The median age of the cohort at enrollment was 15.0 years (mean, 15.3 years; SD, 0.9 years; range, 14–17 years); 85.0% of subjects were African-American, 11.7% were white, and 3.3% were Hispanic. The median follow-up period for the 60 subjects was 2.2 years (mean, 2.2 years; SD, 34 days). Three subjects did not report ever having sexual intercourse; the remaining 57 subjects were sexually active. The median number of lifetime sex partners for these 57 subjects was 2.0 (21.1%, 15.8%, and 49.1% had reported 1, 2, and ≥3 lifetime sex partners, respectively).
Analysis of swab specimens for HPV
A total of 2458 swab specimens were collected (mean, 41.0 swabs/subject; SD, 9.3 swabs/subject; median, 41.5 swabs/subject; range, 18–64 swabs/subject). Of these swabs, 353 were from the cervix, and 2105 were self-obtained vaginal swabs. Swabs that yielded a clear positive band on the strip assay for β-globin for both the high -and low-abundance controls were considered adequate for HPV analysis (). Overall, 2107 (85.7%) of 2458 swabs were adequate for analysis of HPV. Vaginal swabs yielded a higher percentage of β-globin–positive specimens than did cervical swabs (86.4% vs. 81.9%; p = .027). The 14.3% of swabs that did not contain adequate cellular material, as determined by negative amplification of β-globin, may have been a result of poor collection technique on the part of the subject or the nurse. Alternatively, DNA may have been lost during the purification procedure prior to performance of the PCR/reverse blot strip assay.
Cervical and vaginal swab positivity for β-globin, human papillomavirus (HPV), high-risk (HR) HPV, and low-risk (LR) HPV.
Overall, 45.3% of adequate swabs were positive for HPV. Vaginal swabs were more likely to be positive for HPV than were cervical swabs (46.1% vs. 40.1%; p = .003). However, the higher overall HPV prevalence in vaginal swabs was driven by a higher prevalence of LR types. Testing the significance of the proportion differences in the HR/LR strata showed a significant value for LR types (p = .02) but not for HR types (p = .31).
HR HPV types (16, 18, 26, 31, 33, 35, 39, 45, 51, 52, 55, 56, 58, 59, 68, 73, 82, 83, and 84) were detected in 38.6% of all adequate swab specimens (). LR HPV types (6, 11, 40, 42, 53, 54, 57, and 66) were detected in 19.6% ().
HPV 52 was the most frequently detected type, found in 13.6% of all adequate swab specimens (). HPV 16 was found in 11.7% of all specimens, and HPV 59 was found in 6.6% of all specimens. The most frequently detected LR types were HPV 66 (6.1% of adequate specimens) and HPV 6 (5.6%).
Figure 1 Distribution of specific human papillomavirus (HPV) types in cervical swabs, vaginal swabs, and all swabs. The upper rows show the no. of swabs that were positive for each HPV type, and the lower rows show the percentage of swabs that were positive for (more ...)
Several types were detected in 3%–5% of specimens, including HR HPV types 18, 51, 56, 73, and 84 and LR HPV types 53 and 54 (). Other types were detected in <3% of all swabs.
In general, most HPV types were detected in a higher percentage of vaginal swabs than cervical swabs (). HPV types 56, 59, and 68 were exceptions, being detected more often in cervical swabs than in vaginal swabs.
Detection of multiple HPV types in swab specimens
Of all 2107 adequate swab specimens, the number of HPV types detected per specimen ranged from 0 to 8, with 54.7% of the specimens testing negative for HPV. Of the 1818 adequate weekly vaginal swab specimens, the number of HPV types detected was between 0 and 6, with 53.9% testing negative. There were 289 adequate quarterly cervical swabs. The number of HPV types detected per cervical swab ranged from 0 to 8, with 59.9% of cervical swabs testing negative. The mean number of HPV types detected per vaginal swab was 0.99 (SD, 1.43), significantly higher than the mean of 0.79 (SD, 1.21) detected in cervical swabs (p = .047). A single HPV type was detected in 44.1% of all the HPV-positive swabs, and multiple HPV types were detected in the remaining 55.9%. Overall, a similar percentage of cervical swabs contained multiple types compared with vaginal swabs (56.9% of cervical swabs; 55.7% of vaginal swabs). However, ≥4 HPV types were detected in 12 (10.3%) of 116 of HPV-positive cervical swabs, compared with 132 (15.8%) of 838 of HPV-positive vaginal swabs.
High cumulative prevalence of HPV infection in adolescent women
We next considered how often adolescent women (in contrast to all subjects) had detectable infection with a specific HPV type during the study (). Three subjects’ specimens all tested negative for HPV (all 3 reported never having sexual intercourse). Eight subjects had only a single swab specimen that tested positive for any HPV; therefore, these subjects were not considered to be infected by our definition. These single positive specimens were excluded because they may have represented laboratory error or deposition of HPV by a sex partner rather than true infection. Therefore, 11 of the adolescent women were considered to be HPV negative throughout the entire study period, and the remaining 49 subjects were considered to be HPV positive.
Point (enrollment and last study visit) and cumulative prevalence of all human papillomavirus (HPV) detection and detection of high-risk (HR) and low-risk (LR) HPV.
The first and last specimens collected for each subject were analyzed for HPV, and the results were compared with the cumulative detection for every swab specimen collected for each individual subject. HPV positivity increased numerically during the study period, for both HR and LR HPV types (). The cumulative prevalence of all HPV detection was 81.7% ().
depicts the cumulative HPV type distribution among all 60 adolescent women in the study. HPV 52 was detected at some point in 38.3% of all subjects, and HPV 16, the second-most-frequent type, was detected in 31.3% of subjects. Other frequently detected HR types included HPV 59 (found in 23.3% of all subjects), HPV 84 (21.7%), and HPV 18 (20.0%). Frequently detected LR types included HPV 66 (28.3% of all subjects), HPV 6 (25.0%), and HPV 53 (20.0%).
Human papillomavirus (HPV) type distribution among 60 adolescent women. The upper row shows the no. of swabs that were positive for each HPV type, and the lower row shows the percentage of subjects in whom each type was detected.
Frequent detection of multiple HPV types in adolescent women
The mean number of HPV types per HPV-positive subject (n = 49) was 4.9 (SD, 3.3 types; median, 5.0 types; range, 1–14 types). A single HPV type was detected in specimens from 18.4% of these individuals, whereas multiple types were detected in 81.6% who were HPV-positive. Adolescent women with abnormal Pap smears had a mean of 6.0 different HPV types (SD, 3.4 types), compared with a mean of 4.4 types (SD, 2.9 types) in adolescents who had normal Pap smears.
Persistence of HPV
Lengths of type-specific HPV runs were analyzed to gain insight into HPV persistence. The mean number of days of follow-up for subjects with HPV infections was 794 (median, 793 days; SD, 34 days; range, 712–869 days). Subjects who were HPV positive contributed 241 HPV “runs.” Of these subjects, 47 had Pap smear results and thus were included in the survival analysis. The median length of all 239 HPV runs was 168 days. Among all the runs, 166 were of HR HPV types, with a median length of 188 days. There were 73 LR HPV runs, with median length of 89 days. shows Kaplan-Meier estimates of HR and LR HPV persistence (runs). When the impact of the censored HPV runs was taken into account, Kaplan-Meier estimates of the median clearance time were 226 and 170 days, in HR HPV and LR HPV, respectively. Using a Cox regression model with random subject effect, we found that HPV type was significantly associated with run length (p = .034). The risk ratio associated with HR HPV was 0.683. This implied that there was a 31.7% reduction in the probability that an HPV run became undetectable if the run was of an HR HPV. Persistence of any HPV was marginally associated with abnormal Pap smear results (p = .053). shows Kaplan-Meier estimates of runs of HPV in subjects with and without abnormal Pap smear results. The Kaplan-Meier estimates of the median clearance time were 255 and 170 days, in HPV runs with and without abnormal Pap smear results, respectively. The risk ratio associated with an abnormal Pap smear was 0.597, suggesting that the likelihood of an HPV run accompanied by abnormal Pap smear results becoming undetectable was 59.7% of that in a run without an abnormal Pap smear. In other words, HPV runs were more persistent in subjects with abnormal Pap smears. We did not have enough observations to calculate Kaplan-Meier estimates for abnormal/normal Pap smears associated with either HR or LR HPV.
Figure 3 Kaplan-Meier estimates of human papillomavirus (HPV) clearance time in high-risk (HR; oncogenic) and low-risk (LR; nononcogenic) runs. The estimated median clearance time for HR HPV was 226 days, and the estimated median clearance time for LR HPV was (more ...)
Figure 4 Kaplan-Meier estimates of human papillomavirus (HPV) persistence in individual subjects with or without abnormal Pap smear results. The Kaplan-Meier estimates of the median clearance time were 255 and 170 days, in HPV runs with and without abnormal pap (more ...)
High frequency of abnormal cervical cytologic tests in adolescent women
Three subjects who did not report any past or present sexual intercourse did not undergo cytologic testing. Pap smears were available for 54 of the remaining 57 adolescent women in the study. The mean number of Pap smears acquired per subject was 3.0 (SD, 1.6; range, 1–8). Among the 161 Pap smears obtained, results were obtained for 158 (98.1%). Of these, 69.6% were normal, and 30.4% were abnormal (with ASCUS, LGSILs, or HGSILs) (). ASCUS was the most common abnormality, occurring in 28 Pap smears. LGSILs occurred in 19 Pap smears, and HGSILs occurred in 1. Among the 54 subjects with at least 1 Pap smear, 37% had at least 1 abnormal test, and 63% had entirely normal tests ().
Distribution of all Pap smear results in 54 subjects and the most severe Pap smear result during the study period for each individual subject.
The most severe grade of Pap smear abnormality for each of the 54 adolescent women was compared with HR HPV positivity. As expected, Pap smear abnormalities were associated with detection of HR HPV. All 20 subjects with abnormal Pap smears had HR HPV detected at some point during the study period, compared with 24 of 34 subjects with normal Pap smears (p = .009; Fisher’s exact test). The proportions of HR HPV– and LR HPV–positive specimens within the abnormal Pap smear categories were not determined; this was because we used a subject-level summary definition for Pap abnormality, whether or not any HPV-positive specimen was ever of the HR category. The tests for HPV and Pap were different, and dates of specimen acquisition did not always coincide. Therefore, a subject may have had 1 to several Pap tests with different results, as well as many HPV tests with variable results.
Analysis of individual adolescent women
The complete analysis of individual subjects illustrated the acquisition and persistence of specific HPV types during the study. The analysis for 4 adolescent women is shown in .
Figure 5 The complete analysis of 4 individual adolescent woman (subjects A, B, C, and D), shown to illustrate the acquisition and persistence of specific human papillomavirus (HPV) types during the study period. The week in which the specimen was acquired is (more ...)
Subject A was infected with numerous HPV types (). HPV 16 was detected in the swab obtained from her first visit and persisted throughout most of the study period. HPV types 45, 52, 56, 59, and 68 (all HR types) were repeatedly detected. HPV types 6 and 54 (LR types) were also detected. Her most severe cytologic abnormality was LGSILs.
In subject B, HPV 56 was the only type detected, persisting for 9 months (). Cytologic testing revealed HGSILs as her most severe abnormality.
Subject C had no HPV types detected for the first 14 months of the study () but then became infected with numerous types that persisted throughout the study. Cytologic testing showed no abnormalities.
Subject D had no HPV types detected for the first 12 months of the study but then became infected with HPV types 83, 84, and 6, all of which were undetectable after several months (). She then became infected with HPV types 56, 68, and 54, which persisted throughout the remainder of the study. Cytologic testing revealed LGSILs as her most severe abnormality.