Human papillomavirus (HPV) infection can cause cervical cancer, and the cancer burden attributed to this infection varies according to geographical region and the genotype of people who are infected with HPV. It is most prevalent in Africa, Asia, and some European countries, compared to other regions of the world. Thirteen high-risk HPV (HRHPV) types, which are carcinogenic or presumed to be carcinogenic in humans, are frequently detected in patients who develop cervical cancer. For example, HPV types 16 and 18 account for approximately 70% of all cervical cancer cases [1]. Several epidemiological studies have evaluated the genotype-specific prevalences of HPV, factors that influence HPV acquisition, and the risk of cervical cancer development according to HPV infection status [1-4]. Furthermore, there are various factors that influence cervical cancer development, including viral infection and host factors. Thus, the prevention of cervical cancer relies on our understanding of the progression of HR-HPV infections, which involves persistence, clearance, new infections, and other determinant factors. However, most HPV infections are transient and rarely persist [5,6]. Only a few studies in specific countries and populations have considered the HPV genotype-specific incidence, persistence, clearance, and new infections [7-17]. Furthermore, only a few studies have evaluated the natural history of HPV infections in Asian countries, where the prevalence of HPV is high. Moreover, the association with cervical cancer development has been highlighted as a major health issue among women. In Korea, the prevalence of HPV is approximately 20% among women with normal cytology, based on the Papanicolaou test (Pap test) [18]. In addition, cervical cancer is a leading cancer among Korean women, with an age-standardized incidence of 9.5/100,000 women in 2012 [19]. Despite these data, there is a lack of Korean studies regarding changes in HPV infection status and the factors that contribute to these changes. Therefore, the present study aimed to examine HPV type-specific infections, their change patterns, and the factors that were involved in these changes, based on two HPV tests that were performed 2 years apart for Korean women who visited two centers.

Among the 4,588 women who participated in the baseline survey, only 1,224 women were followed-up and included in the present analysis. When we compared the participants from the baseline and follow-up testing, we did not observe any noticeably differences in their socio-demographic, behavioral, and reproductive characteristics, or in their HPV infection rates (S1 Table). The mean patient age at baseline was 47 years (range, 21 to 66 years), most participants were married or cohabitating, 17% of the participants had ≥ 2 sexual partners during their lifetime, and the prevalence of HPV infections was 8.6%. At the 2-year follow-up, the prevalence of HPV infection decreased to 6.0%. All participants and the high-risk group for HPV infection were most likely to live in Busan, have ≥ 2 sexual partners during their lifetime, have a family history of cancer, and have a history of HPV infection at baseline (Table 1).

Among the 105 HPV infections from the baseline, only 13 infections (12.4%) were remained, and the 10 cases of them are HR-HPV types including genotype 33, 45, 16, 35, and 52. Among the 1,119 women who tested negative at the baseline, 54 women (5%) exhibited new HPV infections at the followup. The most frequent HPV genotypes found in new infection cases were genotypes 39, 52, 16, 51, and 58 (Table 2).

Persistent HPV infections were relatively common among unmarried women, women with ≥ 2 more sexual partners during their lifetime, and women with a family history of cancer. Cleared and new infections were relatively common among women with a history of a sexually transmitted disease or Chlamydia infection. Persistent and new infections were more common in Busan, compared to Suwon (Table 3). After adjusting for all potential factors, having ≥ 2 sexual partners exhibited a marginally significant association with new HPV infections (OR, 2.0; 95% CI, 1.0 to 3.9). No factors were significantly associated with HPV persistence and clearance (Table 4).

The present study provided information regarding HPV genotype-specific infections and their change patterns among Korean women. These data are important, because relevant data are sparse in this population, despite the prevalence of HPV infection and incidence of cervical cancer. Furthermore, ours is the first report regarding the natural history of HPV infection among Korean women who visited clinics or hospitals for routine health examinations (e.g., cervical cancer screening), rather than for the diagnosis or treatment of gynecological symptoms. Moreover, we performed both HPV LA and HPV genotyping to confirm the HPV type-specific changes that occurred during the course of the patients’ infections.

Most infections (87.6%) were cleared within 2 years, although 5% of women developed new HPV infections during this time. These results are consistent with findings from previous reports, which suggested that up to 90% of incident infections are cleared within 2 years [5], and that the estimated half-life of HR-HPV infections is 8-10 months [6]. However, HPV persistency varies according to country, and our results revealed a lower persistency (12.4%), compared to results from previous studies in Brazil (19.2%) [9], Italy (49.1%) [16], Denmark (31.4% [21] and 26.9% [12]), the United States (39%) [10], and the Netherlands (44.1%) [22]. These differences may be related to differences in the target populations, who may have different potential risks for persistent infection, and/or differences in the baseline and follow-up testing interval. Nevertheless, the result from a meta-analysis on the patterns of HPV persistence worldwide, the persistence rate tends to decrease with time passed after infection [23]. The association between age and HPV persistence remains controversial. A review of the natural history of HPV infection suggests that age indirectly affects HPV persistence [24], while others studies have reported that younger age is associated with an increased risk of persistence [9,21]. Our results were similar to the persistence rate among young Korean women (17-26 years), despite the differences in the study populations’ ages and the higher rate of new HPV infections among young Korean women [8]. The present study also revealed that the most persistent HPV type was HPV 33, which was followed by genotypes 45, 16, 35, and 52. These results are different with the findings from previous studies, which revealed that HPV 16 and 18 were the most prevalent incident and persistent HPV types [9,16,21,23].

After adjusting for potential risk factors, we did not observe any significant associations with HPV persistence. Some studies have revealed similar findings, although other studies have revealed significant positive [11,12,16,25] or inverse [9,21] associations between age and HPV persistence, including a large retrospective study of Korean women. Another study found that smokers were likely to have persistent HPV infections [22]. In the present study, the number of sexual partners was the only factor to exhibit a marginal association with new HPV infections, and this result is similar to the findings from previous studies [26-28]. The current findings including distributions of HPV genotype-specific infections and their change patterns among Korean women may help improve our understanding of the natural history of HPV infection in the same populations, and facilitate the development of prophylactic vaccines for specific populations.

The present study has several limitations that warrant consideration. First, there is the possibility of selection bias, based on the low follow-up participation rate. However, the participants exhibited similar characteristics at the baseline and follow-up testing, although HPV positivity was slightly more common at the follow-up (10.1%), compared to the baseline (7.7%) (S1 Table). These differences are unlikely to have significantly influenced our findings. Second, the small samples of women who were followed-up and women with HPV (n=105) might have influenced the absence of significant associations between HPV persistence and the potential risk factors in the multivariate analyses. Furthermore, the number of sexual partners not during the last two years when participants followed up but during their life time was included as one of important factors involving HPV infection status and their change. It could be linked to insignificant results on the association between sexual behaviors and change of HPV infection status. Nevertheless, our results regarding HPV change patterns evaluated a large number of Korean women at the baseline (n=4,558), and lifetime sexual behavior could represent overall tendency of sexual behaviors even if it is not the specific sexual behaviors change during the time followed up. Third, detailed data regarding the natural history of HPV infection should be obtained using long-term studies with repeated measurements over shorter intervals (e.g., every 6 months or every year), and we only evaluated HPV infection status after a 2-year interval. Also, information on further diagnosis and treatment such as Pap smear, colposcopic biopsy, and laser therapy during the follow up periods, which might be important factors associated with change of HPV infection status, were not available in this study. Fourth, HPV genotyping was only performed for HPV LA-positive cases at the baseline, although there was disagreement between the test results from HPV LA (6.0%) and HPV HC2 (7.0%) (data not shown). However, the correlation of HPV LA with HR HPV positivity is higher than that with HPV HC2, and this level of disagreement is likely acceptable, based on previously reported results [29,30].

In conclusion, the present study provided information regarding changes in HPV status over a 2-year interval among Korean women. Most infections resolved spontaneously, and only small fractions of the women experienced persistent infections or new HPV infections. The predominant persistent HPV genotypes were different in our study, compared to those from previous studies. We also found a marginally significant association between sexual behavior and HPV infection, but not with HPV persistence or clearance. Further large-scale studies are needed to better understand the natural history of HPV infections and the factors that are associated with HPV type-specific change patterns.