Two Landmark Studies Published in 1976/1977 Paved the Way for the Recognition of Human Papillomavirus as the Major Cause of the Global Cancer Burden

It is a great privilege to write a commentary on two studies that appeared 40 years ago (in 1976/1977) that are, beyond doubt, the two most influential studies ever published in Acta Cytologica during its entire 60-year life span. These two studies, by Meisels and Fortin [1] and Purola and Savia [2],were the first to recognize that dysplastic (the cancer precursor, cervical intraepithelial neoplasia [CIN]) lesions in the uterine cervix are of viral origin, thus introducing the concept that human papillomavirus (HPV) might be the etiological agent of cervical cancer. This groundbreaking new concept paved the way for the emergence of HPV research as one of the most rapidly developing fields of oncology, as witnessed during these past four decades, and culminating in the Nobel Prize in Medicine and Physiology being granted to Prof. Harald zur Hausen in 2008. HPV is among the first human tumor viruses for which all six of Koch's postulates, required for proof of causality (i.e., HPV is the cause of cancer), have been fulfilled. The final proof is the recent demonstration that prophylactic HPV vaccines are indeed effective in preventing (a) viral transmission and HPV infection, (b) benign HPV-induced tumors (genital warts), and (c) CIN, albeit that the evidence of the prevention of cervical cancer by these HPV vaccines still awaits confirmation [3]; the same is true of the eventual prevention of human cancers at other anatomic sites, part of the global burden of oncogenic HPVs [4].

All novel observations and ideas in medical research are based on the previous discoveries made by other scientists (contemporary or past). To fully appreciate the importance of each novel discovery, it is essential to be able to place them in the right context, both retrospectively and prospectively. This commentary will assist the reader by summarizing the existing knowledge on HPV before these two milestone papers appeared [1,2], and describing the incredibly rapid progress that they evoked during the subsequent years and decades. Before this, however, the delicate issue as to which of the two was truly submitted first is discussed, an issue inevitably provoked by the close proximity of the publication of two reports with an obviously identical content in the same journal.

It might sound trivial after 40 years to consider the questions: (1) Which of these two reports [1,2] was submitted first? (2) Did the submission of two papers with a practically identical content truly happen without awareness of each other? (3) Or is only one of them the original and the other one simply a replica?

When two study groups are reporting similar results that are of low or even moderate importance, one would consider such a discussion as being of academic relevance only. This case is different, however, due to the dramatic impact that these two reports had on the subsequent progress in human cancer research that, in less than 3 decades, permanently changed our concepts about the contribution of human tumor viruses to the global cancer burden. Beyond doubt, this development would have been the same, irrespective which of the two studies was published first, and even in the case of only one of them being published in 1976/1977. Thus, from this point of view, this issue appears to be of secondary importance.

Because this controversy was publicly addressed much later by others (most notably by Leopold G. Koss, but in a document that might have remained unnoticed from the majority of our regular readers), this enigma will be briefly addressed here as an example of how several things went wrong 40 years ago. In the history of clinical cytology, however, this is not the only occasion when such things happen, as pointed out by Dr. Koss in his thoughtful commentary on milestone articles [5].

Referring back to the contention over the reports [1,2], the most comprehensive treatise on this subject can be found in a book written in 2004 by Dr. Ervo Vesterinen [6], who was a close collaborator of Drs. Purola and Savia for decades. This book is written in Finnish with the title “Story of the Pap Test.” It contains, as the core document, the commentary of Leopold G. Koss [5], which he wrote in 2003 as a response to Dr. R.G. Lynch, whose commentary on milestone papers in cytology had appeared in the previous issue of the Bulletin of the American Society for Investigative Pathology [7]. In his commentary, Dr. Lynch only discussed the contribution of Meisels and Fortin [1], with no mention of the work of Purola and Savia [2].

In his unsurpassed style, commenting on Lynch's article [7], Leopold G. Koss wrote [5]:

I read, with great interest, the article in the recent ASIP Bulletin (vol. 6, issue 1, pages 10-11) on the topic of condylomatous lesions of the cervix and vagina that led to the study of the role of human papillomavirus in cervix cancer. It is sometimes extremely difficult to establish priorities in important scientific observations that lead to a cascade of events with unpredictable and surprising results. Without wishing to detract, in any way, from the contributions of Dr. Alexander Meisels and his colleagues that were described in the article, the priority of the observations that koilocytes and, hence, precancerous lesions of the uterine cervix, may be caused by viral infection, is attributable to the Finnish observers, Esko Purola and Eeva Savia, whose article on the topic appeared in the subsequent issue of Acta Cytologica, one month after the Meisels paper.

Dr. Koss continued:

Because the priority of these observations became contentious, it was subsequently acknowledged by the Editor of Acta Cytologica (vol. 21, page 483, 1977) that the Purola and Savia paper was submitted to the journal, Acta Cytologica, on May 13, 1975, seven months before the Meisels and Fortin paper, which was received on December 11, 1975. I was the fussy reviewer of the Purola and Savia paper who returned it twice to the authors for improvements. In fact, their paper is, in many ways, more explicit than the Meisels and Fortin contribution. It begins with the statement, “The viral origin of condylomata accuminata in genital warts is beyond doubt.” The paper further deducts that similar lesions of the uterine cervix most likely have a viral origin. I feel badly that the delay in its publication was caused by my interference.

Leo Koss concluded his commentary by stating:

Sic transit gloria mundi.

As described by Vesterinen in the book [6], being an associate editor of Acta Cytologica, Dr. Meisels got his paper published without any such delay, and it appeared in the last issue of 1976 [1], while the Purola and Savia paper was suspended until the first issue of 1977 [2] (for details concerning the reasons for this delay, see [6]). Although the documents by Koss [5 ]and Vesterinen [6] give us a clear picture of which report was submitted first to Acta Cytologica, they do not give us a definite answer to questions (2) and (3) in the first paragraph under this heading. As the original authors are no longer with us, this mystery will most likely remain unanswered, and we leave it up to the discretion of the readers to judge.

The present-day understanding of HPV research and clinical practice is a result of a long history of the pioneering works of a countless number of past and contemporary scientists. The history of papillomavirus (PV) research has an interesting dual nature: (1) basic virological data emerged from animal experiments during the first half of the past century, and (2) an increasing awareness of these viruses as a significant cause of human cancer emerged in the late 1970s.

The history of PV research has attracted scientists for quite a long time, with the first reviews dating back to the early 1950s and 1960s [8,9]. The most recent discourses on this intriguing topic are found in my textbook, published in 2000 [10], and in journal articles from 2008 [11] and 2009 [12]; albeit with a slightly different focus, the significant historical milestones in PV research are listed quite consistently in all of these reviews. The focus of this communication is to describe the impact that the two milestone papers [1,2] have had in the progress of PV science, but it is not feasible to discuss all the historical details from before 1976/1977. To help the orientation of the reader, however, these are listed in Table 1.

From these data, it is obvious that, particularly during the 1970s, the research rapidly progressed towards an understanding of the plurality of HPV and its important role as a human tumor virus (Table 1). The 1970s was heralded by emerging data on the serological response to human wart viruses (HPV). In this respect, the pioneering observations of a Finnish virologist, Seppo Pyrhönen and his colleagues have received far too little attention. They were the first to detect measurable antibodies against pooled, homogenized human wart tissue, detected in 92% of women and 75% of men aged between 15 and 19 years [13]. Their observation on a roughly inverse correlation between the number and duration of skin warts and the antibody levels was also important [13,14]. While studying patients with systemic lupus erythematosus, who showed HPV antibodies less frequently than their immunocompetent counterparts, the authors concluded that this lack of humoral response to HPV might explain the increased frequency of warts in these patients [15]. They also speculated on the antigenic relatedness between the HPVs in genital and skin warts, as suggested by the less frequent occurrence of HPV antibodies in patients with genital warts only. While detecting measurable HPV antibodies in students reporting not ever having had visible wart lesions, they made the assumption that subclinical HPV infections must be quite common. They also concluded that there must be more than one HPV type [15,16].

Beyond doubt, the HPV research of today owes much to one of the pioneers in the field, Harald zur Hausen, who turned his interest from HSV to HPV in the early 1970s. In the first of his classical series of four works from 1974-1976, attempting to detect virus-specific DNA in human tumors, he completed nucleic acid hybridizations with the complementary RNA of the human wart virus [17]. It soon became evident that different viruses are responsible for cutaneous common warts and genital warts. He also formulated the hypothesis on HPV as the etiological agent of cervical cancer [18,19], most comprehensively presented in his classical review of 1977 [20].

Thus, by 1976/1977, the time had ripened to translate this virological knowledge to the clinic, and this is exactly what Purola and Savia and Meisels and Fortin did in their reports. One cannot avoid an impression, however, that they did this unintentionally, because none of the preceding virological (molecular) studies from the past decades (Table 1), nor the more recent ones by zur Hausen [17,18,19] and Pyrhönen [13,14], are included in their references or discussed to any extent.

The period 1976/1977 was unrivalled in the history of HPV research in many respects, not only because of the two abovementioned contributions, but also because of the major progress made in basic research, most notably by the groups of Harald zur Hausen (Freiburg, West Germany) and Gerard Orth (Paris, France). In a series of studies, these two groups established the plurality of HPV, by disclosing and characterizing the first 4 HPV types in skin warts, numbered HPV1-4 [21,22,23]. Serological evidence was soon provided to support the concept of plurality [15,16], because there seemed to be no link between HPV1-4 and the new HPV type (to be characterized later as HPV6) found in condylomata acuminata (CA), laryngeal papilloma, or any of the malignant squamous cell lesions tested [24].

From the clinical point of view, the major breakthrough was indeed provided by the reports of Purola and Savia and Meisels and Fortin [1,2], by describing the koilocytotic cells in cervical Pap smears derived from flat epithelial lesions that are frequently associated with cervical precancer lesions. In addition to these characteristic koilocytes, they accurately described two other cell types that regularly exfoliate from genital CA: dyskeratotic superficial cells and “condylomatous” intermediate cells. They were the first to realize that by observing the cytopathic effects of a virus under a light microscope, one could probably “see” the evidence for an etiological agent of cervical cancer and its precursor lesions on biopsies and Pap smears.

Like most novel innovations, it took quite a while until this revolutionary idea became more generally accepted among cytopathologists. I was possibly one of the first to realize the significance of this observation. I therefore have deep personal feelings when speaking about these innovative reports, discussed in an issue of CytoPaths [25]. In fact, February 1977 was my first month as Resident at the Department of Pathology, Jorvi Hospital (now Helsinki University Hospital), and the two most recent issues of Acta Cytologica were brought to my attention as “recommended reading” by my tutors and mentors. Having familiarized myself with the idea, as a young man with little experience in clinical pathology, I became curious enough to start collecting all the Pap smears and biopsies of cervical cancer and precancer lesions from the Department archives. The work progressed slowly, however, because the hospital was brand new (<1 year old) and not many cases had accumulated. Finally, however, enough cases were available (n = 184) to make the analysis, which fully confirmed the observations of Meisels and Fortin and Purola and Savia [1,2]. Illustrative of this is the fact that, a report published in 1979 in Archives of Gynecology [26] was the first to reproduce the original findings in a similar setting of the light-microscopic examination of cervical precancer lesions. Even the next year (1980), when my first review on the subject was published, my reference list was very scanty indeed, mostly comprising older and more recent virological studies [27]. Well before that, however, we submitted another cytological study to Acta Cytologica, which was readily accepted but, at the time, the journal was so crowded with papers that it took almost two full years to get it published [28].

To make the 40-year history of PV/HPV research more tangible, Table 2 lists the key discoveries made since that unrivalled time in 1976/1977. A short note follows here of all the cited discoveries so as to better cover the title of this commentary.

Della Torre et al. [29 ]and Hills and Laverty [[30 ]were the first to detect viral particles within the nuclei of koilocytes on transmission electron microscopy (TEM). These viral particles were identical to those described in CA in the late 1960s [31,32], and in skin even earlier, in 1949 [33]. This confirmed that these newly described flat and endophytic epithelial lesions of the uterine cervix are manifestations of HPV, although distinct from the classical CA in their light-microscopic appearance. The circle was completed in 1979, when an invasive cervical squamous cell carcinoma (SCC) was described with an abundance of koilocytes in the biopsies [34].

In 1978, two new HPV types associated with epidermodysplasia verruciformis (EV) lesions were discovered by Orth et al. [35], paving the way for the subsequent detection of a large number of the so-called “EV-specific” HPV types recognized today. In parallel with the incited interest in HPV lesions of the genital tract and skin, the suspected HPV origin of two additional lesions was confirmed: (1) juvenile-onset laryngeal papilloma, and later (2) adult-onset laryngeal papilloma. Quick et al. [36,37] described epithelial atypia in these lesions, with the possible implications of their known risk for malignant transformation. Soon, conclusive clinical and virological evidence on similarities between genital condylomas and laryngeal papillomas was provided [38]. Within the next two years, HPV involvement in laryngeal SCC was suggested, based on the morphological features and detection of HPV antigens by immunohistochemistry (IHC) in up to 36% of the 116 cases [39,40]. When subjected to meta-analysis in 2016, the literature worldwide included 179 studies, comprising 7,437 laryngeal SCCs analyzed, with a pooled HPV prevalence of 26.9% (95% CI 24.2-29.7%; random-effects model) [41].

Before the introduction of DNA technology for general use, morphology, TEM and IHC were the diagnostic tools to provide evidence for HPV involvement in genital and extragenital squamous cell lesions. Prompted by the encouraging findings in laryngeal SCC, we focused our interest on similar lesions in the lower respiratory tract, i.e., bronchial SCC [42]. Using these simple means, we described koilocytotic cells and other morphological evidence (confirmed by IHC) suggestive of HPV involvement in bronchial SCCs, first in a single case [42], and subsequently in a series of 104 SCCs, 34.6% of which were interpreted as suggestive of HPV [43]. These observations paved the way to a more widespread thinking about HPV as the potential etiological agent in human SCCs at mucosal sites other than the genital tract and skin. These pioneering data seem to have withstood the test of time, as suggested by a meta-analysis published 30 years later (in 2012), where 100 eligible studies with 7,381 lung cancers showed a pooled HPV prevalence of 34.8% (95% CI 33.3-36.3%) [44.]

Mandatory DNA techniques also developed fast. Dr. Howley's group [45 ]was the first to adopt gene cloning and hybridization under nonstringent conditions for PV research, showing the highly conserved nucleotide sequences in PVs. The decade was concluded by an introduction of the first unanimously agreed classification of PVs, based on the decisions of the First Workshop on Papillomaviruses [46]. This first classification was based exclusively on the sequence homology between the different PV isolates: if there was a <50% crosshybridization with the known PV types, the viral DNA was assigned a new type. This classification prevailed until the 1990s, when it was revised, and finally, in 2004, PVs were classified as a taxonomic family of their own [47].

The 1980s witnessed incredible and dramatic progress in all fields of PV research, mostly because of the development of molecular cloning and related techniques [45]. This speed is best illustrated by the rapid discovery, cloning, and characterization of an ever-growing number of new HPV types, for which there is no end in sight, even today [48]. This development was started in 1980 by Gissmann and zur Hausen [49], who isolated, characterized, and cloned a new virus, which proved to be the etiological agent of classical CA and was designated as HPV6 [50]. Characterization of the first of these genital HPV types led to the isolation of its closest relative from laryngeal papilloma receiving the label HPV11 [51]. All attempts to detect homologous DNA in laryngeal SCCs failed, however [51].

One of the absolute highlights was the isolation and characterization of a new HPV type from cervical cancer by Dürst et al. [52] in 1983, which has subsequently proved to be the single most important HPV type of all, namely HPV16. While tested in a series of biopsies from cervical, vulvar, and penile cancer, >60% of cervical cancer samples were found to hybridize with HPV16 DNA, the corresponding figures for vulvar and penile cancer being 28.6 and 25%, respectively. In contrast, practically none of the benign CA lesions were shown to contain HPV16 DNA, which led the authors to suggest that HPV16 is an HPV type characteristic of SCC of the genital tract. This was soon confirmed in a prospective study, where HPV16 showed an extremely high propensity for malignant transformation [53].

In 1984, HPV18 was isolated and characterized from cervical carcinoma [54]. Importantly, HPV18 DNA was also found in several cell lines derived from cervical cancer, including the HeLa, KB, and C4-1 lines [54]. In 1986, Lörincz et al. [55] isolated HPV31, and HPV33 was molecularly cloned and characterized by a French study group in the same year [56].

Although it may appear egocentric to include too many of my own contributions in this list, it is difficult to avoid mentioning October 1981, which signified the starting point of the first prospective follow-up (cohort) study of women with cervical HPV infection (1981-1998 in Kuopio, Finland). By 1985, we had established that the natural history of cervical HPV lesions was identical with that of classical CIN lesions. In the same year, the inherent potential of HPV16 and HPV18 lesions to progress to invasive cancer was firmly confirmed in our cohort [53]. The first epidemiological study on the risk factors of genital HPV infections appeared at the same time, confirming an early onset of sexual activity, the number of sexual partners, mode of contraception as well as smoking to be key risk factors for HPV infections [57].

Even if running the prospective cohort study was our main effort, our group did not lose interest in exploring the potential HPV involvement of SCC at other anatomical sites. This search led to a description of such evidence in two distinct entities, squamous cell papilloma and carcinoma of the esophagus in 1982 [58,59], followed a year afterwards by yet another squamous cell lesion, inverted papilloma of the nasal cavity/paranasal sinuses [60]. The latter represent relatively rare tumors, but are of clinical importance due to their frequent recurrence and a definite risk for malignant transformation. Assessed 30 years afterwards by meta-analysis of the published literature, both these observations [58,59,60] have gained widespread acceptance in the scientific community. Indeed, esophageal SCC is the most intensely studied extragenital HPV manifestation, >10,000 samples of which have been analyzed in over 150 published studies [61]. Highly interestingly, the pooled HPV prevalence of 37.2% (95% CI 36.0-38.4%; fixed-effects model) is not far off 40% (24/60), as originally reported [59]. Concerning sinonasal papillomas, 76 eligible studies were published until 2012, with 1,956 cases analyzed, showing a pooled HPV prevalence of 42.1% (95% CI 35.9-48.5%) [62].

These strenuous years also witnessed the extension of clinical HPV research into yet another group of squamous cell lesions that subsequently gained a substantial clinical importance, i.e., the first evidence of HPV involvement in benign [63], premalignant, and malignant [64] SCCs of the oral mucosa. At the time, it proved to be extremely difficult to get the reports on such a new idea (i.e., HPV as an etiological agent of oral cancer) accepted in any journals of oral medicine, which resulted in a marked delay in the publication of these two pioneering studies [64,65]. During the past 30 years, however, HPV as the causal factor of a subset of head and neck cancer has become an accepted fact [66,67], and this is the first specific group of human cancers where HPV testing has become a routine practice, because it is shown to have a marked impact in a patient's treatment and survival [68].

The mid 1980s also generated significant achievements in the methodological development of HPV research. Two innovations deserve a mention: (1) the design of the so-called Kreider model [69], and (2) the development of different hybridization methods, in situ hybridization (ISH) being particularly innovative [70,71]. In 1985, Kreider et al. [69] conducted experiments where they succeeded in inducing squamous cell lesions consistent with CA in epithelial cells derived from a normal uterine cervix after exposure to HPV11 from a CA lesion. This was the first demonstration of a morphological transformation of human tissues with HPV under experimental conditions. The only disadvantage of the model was the failure of the grafted epithelium to support the replication of the oncogenic HPV16 and HPV18. ISH, developed simultaneously by two independent groups in 1985 proved to be well accepted among pathologists, because, for the first time, it allowed a direct visualization of the hybridization signals in the epithelial cells, on both biopsies and Pap smears [70,71]. The originally used radioactive label was soon replaced by a biotin label which further increased the acceptability of the ISH technique [72].

PV/HPV research had made such remarkable progress that it was timely to publish the first textbook on the subject that would share the current state of the art with a wider readership. This project was realized in 1985-1987, when I had the privilege to work with two giants in HPV research (Lutz Gissmann and Leopold G. Koss) as the editors of a textbook, all 17 chapters of which were written by the foremost PV/HPV scientists. In 1987, once published, this first textbook on PVs became the standard reference for several years [73].

In parallel with the major advances made in research technology, the basic understanding of the molecular mechanisms of how PVs induce malignant transformation increased substantially via several innovative experiments. In 1986, Yasumoto et al. [74] described the transformation of a rodent cell line (NIH3T3) by HPV16. Using a recombinant HPV16 DNA (pSHPV16d), which contains a head-to-tail dimer of the full-length HPV16 genome, they could induce morphologic transformation, and, indeed, the transformed cells proved to be tumorigenic in nude mice. Subsequently, the transformation of NIH3T3 cells has provided a useful model for analyzing the functions of HPV16.

Equally important are reports from 1987 by Pirisi et al. [75] and Dürst et al. [76], where human keratinocytes and fibroblasts isolated from foreskin were transformed by transfection with recombinant HPV16 DNA. The transformed cells exhibited an extended (fibroblasts) or indefinite (keratinocytes) life span compared with that of normal controls. Such immortalized cell lines represented a unique system to study the interaction of HPV with its natural target cell in vitro.

Yet another technical innovation, albeit not an original development, was made in 1988, when the organotypic-raft culture was modified for HPV research in Dr. Laimins' laboratory [77]. Using a cell culture system for keratinocytes which allows the stratification and production of differentiation-specific keratins, the authors examined the effects of HPV16 on the differentiation capabilities of human epithelial cells. Not unexpectedly, the histological abnormalities induced by HPV16 closely mimicked those seen in CIN lesions.

While approaching the 1990s, two key discoveries in basic HPV research deserve their place on the list, because they contributed significantly to our understanding of the basic mechanisms of virus-host cell interactions. In 1989, prompted by the previous observations that the Rb1 (retinoblastoma susceptibility) gene product, p105Rb, forms stable complexes with the oncoproteins of the adenovirus E1A protein and the SV40 large T antigen, Dyson et al. [78] demonstrated that the HPV16 E7 oncoprotein can form similar complexes with p105Rb. While a similar mechanism in transformation is used by these three DNA viruses, this finding strongly implicated Rb-binding as a key molecular event in HPV-induced carcinogenesis.

This discovery was followed by a logical approach to explore if another tumor suppressor gene, p53, and its protein product will bind to other oncoproteins of HPVs. In the same year (1989), accordingly, Werness et al. [79] showed that the E6 protein of HPV16 is capable of binding to the cellular p53 protein. Because of the fact that the wild-type p53 protein also forms complexes with the SV40 large T antigen and the E1B 55 kD protein of adenovirus type 5, an analogy was established between E7-Rb and E6-p53 complexing. Undoubtedly, this was strong evidence in favor of the concept that HPVs, adenoviruses, and SV40 utilize similar cellular pathways in their cell transformation [78,79]. Since these experiments, more than 20 human proteins have been identified as interacting with either HPV16 E6 or E7 proteins [10,11,12].

In 1992, a highly innovative discovery was made which subsequently opened up completely new perspectives on two important areas of HPV research: (i) serology and (ii) vaccination. This was the description of the technique for producing virus-like particles (VLP) in vitro by Kirnbauer et al. [80], who succeeded in expressing the L1 major capsid proteins of BPV1 and HPV16 in insect cells (using a baculovirus vector), and analyzing their conformation and immunogenicity. The L1 proteins were expressed at high levels and, surprisingly, assembled into structures that closely resembled PV virions in appearance [80]. These self-assembled BPV L1 VLPs also mimicked intact BPV virions functionally, while being capable of inducing neutralizing antibodies in rabbits. Thus, the L1 protein seems to have an intrinsic capacity to assemble into empty capsid-like structures with immunogenicity similar to that of infectious viral particles. This novel L1 VLP preparation was immediately recognized as a potential candidate for serological tests to measure antibodies to conformational virion epitopes as well as for developing a vaccine to prevent HPV infections [80].

Subsequent experimental and human studies on these cell lines resulted in the development of the first generation of prophylactic vaccines against the HPV types 6, 11, 16, and 18 (Gardasil®, Merck) in 2006, and against HPV16 and HPV18 (Cervarix®, GSK) in 2007. The vast literature accumulated on prophylactic HPV vaccines during the past 10 years falls outside the scope of this paper (please refer to reviews on the topic [3]).

The latest development in the field of prophylactic HPV vaccines represents the 9-valent GARDASIL®9 which obtained FDA approval in 2014 [3], and is indicated for females 9-26 years of age for the prevention of (1): cervical, vulvar, vaginal, and anal cancers caused by HPV types 16, 18, 31, 33, 45, 52, 58; (2) precancerous or dysplastic lesions caused by HPV types 6, 11, 16, 18, 31, 33, 45, 52, 58; and (3) genital warts caused by HPV6 and HPV11. It is also indicated for males from 9 to 26 years of age for the prevention of: (1) anal cancer caused by HPV types 16, 18, 31, 33, 45, 52, 58; (2) precancerous or dysplastic lesions caused by HPV types 6, 11, 16, 18, 31, 33, 45, 52, 58; and (3) genital warts caused by HPV6 and HPV11 [3]. Importantly, GARDASIL 9 does not eliminate the necessity for girls to continue to participate in cervical cancer screening later in life, and recipients of this vaccine should not discontinue anal cancer screening if this has been recommended by a health care professional.

According to the manufacturer (Merck), GARDASIL 9 has not been demonstrated to provide protection against diseases from vaccine HPV types to which a person has previously been exposed through sexual activity [3]. Similarly, this vaccine (or Gardasil and Cervarix) is not a treatment for external genital warts (CA), or cervical, vulvar, vaginal, and anal cancers and intraepithelial neoplasias. It protects only against those cervical, vulvar, vaginal, and anal cancers caused by HPV types 16, 18, 31, 33, 45, 52, and 58 [3].

With the current understanding of the role of HPVs in human carcinogenesis, and in view of the WHO incidence statistics for the 15 major malignancies worldwide, it can be estimated that oncogenic HPV types might be involved in the development of up to 10-15% of all human malignancies [4]. How much of this global cancer burden can be eventually prevented by the currently available prophylactic HPV vaccines remains to be seen during the next few decades. This possibility clearly exists already today, thanks to the astounding progress in HPV research since 1976/1977. Much of this development is indebted to the pioneering works of Purola and Savia [2] and Meisels and Fortin [1], who showed us that the same virus that was known for decades to induce benign genital warts was indeed capable of inducing squamous cell precancer lesions and cancer.

The author has no conflicts of interest to declare.