual countries on the best available PCV for their pediatric population, and improved modeling of optimal future PCV designs.

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Keywords:

Burkina Faso; pneumonia; serotype; Streptococcus pneumoniae; urinary antigen detection assay

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