This population-based prospective cohort study of 7482 women with normal cytology from the general population, with follow-up for up to 13.4 years through a routine screening system with virtually no loss to follow-up, provides estimates of the absolute risks for high-grade cervical lesions (ie, CIN2, CIN3, or worse) after infection with specific high-risk HPV types and estimates of the long-term absolute risk for high-grade CIN or cancer) after persistent infection with various high-risk HPV types. We found that HPV16 was the HPV type with the greatest carcinogenic potential. More than 25% of women with normal cytological findings who were HPV16 DNA positive at study baseline developed CIN3 or worse within 12 years. Although women with normal cytology who were positive for other high-risk HPV types, including HPV18, HPV31, HPV33, and HPV58, also had high risks for developing high-grade cervical lesions, those risks were much lower than those associated with HPV16 DNA positivity.
It can be difficult to determine the risk of developing high-grade cervical lesions that is associated with a specific HPV type in women with multiple HPV infections because the risk might be influenced by co-infection with other HPV types. Therefore, we also examined the absolute risk of high-grade cervical lesions after a single infection with a specific HPV type in an attempt to isolate the effect of the individual HPV type. Women who had a single infection with HPV16 (26%) or HPV18 (15.4%) had the greatest absolute risk for subsequent CIN3 or worse, followed by women who had a single infection with HPV33 (12.8%), HPV31 (9.8%), HPV35 (9.1%), HPV58 (8.3%), or HPV45 (6.4%). It is noteworthy that CIN3 or worse was not observed during the follow-up time up to 13.4 years after a single infection with five other high-risk HPV types (ie, HPV39, HPV59, HPV68, HPV53, and HPV66). Our results for HPV16 and HPV18 are similar to those of cohort studies in the United States with 10 years of follow-up (3
) and in Costa Rica with 5–7 years of follow-up (2
), in which HPV16 was consistently the HPV type that was associated with the highest risk for CIN3 or worse, followed by HPV18. In a recent cohort study from Sweden (6
), the absolute risk of CIN3 or worse that was associated with HPV18 was much lower than the absolute risk associated with HPV16, and women with an HPV33 infection had the same high absolute risk of CIN3 or worse as women with an HPV16 after 4 years of follow-up. In our previous report on the absolute risk of CIN3 or worse in women with normal cytology who concurrently tested positive for high-risk HPV in the HC2 assay, we found that a single positive test is a good predictor of high-grade CIN (9
). In this study, the detection of HPV16 DNA, and of HPV18 DNA, allowed a better stratification of women with regard to their subsequent risk of CIN3 or worse. In line with this finding, it has been suggested that women who test positive for HPV type 16 or 18 could benefit from immediate colposcopy (16
Because viral persistence rather than transient viral infection plays a crucial role in cervical carcinogenesis (17
), it could be hypothesized that measures of viral persistence would increase the accuracy of cervical cancer screening systems. Few studies, to our knowledge, have addressed the longer-term absolute risk of CIN3 or worse among women with a persistent HPV infection (11
). Previously, we found that being positive twice for high-risk HPV infection measured by HC2 tests (ie, persistence of a prevalently detected high-risk HPV infection) 2 years apart in women who initially had normal cytology was a substantially better predictor of risk for subsequent CIN3 or worse compared with one positive test during an 11-year follow-up (9
). Here we have updated this finding with more than two additional years of follow-up. Although our results indicate that persistence of prevalently detected high-risk HPV DNA positivity (by HC2) can be used to stratify women into different categories of risk for high-grade CIN, this study also showed that even one newly detected HPV16 infection was associated with virtually the same absolute risk of CIN3 or worse as were two positive high-risk HC2 tests. This finding points to the relative value of HPV genotyping for defining individual risk compared with HPV DNA detection tests that are based on a cocktail of probes and thus cannot differentiate the importance of specific HPV types with regard to the risk for cervical cancer. This result is somewhat in contrast to those of Koshiol et al. (18
), who reported that the persistence of specific HPV types was not more strongly associated with the risk of CIN3 or worse compared with repeated positive tests for high-risk HPV without type distinction. The apparent discrepancy between these findings could partly be explained by the fact that the women in this study were younger than those in most other studies, including, possibly, those in the study of Koshiol et al., and younger women are more likely than older women to have a prevalent transient HPV infection detected, to clear the infection, and to acquire new infections. Consequently, HPV type–specific persistence may be a better risk stratifier (compared with persistence of high-risk HPV infection measured by the HC2 test) in younger women than in older women because a higher proportion of older women who test positive twice for high-risk HPV by HC2 (persistence of prevalently detected HPV infection) will, in fact, have persistence of a specific HPV type. This hypothesis is supported by findings of a recent study from Costa Rica (11
) that was similar to this study.
We also examined the importance of the individual HPV types by looking at the combined picture of prevalence, the tendency of various specific high-risk HPV types to persist, and progression to CIN3 or worse given persistence. We found that the phylogenetic group predicted both tendency to persist and carcinogenic potential because species in the alpha 9 group were more likely to persist and to progress when they persisted compared with those in the alpha 7 or alpha 5 and 6 groups. Again, HPV16 had the highest rates of prevalence and persistence and the greatest probability of leading to progression when it persisted. The estimated probability of developing CIN3 or worse at 12 years was 47.4% (95% CI = 34.9% to 57.5%) among cytologically normal women with a persistent HPV16 infection.
Persistence alone, however, may not be unequivocally and strictly related to carcinogenicity given that we observed a relatively high rate of persistence for HPV types that are only rarely found in cervical cancer and that the persistence of some HPV types (eg, HPV53, HPV56, HPV59, HPV66, and HPV68) was never or only rarely followed by CIN3 or worse. Given these findings, clinically useful algorithms for HPV persistence in primary cervical cancer screening are not entirely straightforward and thus difficult to define, and other issues will have to be resolved before HPV persistence can be taken into account in cervical cancer screening programs. For example, although HPV16 persistence in particular is a strong predictor of risk of high-grade cervical lesion or worse, women might not agree to return for retesting in 1–2 years to assess potential HPV persistence, and compliance with follow-up might decrease. An alternative would be to use a single detection of one or more of the most high-risk HPV types, such as HPV16 and/or HPV18 and perhaps HPV31 and HPV33 as well, and for infections with the other high-risk HPV types, a single combined test for infections could be used, given our finding that the subsequent risk for CIN3 or worse for them remains low for several years (). It is reassuring that HPV DNA–negative women retained their low risk of CIN3 or worse for many years, suggesting that frequent screening of these women is unnecessary.
The strengths of this study include the relatively large cohort, the population-based study design, the long follow-up time, and the virtual absence of loss to follow-up because of the existence of the unique personal identification number and the complete nationwide registers used for the follow-up. This study also has some limitations. First, due to small numbers, we were unable to examine all HPV types individually as single-type infections in all analyses, which may have resulted in an overestimation of the rates of progression after persistence of some lower-risk HPV types. Second, because we measured HPV only twice, 2 years apart, we could not assess the duration of persistent infection and its role in the risk of progression, which has been suggested previously (18
). The prevalently detected infections in this study had persisted for an unknown length of time. The duration of a persistent HPV infection is important to take into consideration if HPV testing is going to be used as a risk stratifier in screening, especially in screening programs with a short screening interval, because the likelihood of continued persistence increases with the length of time that the infection has already persisted and, again, is associated with the risk of progression. In addition, because we had no information on HPV variants, some of the infections that were defined as persistent might have been re-infections with the same HPV type, which would have resulted in an underestimation of the risk of CIN after HPV persistence. Finally, because we used a passive approach to follow up the women after the two active study examinations, this investigation is based on everyday clinical management rather than on management performed in a randomized clinical trial. This distinction might be important in the case of intensive screening and management of low-grade lesions because, in this case, the natural history may change, which could possibly lead to an underestimation of the occurrence of CIN3 or worse. However, as previously reported (9
), cervical cancer screening in Denmark is recommended every 3 years from the age of 23 years, and the management of low-grade lesions tends to be less aggressive than it is in countries such as Germany and the United States.
In summary, this relatively large prospective cohort study clearly demonstrates that infection with HPV16 is the most prevalent and has the greatest tendency to persist and the highest probability for progression when it persists. HPV18, HPV31, and HPV33 were the high-risk HPV types other than HPV16 that was associated with high absolute risks for progression. The main predictor of subsequent risk of CIN3 or worse was HPV16 persistence. One positive test and persistence (ie, two positive tests) for high-risk HPV types other than HPV16, HPV18, HPV31, and HPV33 were associated with low absolute risks of CIN3 or worse that lasted for years, indicating that a different follow-up algorithm is needed for women positive for these HPV types. Although this study contributes findings that may be important for designing more effective screening programs, several issues remain to be resolved before HPV persistence can be used in primary screening, including a standardized definition of persistence and a standardized follow-up algorithm that will not jeopardize compliance. Until other markers of HPV persistence or progression using a single measurement, are identified, type-specific HPV testing may be useful in stratifying risk in cervical cancer screening. Our results indicate that some of the HPV types that are currently classified as high risk show little potential for progression to high-grade lesions, even in cases of persistent infection; for example, our results for HPV66, which showed that persistence of this HPV type was not followed by any cases of CIN3 or worse, are in agreement with the recent re-assessment of the carcinogenicity of HPV types by an International Agency for Research on Cancer Working Group (19
Finally, it should be mentioned that although there is an increased risk of developing CIN2 or worse associated with specific high-risk HPV types, most CIN2, if left untreated, will never progress to invasive cancer, and, in some women, notably younger women, these lesions can regress. This information is now of considerable importance given the demonstration of some obstetrical morbidity associated with definitive treatment of these lesions (20
). By contrast, the risk of CIN3 progressing to cancer is substantial if left untreated (22
This study provides the first population-based data to our knowledge on the long-term absolute risk of high-grade CIN after (prevalently detected) persistent infection with individual high-risk HPV types. Our findings may facilitate the interpretation of the results of screening trials, may be useful in the development of more specific HPV tests, and may identify issues that need to be resolved to obtain the greatest clinical value from HPV testing. Finally, these results may be of value in the development of new generations of prophylactic HPV vaccines and may influence the current classification of 13 class I carcinogenic high-risk HPV types.