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Keywords:

  • human papillomavirus;
  • human papillomavirus types;
  • high-grade cervical lesions;
  • health disparities;
  • geocoding

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. FUNDING SOURCES
  8. CONFLICT OF INTEREST DISCLOSURE
  9. REFERENCES

BACKGROUND

Current vaccines protect against 2 human papillomavirus (HPV) types, HPV 16 and 18, which are associated with 70% of cervical cancers and 50% of high-grade cervical lesions. HPV type distribution was examined among women with high-grade lesions by individual and area-based measures of race, ethnicity, and poverty.

METHODS

This analysis included 832 women aged 18 to 39 years reported to a surveillance registry in Connecticut during 2008 to 2010. Diagnostic specimens were obtained for HPV DNA testing. Individual measures were obtained from surveillance reports, medical records, and patient interviews. Cases were geocoded to census tracts and linked to area-based measures of race, ethnicity, and poverty. Statistical analysis included use of generalized estimating equations.

RESULTS

Overall, 44.8% of women had HPV 16/18. In a multivariate model controlled for confounding by age and diagnosis grade, black race (adjusted prevalence ratio [aPR] = 0.54, 95% confidence interval [CI] = 0.34-0.88), Hispanic ethnicity (aPR = 0.59, 95% CI = 0.40-0.88), and higher area-based poverty (aPR = 0.59, 95% CI = 0.40-0.87) were associated with a lower likelihood of HPV 16/18 positivity. Black and Hispanic women were less likely to have HPV 16/18 than white women across all levels of area-based measures.

CONCLUSIONS

Black race, Hispanic ethnicity, and higher area-based poverty are salient predictors of lower HPV 16/18 positivity among women with high-grade cervical lesions. These data suggest that HPV vaccines might have lower impact among black and Hispanic women and those living in high poverty areas. These findings have implications for vaccine impact monitoring, vaccination programs, and new vaccine development. Cancer 2013;119:3052—3058. © 2013 American Cancer Society.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. FUNDING SOURCES
  8. CONFLICT OF INTEREST DISCLOSURE
  9. REFERENCES

Nearly 12,000 women in the United States are diagnosed with cervical cancer each year despite recent declines in incidence and mortality due to increased availability of screening with Papanicolaou tests.[1] Furthermore, racial, ethnic, and socioeconomic disparities persist with black and Hispanic women and women living in poverty experiencing higher rates of cervical cancer morbidity and mortality.[2] For example, incidence rates of invasive cervical cancer among black, Hispanic, and white women from 1998 to 2003 in the United States were 12.6, 14.2, and 8.4 per 100,000, respectively, and mortality rates show a similar disparity.[3] Women who live in lower socioeconomic areas have higher rates of late-stage diagnoses and lower survival rates compared with women living in high socioeconomic areas.[2]

In addition to being the most common sexually transmitted infection in the United States, genital human papillomavirus (HPV) is now recognized as a necessary cause of cervical cancer.[4, 5] Of the more than 50 anogenital HPV types, approximately 18 are considered high-risk because of their strong and consistent association with cervical cancer.[6] Based on studies conducted in various settings worldwide, persistent infection with high-risk HPV types 16 or 18 (HPV 16/18) causes approximately 70% of cervical cancer cases and 50% of high-grade cervical lesions.[7]

Since 2006, the US Food and Drug Administration has approved 2 prophylactic HPV vaccines that protect against HPV 16 and 18 (HPV 16/18) and are now recommended for routine use among adolescents.[8] The quadrivalent vaccine that is predominantly used in the United States also protects against HPV types 6 and 11 that are associated with 90% of genital warts. Both vaccines have high proven efficacy in clinical trials of >95% against vaccine-type (HPV 16/18)–associated disease in HPV-naive women and thus have high potential for population-level impact. Several efforts are currently underway to monitor vaccine impact, and recent data from Australia, Sweden, and the United States show early signs of reductions in warts and possibly cervical abnormalities.[9-12]

Although HPV 16/18 is responsible for the highest proportion of cervical cancers worldwide, HPV type distributions in cervical cancer vary by age and geographic region, as revealed by numerous studies, including meta-analyses.[7, 13-15] Fewer studies have described HPV type distribution in high-grade cervical lesions. High-grade cervical lesions are necessary precursors to cancer that arise from viral persistence of an HPV infection.[16, 17] Although the transformation process is heterogeneous and disentanglement of risk factors is difficult, epidemiologic data indicate that most primary HPV infections, up to 90%, are cleared within 2 years by host immune response. When persistence does occur, it has been estimated that anywhere from 14% to 56% of high-grade lesions will persist or progress to more invasive disease via clonal expansion of transformed cells. A small body of research indicates that the type distribution in high-grade lesions varies by diagnosis, age, and race/ethnicity.[18-20] The only study to examine race/ethnicity (the multisite HPV-IMPACT project[19]) used individual-level data, yet the use of area-based measures to assess and monitor socioeconomic inequalities has been shown to be a rigorous and accurate way of monitoring health disparities in the United States.[21] Area-based measures serve not only as proxies for individual-level data but are also meaningful measures in their own right, because they capture the socioeconomic conditions in which people live. We are not aware of any studies that have examined HPV type distribution by area-based sociodemographic measures to examine disparities. Therefore, the objective of this analysis was to examine the prevalence of HPV 16/18 among a sample of women not yet affected by HPV vaccines by both individual and geographic measures of race, ethnicity, and poverty.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. FUNDING SOURCES
  8. CONFLICT OF INTEREST DISCLOSURE
  9. REFERENCES

Surveillance System and Individual Measures

The HPV-IMPACT monitoring system was established in 2008 and is a collaboration between the Centers for Disease Control and Prevention (CDC) and 5 sites of the Emerging Infections Program network with the purpose of monitoring the impact of HPV vaccine on high-grade cervical lesions in the United States. This project has been reviewed by local, state, and federal institutional review boards and deemed exempt from human subjects approval as public health surveillance. Methods have been previously reported.[22, 23] Briefly, at the Connecticut site, statewide reporting of cervical intraepithelial neoplasia grade 2 or 2/3 or 3 and adenocarcinoma in situ (CIN2/3/AIS) is mandated by the state health department for public health surveillance, and all 34 pathology laboratories in the state are currently in compliance with this requirement. Reports include diagnostic and patient demographic information. For women aged 18 to 39 years residing in New Haven County, Connecticut, enhanced surveillance activities include medical chart reviews and patient interviews to collect HPV vaccine histories and demographic and health history information as well as residual diagnostic specimen collection for HPV DNA testing.

Age was initially examined in 5 categories (18-19, 20-24, 25-29, 30-34, and 35-39 years) and then dichotomized as 18 to 29 and 30 to 39 years after verifying similarities between adjacent categories and to increase statistical power. Diagnosis was classified as CIN 2 or CIN 2/3, 3, and AIS; the higher-grade diagnoses were combined due to relatively small numbers in some categories (eg, AIS). The individual-level race/ethnicity measure included mutually exclusive categories for black, Hispanic, and white; women with unknown or other race/ethnicity were excluded from analyses. Health insurance was composed of 2 categories: 1) private insurance that included HMO, PPO, managed care plans, or Veterans Administration insurance, and 2) public insurance that included Medicaid or no insurance. These groupings were chosen to create a marker of socioeconomic status.

HPV DNA Testing

Residual cervical tissue specimens from diagnostic paraffin-embedded tissue blocks were requested as part of enhanced surveillance for type-specific HPV DNA testing. For women with multiple tissue blocks, the slides were re-reviewed by a pathologist, and the tissue blocks representative of the highest grade lesion were chosen. Tissue for analysis was prepared at either the diagnostic laboratory or at a central pathology laboratory by cutting serial sections of the tissue block. First and last sections were stained with hematoxylin and eosin and 2 intervening 10-μm unstained sections were placed in sterile microfuge tubes for DNA extraction.

HPV typing procedures conducted at the CDC have been previously described.[19] Briefly, DNA was extracted using DNeasy (Qiagen, Valencia, Calif) and tested immediately or stored at −20°C. Extracted DNA was tested with the Linear Array HPV Genotyping Assay (LA; Roche Diagnostics, Indianapolis, Ind). This assay detects 37 HPV types (6, 11, 16, 18, 26, 31, 33, 35, 39, 40, 42, 45, 51, XR(52), 53, 54, 55, 56, 58, 59, 61, 62, 64, 66, 67, 68, 69, 70, 71, 72, 73, 81, 82, 83, 84, 89, IS39). If the XR probe and the cross-hybridizing HPV types 33, 35, or 58 were positive, the presence of HPV 52 was determined with a specific quantitative polymerase chain reaction assay. Inadequate or negative samples were tested with INNO-LiPA HPV Genotyping Extra Assay (Innogenetics, Gent, Belgium). This assay detects 28 HPV types (6, 11, 16, 18, 26, 31, 33, 35, 39, 40, 43, 44, 45, 51, 52, 53, 54, 56, 58, 59, 66, 68, 69, 70, 71, 73, 74, 82).

Geocoding and Geographic Measures

Using patient residential addresses obtained from surveillance reports, cases were geocoded to the census tract level, which is a useful measurement of area for examining health disparities.[24] Census tracts are small (∼2500 to 8000 residents), relatively permanent subdivisions of counties that are designed to be homogeneous with respect to population characteristics, economic status, and living conditions. Cases reported between January 1, 2008, (beginning of the surveillance period) and December 31, 2010, were geocoded using a 2-step process. First, we used ArcGIS version 9.2 software (ESRI, Redlands, Calif). We considered an address successfully matched when the match score was at least 80. We then used the Federal Financial Institutions Examination Council database to manually match remaining valid addresses to census tracts. Census tract level measures of race and ethnicity were obtained from US Census 2010 data as percentage of the population in each census tract that was black and Hispanic, respectively. Because the US Census asks 2 separate questions for race and ethnicity, these categories were not mutually exclusive and considered as 2 separate variables. The poverty measure was obtained from the US Census 2006-2010 American Community Survey 5-year estimates as the percentage of population in each census tract living below the federal poverty level. These measures were chosen to correspond to the individual-level measures that were available. We initially considered 4 levels for the area-based measures of race, ethnicity, and poverty: < 5.0%, 5.0% to 9.9%, 10.0% to 19.9%, and ≥20% based on previous work.[21, 23] However, due to small sample sizes in some cells for the analysis of interaction effects (described below), the lower 3 categories were combined so the final analysis included levels of <20% and ≥20%.

Statistical Analysis

The data set was stripped of duplicates (de-duplicated) by selecting the first diagnostic report for women who had more than one diagnosis reported during this time. Women who were determined to have received at least one dose of HPV vaccine prior to their CIN2/3/AIS diagnosis by either interview or medical record review were excluded.

The primary outcome for analysis was presence of HPV 16/18 DNA detected in the cervical biopsy specimens. Women who had multiple HPV types detected were classified as having HPV 16/18 if one of the types included HPV 16 or 18. To determine unadjusted associations between individual characteristics and the proportion of CIN2/3/AIS with HPV 16/18, binary logistic regression modeling was conducted. For race/ethnicity, pairwise comparisons were conducted for black and Hispanic women compared with white women. For this and all subsequent analyses, prevalence ratios were calculated from the odds ratios generated by the binary logit models using formulas provided by Zhang and Yu.[25] For the area characteristics, generalized estimating equations were used to account for the correlation between women residing in the same census tracts.

To examine interaction between the individual and area characteristics, stratified analyses were conducted and multiplicative interaction terms were modeled. Associations between individual characteristics and the proportion of lesions with HPV 16/18 were estimated at the different levels of area-based characteristics using binary logistic regression. P values for the statistical significance of the interaction were obtained from generalized estimating equations models that included the cross-product of the individual and area measures. A total of 6 models were run: one for each of 2 individual measures (race/ethnicity and insurance status) by each of 3 area measures (race, ethnicity, and poverty).

To determine the independent effects of individual and area characteristics, generalized estimating equations were used to account for correlation by census tract. All models included terms for measures previously shown to be associated with HPV 16/18, including age and diagnosis grade, thereby producing adjusted effect estimates that were controlled for potential confounding.[19] First, a full model was run that included the 2 individual and 3 area measures. Then, backward selection of variables was conducted by removing the least significant variables one at a time, and repeating this process until all variables remaining in the model were statistically significant at P < .05.

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. FUNDING SOURCES
  8. CONFLICT OF INTEREST DISCLOSURE
  9. REFERENCES

During 2008 to 2010, a total of 1869 New Haven County women aged 18 to 39 years were diagnosed with CIN2/3/AIS and reported to the Connecticut HPV-IMPACT surveillance system, of whom 1831 (98.0%) were successfully geocoded. Of these women, 966 (52.8%) had HPV typing completed. Reasons why specimens were not yet typed included pending requests for specimen to pathology laboratories (n = 64, 3.5% of total), pending receipt of specimen from pathology laboratories (n = 468, 25.6%), pending shipment to the CDC (n = 84, 4.6%), or pending specimen processing at the CDC (n = 104, 5.7%). Pathology laboratories deemed specimens insufficient to process for typing in 145 cases (7.9%). An additional 134 women were excluded because their specimen tested negative for all HPV types (n = 39, 4.0% of typed specimens) or because they received HPV vaccine prior to diagnosis (n = 95, 9.8%), resulting in a sample size for analysis of 832. Of these women, race/ethnicity and insurance status were known for 671 (81%) and 798 (96%), respectively. Women who were included were significantly more likely to have higher grade lesions and be older, although the magnitudes of these differences were less than 10% (Table 1).

Table 1. Comparison of Reported CIN2/3/AIS Cases Among Women Aged 18 to 39 Years in New Haven County, Connecticut, During 2008-2010 by Inclusion Status (n = 1831 Unless Otherwise Noted)
 IncludedNot Includeda 
Characteristicn = 832 (45.4%)n = 999 (54.6%)P
  1. Abbreviations: AIS, adenocarcinoma in situ; CIN2/3, cervical intraepithelial neoplasia 2/3; HPV, human papillomavirus.

  2. a

    Reasons for exclusion included specimens not typed (n = 865), specimens did not have any HPV detected (n = 39), or women had received HPV vaccine prior to diagnosis (n = 95).

Race/ethnicity (n = 1400)   
Black101 (45.1)123 (54.9).535
Hispanic153 (50.0)153 (50.0) 
White418 (48.0)452 (52.0) 
Health insurance (n = 1723)   
Public/none285 (44.3)358 (55.7).201
Private513 (47.5)567 (52.5) 
Age, y   
18-29536 (43.8)689 (56.2).040
30-39296 (48.8)310 (51.2) 
Diagnosis   
CIN2517 (42.7)693 (57.3).001
CIN 2/3, 3, or AIS315 (50.7)306 (49.3) 
Proportion black   
≥20%238 (44.7)295 (55.3).665
<20%594 (45.8)704 (54.2) 
Proportion Hispanic   
≥20%274 (44.4)343 (55.6).528
<20%558 (46.0)656 (54.0) 
Proportion in poverty   
≥20%182 (45.4)219 (54.6).981
<20%650 (45.5)780 (54.5) 

A total of 373 women (44.8%) had HPV 16/18 DNA detected, including 327 (39.3%) with HPV 16 only, 41 (4.9%) with HPV 18 only, and 5 (0.6%) with both HPV 16 and 18 detected. In the unadjusted main effects analysis, black and Hispanic women had a lower proportion of HPV 16/18 associated lesions compared to white women (P = .003 and P = .009, respectively); there was no significant difference by insurance status (P = .471) (Table 2). For the area characteristics, the prevalence of HPV 16/18 was lower in areas with ≥20% of the population black, Hispanic, and living in poverty compared with <20% (P = .002, P = .079, and P = .034, respectively).

Table 2. Prevalence of HPV 16/18 in CIN2/3/AIS Lesions: Unadjusted Main Effects of Individual- and Area-Level Characteristics (n = 832 Unless Otherwise Noted in Parentheses)
  Prevalence Ratio 
CharacteristicPrevalence(95% CI)P
  1. Abbreviations: AIS, adenocarcinoma in situ; CI, confidence interval; CIN2/3, cervical intraepithelial neoplasia 2/3; HPV, human papillomavirus.

Overall44.8 (373/832)
Individual-level characteristics   
Race/ethnicity (n = 671)   
Black34.7 (35/101)0.50 (0.32, 0.78).003
Hispanic39.2 (60/153)0.61 (0.42, 0.88).009
White51.6 (215/417)1.0 
Health insurance (n = 798)   
Public/none43.2 (123/285)0.90 (0.67, 1.20).471
Private45.8 (235/513)1.0 
Age, y   
18-2948.5 (260/536)1.27 (1.07, 1.51).004
30-3938.2 (113/296)1.0 
Diagnosis   
CIN 236.0 (186/517)0.61 (0.52, 0.70)<.001
CIN 2/3, CIN3, or AIS59.4 (187/315)1.0 
Area-level characteristics   
Proportion black   
≥20%36.6 (87/238)0.62 (0.46, 0.85).002
<20%48.1 (286/594)1.0 
Proportion Hispanic   
≥20%40.5 (111/274)0.77 (0.57, 1.03).079
<20%47.0 (262/558)1.0 
Proportion in poverty   
≥20%37.9 (69/182)0.69 (0.50, 0.97).034
<20%46.8 (304/650)1.0 

To determine the independent effects of individual and area-based measures, multivariate modeling adjusted for potential confounding by age and diagnosis was conducted. Race/ethnicity and area poverty remained significantly associated with lower prevalence of HPV 16/18. Black and Hispanic women had a lower prevalence of HPV 16/18 in lesions compared with white women (adjusted prevalence ratio [aPR] = 0.54, 95% confidence interval [CI] = 0.34-0.88 and aPR = 0.59, 95% CI = 0.40-0.88, respectively; P = .01 for both). Women living in areas with ≥20% of the population in poverty also had a lower prevalence of HPV 16/18 compared with women in areas with <20% in poverty (aPR = 0.59, 95% CI = 0.40-0.87, P = .007) (Table 3).

Table 3. Correlates of HPV 16/18 Prevalence in CIN2/3/AIS Lesions: Adjusted Associations Between Individual- and Area-Level Characteristics (n = 671)
 Adjusted Prevalence Ratio 
Characteristic(95% CI)aP
  1. Abbreviations: AIS, adenocarcinoma in situ; CI, confidence interval; CIN2/3, cervical intraepithelial neoplasia 2/3; HPV, human papillomavirus.

  2. a

    Results from multivariate generalized estimating equations model including all variables in the table. Area-level characteristic assessed with trend tests.

Individual-level characteristics  
Race/ethnicity  
Black0.54 (0.34, 0.88).010
Hispanic0.59 (0.40, 0.88).010
White1.0 
Age, y  
18-291.73 (1.23, 2.44).001
30-391.0 
Diagnosis  
CIN20.34 (0.25, 0.48)<.001
CIN 2/3, 3, or AIS1.0 
Area-level characteristics  
Proportion in poverty  
≥20%0.59 (0.40, 0.87).007
<20%1.0 

Because of the significant independent main effect of individual race/ethnicity, we examined the effects of race/ethnicity at each level of the area-based measures to explore possible interaction (Fig. 1). The same pattern of lower prevalence of HPV 16/18 among lesions in black and Hispanic women compared with white women was consistent across levels of area characteristics. There were no statistically significant interactions between race/ethnicity and any of the area characteristics (P > .20 for each) reflecting a widespread pattern of lower HPV 16/18 prevalence among black and Hispanic women in all areas of different racial, ethnic, and poverty compositions. Because the main effect of insurance status was not significant, the pattern of interactions is not presented but statistical modeling of interaction terms revealed no significant interactions between insurance status and any of the area characteristics (P > .20 for each).

image

Figure 1. Prevalence of human papillomavirus types 16 and/or 18 (HPV 16/18) in cervical intraepithelial neoplasia 2/3 (CIN2/3) and/or adenocarcinoma in situ (AIS) lesions among black, Hispanic, and white women is shown by area-level measures of race, ethnicity, and poverty.

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DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. FUNDING SOURCES
  8. CONFLICT OF INTEREST DISCLOSURE
  9. REFERENCES

Analyzing the distribution of HPV types by sociodemographic factors is important for monitoring the impact and determining the prevention potential of HPV vaccines, both overall and among women who bear a disproportionate burden of cervical cancer. This analysis expands current knowledge about disparities in HPV 16/18 prevalence in high-grade cervical lesions among a population before vaccine impact, by including examination of area-based measures in addition to individual sociodemographic characteristics. A key finding from this study is the significantly lower proportion of CIN2/3/AIS high-grade lesions with HPV 16/18 among women living in areas of greater poverty. This finding is consistent with a growing body of literature about the robust association between poverty and differences in health and raises the concern that current vaccines may have a lower impact among women living in impoverished areas.[24, 26] Reasons for differences in HPV type distribution by area poverty are not clear, but they may reflect complex associations between individual and area-based measures or the presence of geographically defined sexual networks. Monitoring vaccine impact on high-grade lesions by geographic measures of poverty will be an important part of ongoing assessments.

These findings do not differ from the 5-site HPV-IMPACT project with respect to a difference in vaccine types by individual race and ethnicity: HPV 16/18 was less likely to be detected in lesions among black and Hispanic women compared with those of white women.[19] This finding raises a similar concern to that for poverty, namely, that racial/ethnic minorities may experience lower benefit in terms of vaccine protection against high-grade cervical lesions. Given that black, Hispanic, and women of lower socioeconomic status are also less likely to have regular cervical cancer screening and to be disproportionately impacted by cervical cancer, these data raise concern that disparities will widen if effective preventive interventions other than HPV vaccine are unavailable or unequally accessed.[2, 27, 28] We also observed a higher HPV 16/18 prevalence in higher grade lesions and among younger women that supports other reported findings in the literature.[18, 19] The association with higher grade lesions was expected, given the well-established associations between HPV 16/18 and invasive carcinomas.[16, 17] Although reasons for the association with younger age are less clear, this finding may be related to the hypothesis that HPV 16/18–associated lesions progress more rapidly.[29]

A nonavalent vaccine is currently in development that protects against additional types (HPV types 31, 33, 45, 52, and 58) and holds promise for greater protection that may provide additional benefits for the groups that are disproportionately impacted by types other than HPV 16/18 (as described elsewhere[19]).[30, 31] In the meantime, widespread vaccination programs that increase uptake and completion rates of the 3-dose series can benefit all women, including racial/ethnic minority and low-income populations who have higher cervical cancer incidence and mortality, by protecting those who would acquire infections caused by HPV 16/18.

The following limitations should be noted. First, a large number of women were excluded from analysis, primarily because their specimens had not undergone DNA testing, and those who had typed specimens were somewhat different from excluded women with respect to age and diagnosis. Furthermore, missing race, ethnicity, and insurance data for some women resulted in further exclusions. Therefore, the possibility of some selection bias cannot be ruled out. Second, some women had both HPV 16/18 and another HPV type detected (11% of the total sample), and we cannot know which HPV type caused the detected lesion, thus making it difficult to assess the potential for vaccine impact with certainty. Third, the extent to which our findings may generalize to other populations is not known. Fourth, because different HPV types progress to cervical cancer at different rates, it is difficult to say how these results may apply to monitoring impact on invasive disease. However, notable strengths of this analysis include high case ascertainment of a common diagnosis through a mandatory statewide surveillance system, exclusion of women who had received HPV vaccine in order to focus on pre–vaccine-impact associations, and the first examination of area-based measures.

This study adds to the evidence about HPV type distribution in high-grade cervical lesions during the pre–vaccine-impact era that is needed to monitor future vaccine impact. Our findings suggest that monitoring impact by both individual race/ethnicity and area-based poverty will be important. Second-generation vaccines directed against additional HPV types might provide additional benefit for some racial/ethnic and socioeconomic groups.

FUNDING SOURCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. FUNDING SOURCES
  8. CONFLICT OF INTEREST DISCLOSURE
  9. REFERENCES

This work was funded by Cooperative Agreement CIU01000307 from the Centers for Disease Control and Prevention. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of CDC.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. FUNDING SOURCES
  8. CONFLICT OF INTEREST DISCLOSURE
  9. REFERENCES
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