Invasive cervical cancer (ICC) is the third most common cancer among women worldwide, with an estimated incidence of 553,119 new cases and 288,109 deaths in 2010 1 . Persistent infection with one of the oncogenic human papillomavirus (HPV) genotypes is required to cause ICC 2 3 4 5 . More than 150 HPV genotypes have been identified and about 40 are known to infect the genital tract 6 7 .

To date, HPV genotypes identified as causing ICC have belonged to a few genetically related “high-risk” species of the mucosotropic α-genus (α-5, -6, -7, -9 and -11) 8 but other HPV genotypes could be involved 9 . Dichotomous classification into low- and high-risk HPV genotypes has often been used previously 4 10 . Alternatively, the International Agency for Research on Cancer (IARC) classified individual HPV genotypes into more categories based on available epidemiologic and mechanistic evidence of their carcinogenicity for cancer at any site. Thus, 12 HPV genotypes (HPV-16, -18, -31, -33, -35, -39, -45, -51, -52, -56, -58 and -59) are classified as “carcinogenic to humans” (Group 1), HPV-68 as “probably carcinogenic” (Group 2A) and 12 other HPV genotypes as “possibly carcinogenic” (Group 2B) 8 11 .

Two vaccines targeting HPV-16 and -18, which account for 70% of cervical cancers worldwide 12 13 , are currently available. Vaccination impact on the cervical cancer incidence remains uncertain, especially because genotype-specific prevalences of vaccine and non-vaccine genotypes might change after vaccine introduction through vaccine-induced cross-protection or genotype replacement 14 15 16 . The number of ICC cases associated with a given HPV genotype depends both on its prevalence in the general population and its oncogenic potential, which can be defined as the inherent and differential abilities of each genotype to trigger malignant transformation and induce cervical cancer 17 . Within categories of IARC-classified carcinogenic HPV genotypes, the risk of progression to ICC might differ by HPV genotype. Therefore, ranking the oncogenic potentials of HPV genotypes, independently of their respective prevalences, is challenging but essential to guide the formulation of second-generation polyvalent HPV vaccines and HPV-DNA–based screening tests.

This study was undertaken to rank HPV genotypes as causal agents of ICC according to their relative oncogenic potentials assessed by means of meta-analyses of published observational data. Oncogenic potentials herein are expressed using HPV-16 as the reference, since it has been recognized as the most carcinogenic HPV genotype 8 11 18 .

Conducting meta-analyses of published epidemiologic studies enabled us to rank 32 HPV genotypes according to their oncogenic potential relative to HPV-16. All HPV genotypes studied had much smaller estimated oncogenic potentials than HPV-16, reinforcing its being considered the “most potent HPV genotype” 8 11 . Following HPV-16, the HPV genotypes with the highest oncogenic potentials were, in descending order of their pooled ORs: HPV-18, -45, -69, -58, -31, -33, -34, -67, -39, -59, -73 and -52.

Globally, it is reassuring that the HPV genotypes accorded the highest relative oncogenic potentials in our analyses were indeed IARC-classified as carcinogenic. Moreover, the genotypes with the highest ORs and relatively narrow CIs, namely HPV-18, -31, -33, -39,-45, -52, -58 and -59, all belong to the α-7 and α-9 species and are IARC-classified as carcinogenic to humans with sufficient evidence 8 11 . Pertinently, all these genotypes except HPV-18 showed largely overlapping CIs, while HPV-18 (α-7) ranked closest to, but distinct from, HPV-16 (α-9), supporting its being considered separately from the other potentially oncogenic genotypes characterized by a risk continuum without a clear break point (Figure 2).

Surprisingly, HPV-35, -51 and -56, also IARC-classified as carcinogenic and commonly detected in ICC 57 , ranked lower in our analysis (OR ≤0.15). The same was observed for HPV-68 (α-7), currently IARC-classified as probably carcinogenic and among “probably high-risk genotypes” 10 . So, based on epidemiologic data alone, our analysis would suggest milder oncogenic potentials for HPV-35, -51, -56 and -68 than inferred from the IARC classification, although we acknowledge that more evidence (e.g., mechanistic) needs to be considered 9 .

Notably, our results provide insights into the oncogenic potentials of several genotypes currently IARC-classified as probably oncogenic in humans. Our meta-analytic assessment of the oncogenic potentials of HPV-69 and -82 (both α-5 species), -30 (α-6), -67 (α-9), and -34 and -73 (α-11) was based on small numbers of cases, which yielded particularly wide CIs. However, they ranked among carcinogenic HPV genotypes, which could suggest stronger oncogenic potentials than assumed so far. To date, evidence for HPV-30, -34 and -69 has relied on their phylogenetic analogy to other HPV genotypes, while HPV-67, -73 and -82 were positively associated with cancer but lack strong mechanistic evidence 8 11 . In contrast, HPV-53, -66 and -70, also placed in the probably carcinogenic subgroup 8 11 , had lower relative ORs in our analyses. Hence, overall, our analysis of available epidemiologic data provided more discrepant results for the probably carcinogenic genotype distribution.

Conversely, little to no mechanistic evidence supports that HPV-6 and -11 (both α-10 species), which commonly cause benign genital warts, can contribute to carcinogenesis and they remain unclassifiable as to their carcinogenicity in humans 8 11 . Our meta-analyses consistently ranked both at the end of the distribution with estimated pooled ORs ≤0.15. We should mention that our HPV-16 reference model did not allow us to disentangle less oncogenic from non-oncogenic genotypes.

Finally, no epidemiologic evidence suggests cervical oncogenicity for HPV-40 and -44 11 . In phylogenetic terms, these genotypes belong rather to non-oncogenic species (α-8 and -10, respectively) 58 and have been considered “low-risk” genotypes 10 . In our main analysis, these two genotypes ranked before HPV-6 and -11. However, their estimated ORs were based on limited data and their classification was not robust in the sensitivity analyses (Additional file 4). Taken together, our results do not support HPV-40 and -44 oncogenic potentials.

Strengths of our study derive from methodologic choices. To date, the assessment of the HPV-genotype–specific oncogenic potential in cervical cancer has mainly been based on HPV-genotype–prevalence data among cases 13 57 59 60 . However, that knowledge alone may be insufficient to fully appreciate each genotype’s oncogenic potential. For a given HPV genotype, low frequency in ICC (corresponding to a small etiologic fraction) could reflect low prevalence in the general population or low oncogenic potential. In our study, HPV-genotype ranking according to their prevalences in cases visibly differed from that according to their estimated relative oncogenic potentials. For example, HPV-39 and -59 (both α-7), about four times less prevalent than HPV-52 (α-9), had higher oncogenic potentials estimated by their pooled ORs (Table 2); yet all three genotypes are IARC-classified as carcinogenic.

To our knowledge, the risks associated with the different HPV genotypes have rarely been assessed and HPV-negative, rather than HPV-16–positive, subjects served as the reference group to calculate ORs 4 10 with at least one exception 61 . Our similar third sensitivity analysis found lower OR estimates of the same order of magnitude as those previously published 10 , e.g., respectively, 136.7 versus 281.9 for HPV-16 and 99.1 versus 222.5 for HPV-18. Notably, that third analysis showed no clear break point between HPV-18 and the other genotypes, with ORs decreasing progressively from HPV-16 to the end, unlike our main analysis. The choice of this reference group may be questioned because it takes uninfected cases into account for OR calculation, even though it is currently accepted that persistent HPV infection is required to cause ICC 2 3 4 . With few or no HPV-negative cases of cervical cancer expected, estimation of ORs and their CIs may become problematic. Therefore, we chose the unusual approach of using HPV-16–infected subjects as the reference category. HPV-16’s high oncogenic potential is well-documented 8 11 , this genotype is highly prevalent in cases 13 57 and often identified in women with normal cytology 62 63 . In our opinion, considering HPV-16–infected women as the reference group seemed more consistent with the natural history of cervical cancer and could be more appropriate for estimating HPV-genotype oncogenic potentials, regardless of their prevalence. However, the control group’s baseline risk of developing ICC cannot be considered low, meaning that ORs cannot be directly interpreted as an accurate estimate of the relative risk, even though they can be used to rank genotypes. Alternatively, estimating ORs relative to an established low-risk genotype, e.g., HPV-6, was limited by the small, if not inexistent, numbers of ICC cases positive for such a genotype.

Herein, we combined study ORs using the random-effects model, as sometimes recommended to perform meta-analyses of published data 64 . This approach implies wider CIs than in a fixed-effect model because, in addition to random fluctuations, the random-effects model allows for variability of the real risk. However, sensitivity analyses showed our results to be consistent with those obtained using Peto’s method (Additional file 4), thereby indicating that the wide CIs mostly reflected the scarcity of epidemiologic data, rather than the choice of statistical models.

Some authors questioned the use of CC in the random-effects model, when the underlying risk varies among studies 65 . Our sensitivity analyses with a halved CC factor differed only slightly from our main results. In contrast, applying no CC raised estimation difficulties preventing the calculation of two pooled ORs (Additional file 4). Nevertheless, our choice is supported by the consistencies, both external (with the literature) and internal (across other sensitivity analyses), of our findings after correction.

Our meta-analysis has several limitations that warrant being mentioned. First, we applied stringent selection criteria, including only studies with sufficient numbers of HPV-positive cases and controls. That choice rendered the several large investigations conducted in North America ineligible 66 67 , which is consistent with 85% of ICC cases occurring in developing countries 1 , and HPV-vaccine trials being conducted more frequently in Asia-Pacific, Europe or Latin America than North America 68 69 . Nevertheless, although the distributions of HPV genotypes vary across populations 18 57 59 63 , no evidence indicates that HPV-type–specific oncogenic potential could differ according to geographic area. Moreover, the continent did not explain heterogeneity in our meta-analyses.

Second, basing this study on summary data meant we could not control for age, despite its being a critical variable, closely associated with HPV infection, clearance, persistence and progression. Age information was frequently missing and rarely available for HPV-positive cases and controls specifically. Controls tended to be 10 years younger than cases on average, possibly reflecting different stages in the natural history of cervical cancer. The peak prevalence of cervical HPV infection coincides closely with first-time sexual intercourse, at around 20 years of age, while that of ICC occurs at 40–50 years 70 . It was reassuring that the comparability of age distributions between cases and controls did not clearly explain heterogeneity in our meta-analyses.

Third, the small number of cases infected with some HPV genotypes hindered precise estimations of their oncogenic potentials. This paucity is partly due to our strict definition of cases as having ICC. This choice was motivated by the natural history of cervical cancer, according to which precancerous lesions, even high-grade cervical intraepithelial neoplasia, may regress in a substantial proportion of cases 71 . Previous studies 17 72 might have been more permissive, assimilating high-grade lesions and ICC cases, especially longitudinal studies, often limited by the low numbers of ICC during the follow-up. Moreover, clinical management guidelines also recommend the excision of precancerous lesions, and will continue to do so as long as whether these would regress or progress cannot be foreseen 73 .

Fourth, we did not distinguish between ICC histologic types, even though HPV-18 could be more prevalent in adenocarcinomas than squamous cell carcinomas 13 . However, the HPV-genotype–specific distribution according to histologic type was seldom reported in selected studies. When histologic type was reported, most were squamous cell carcinomas, which is the most common histologic cervical cancer type 74 .

Fifth, our analysis was limited by the variety of sample types and HPV assays, as in previously reported meta-analyses of HPV-genotype–specific prevalences 57 59 60 . Although all HPV-detection methods were PCR-based, sensitivity and specificity of PCR protocols varied across studies and numerous HPV genotypes were not detected by some of them. However, each study used the same HPV-typing method for cases and controls, so it is unlikely that the differences among studies affected our estimates. Moreover, the heterogeneity in our meta-analyses was not explained by the HPV-detection threshold for cases.

Finally, because the components of multiple infections were seldom available, the oncogenic potential of each HPV genotype was assessed without distinguishing between single or coinfection. Thus, the oncogenic potentials of some HPV genotypes might have been overestimated in our meta-analysis if they had been coinfection partners with established high-risk genotypes, e.g., HPV-16 or -18, and were wrongly accorded equal weight in cancerous lesions even though the high-risk genotype was solely responsible for the lesions 11 . That possibility could explain HPV-11’s unexpectedly higher oncogenic potential. Moreover, for studies that did not report coinfection, misattribution of the causal HPV genotype could bias the estimated oncogenic potentials of coinfecting HPV genotypes either way 75 . A new generation of molecular studies involving lesion microdissection and HPV-E6/E7 expression could provide valuable information to assess more specifically each HPV genotype’s oncogenic potential 9 76 .

Our results provide further evidence reinforcing the high oncogenic potentials of genotypes HPV-18, -31, -33, -45, -52 and -58, already classified as high-risk for ICC. They also highlight the need to include in detection kits HPV-34, -67, -69 and -73, for which epidemiologic data are currently lacking, and to further examine their possibly underestimated oncogenic potentials. Moreover, although HPV-39 and -59 belong to the same α-7 species as HPV-18, they are not, at present, included in a future nonavalent anti-HPV vaccine (HPV-6, -11, -16, -18, -31, -33, -45, -52 and -58) 77 . Those genotypes may deserve further consideration, owing to accumulating evidence (relatively precise estimates) and their classification among the 10 most oncogenic genotypes after HPV-16 in our meta-analyses. Pooling individual data from presently available and future studies investigating these genotypes would allow more robust estimates, especially if controlled for age. Overall, such findings may have important implications for the prevention of cervical cancer and could help guide HPV-based–screening programs 78 and the composition of the second-generation anti-HPV vaccines 79 .

CC: Continuity correction; CI: Confidence interval; HPV: Human papillomavirus; IARC: International Agency for Research on Cancer; ICC: Invasive cervical cancer; OR: Odds ratio; PCR: Polymerase chain reaction.

The authors declare that they have no competing interest.

EB conducted the literature review, study analyses and wrote the first draft of the manuscript. ACMT and MPS supervised the study and participated in the literature review, analysis and writing. MF, IH, EDA and DG critically revised the manuscript and contributed to its Introduction and Discussion. ACMT and DG designed the study’s analytic strategy. All authors read and approved the final manuscript.