All sexually active men and women are susceptible to acquiring genital HPV infection. The life-time risk of HPV infection is up to 80%. Most of these infections (up to 90%) are cleared within 2years (12–24months) and only a few become persistent [3]. The first HPV infection often occurs soon after the first sexual intercourse (FSI), the cumulative rate of genital HPV infection being approximately 50% over 3years after the first intercourse [4,5]. The prevalence of HPV infection is highest among young women (18–24years) soon after the onset of sexual activity, and falls gradually with age until the 4th-5th decade, when a second peak occurs for hormonal changes during perimenopause, followed by another decline [6]. Young subjects are exposed to HPV infection more often because of their sexual behaviour, and young women are more vulnerable than older women because the transformation zone is located on the ectocervix (cervical ectopy) [7].

The likelihood of developing precancerous lesions increases with persistent infections [8]. Persistent infection with an oncogenic type of HPV is a prerequisite to developing cervical cancer [9] and can determine cancer in other anatomical sites, such as the anus, vagina, vulva, oral cavity and oropharynx [10,11].

The most frequent risk factors correlated with the infection and persistence of HPV in the population are the following: initiation of sexual activity at a young age, the number of life-time sexual partners, frequency of sex or other intimate skin-to-skin contact, the sexual histories and behaviours of sexual partners, cigarette smoking, parity, the use of some types of oral contraceptives and alcohol consumption [2].

It is known that, among 150 different HPV types, only about 60 are able to infect the ano-genital area and that vaccination is a useful tool for preventing HPV infection caused by types more frequently involved [12-17]. For some years, two HPV vaccines have been on the market in Europe (EU): a quadrivalent vaccine (Gardasil®), licensed in September 2006, and a bivalent vaccine (Cervarix®), licensed in September 2007. Both vaccines have a prophylactic indication and are able to prevent pre-cancerous lesions (CIN II +) and cancers due to persistent infection with HPV-16 or HPV-18. Furthermore, both vaccines have been shown to elicit cross-protection against other high-risk HPV types [13-17]. Gardasil® also offers protection against genital warts caused by HPV-6 and HPV-11.

As HPV infection is exclusively contracted through sexual activity, it was recommended that adolescent females should be vaccinated against HPV before their sexual debut, as the main target population. Therefore, in March 2008, the Italian Ministry of Health recommended that regional authorities should start a campaign of free HPV vaccination for 12-year-old girls [18]. Since the introduction of HPV vaccination, many studies have been conducted with the aim of helping decision-makers choose the best vaccination policy [19-24].

Another important issue concerns the coverage rate, since only by achieving high coverage can circulation of the virus be rapidly reduced, thereby exerting a greater impact on diseases related to HPV genotypes. Indeed, in its HPV immunization plan, the Italian Ministry of Health has set the goal of achieving 95% coverage within 5years of initiating the vaccination program in 12-year-old girls [18].

The study was carried out from May 2008 to May 2011.

The study involved 7449 volunteers aged 14–24years (2386 males and 5063 females). We excluded 151 questionnaires from the analysis on the basis of quality control. Thus, 7298 questionnaires were analyzed (4962 females and 2336 males).

Out of 7298 subjects studied, 3334 (2222 females and 1112 males) were in the 14-16-year age-group, 2784 (1908 females and 876 males) were aged between 17 and 19years, and 1180 (7832 females and 348 males) belonged to the 20–24year age-group. Out of 7298 subjects studied, 6861 (4659 females and 2202 males) were Italians and 437 (303 females and 134 males) were foreigners.

Since the answers of the foreign students did not differ from those of the Italian students, a combined statistical analysis was performed. Likewise, as no statistically significant differences emerged among the answers given by the students from the five Italian cities, this statistical analysis was also combined.

A total of 55.3% of females (95% CI 53.9 – 56.7) and 52.5% of males (95% CI 50.5 – 54.5) reported regular sexual activity. No statistically significant differences were found on considering gender.

The study considered only subjects who declared regular sexual activity after their sexual debut. However, 21 females and 15 males who declared having had a sexual debut reported irregular sexual activity. These volunteers have been included in the group of subjects “NO regular sexual activity” (Table ​(Table11).

Table ​Table11 shows regular sexual activity and age at sexual debut by age-group and gender. It also reports the sexually active subjects who stated regular sexual activity prior to their 15^th birthday. The percentage of subjects in each age-group who were sexually active before their 15^th birthday was calculated in order to better assess the changes in sexual habits among the young.

With regard to regular sexual activity, a significant (p<0.001) increasing trend in proportions by age was observed in both sexes.

Among females, the mean age at sexual debut was 15.7±1.6 and the median age was 16 (25^th and 75^th percentiles=15-17). Among males, the mean age at sexual debut was 15.6±1.6 and the median age was 16 (25^th and 75^th percentiles=15-17).

Comparison of the mean ages at sexual debut among the age-groups of respondents of the same sex was carried out by means of analysis of variance (ANOVA). This revealed a highly significant difference (p<0.001), indicating that, among the subjects who declared sexual activity, younger subjects debuted earlier than their older counterparts. Most females had their first sexual intercourse with a partner 2.4years older, while males first had intercourse with a partner 0.5years older (data not shown).

Table ​Table22 shows the characteristics of sexual behaviour by gender and age-group among sexually active subjects. The number of lifetime sexual partners increases with age in both sexes, and males report more multiple partnerships than females. No difference was found in the number of partners prior to the respondent’s 15^th birthday, nor in the number of sexual partners in the last year among the different age-groups.

Figure ​Figure11 reports the numbers and percentages of males and females, broken down according to the risk of acquiring HPV infection. Among males, 434 (38.2% 95%CI 35.8-40.5), 232 (20.4% 95%CI 18.4-22.3) and 470 (41.4% 95%CI 39.9-43.8) subjects fell into the low-risk, medium-risk and high-risk groups, respectively. Among females, 1479 (55.0% 95%CI 53.1-56.9), 524 (19.5% 95%CI 18.0-20.9) and 686 (25.5% 95%CI 23.8-27.1) subjects fell into the low-risk, medium-risk and high-risk groups, respectively.

Table ​Table33 shows the percentage of subjects who used a contraceptive method during their sexual debut by gender and age-group. Overall, the use of a contraceptive method at sexual debut was reported by 62.2% (95% CI 59.5-64.9) of males; 58.9% (95% CI 56.1-61.6) indicated using a condom. Among females, 70.3% (95% CI 68.6-72.0) reported using a contraceptive method at sexual debut; 67.2% (95% CI 65.9-69.3) specified using a condom. Moreover, a significant (females: p<0.001; males: p<0.05) increasing trend in proportions of the use of contraceptive methods by age was observed in both sexes.

With regard to the use of contraceptive methods during the last year, 63.6% (95% CI 60.7-66.5) of males responded affirmatively, and 42.6% (95% CI 39.6-45.6) reported using a condom. Among females, 62.8% (95% CI 60.9-64.7) responded affirmatively: 42.8% (95% CI 40.8-44.7) reported using a condom and 21.2% (95% CI 19.6-22.8) oral contraception.

More than two thirds of the students involved in this study were females, who constitute the target population of HPV vaccination. The results yielded by this sample therefore enabled the suitability of the current vaccine-tion strategies in Italy to be evaluated indirectly, in that sexual activity is the most consistent predictor of the probability of acquiring HPV infection.

Exposure to HPV infection is determined by risks factors such as an early sexual debut, a high number of lifetime sexual partners, a high number of recent or current sexual partners and the sexual histories and behaviours of sexual partners [2,25]. Condom use has a protective effect, although its effectiveness in reducing HPV acquisition has not been completely clarified [32,33].

The results of our study highlight the early start of sexual activity among young students in Italy, with a mean age at sexual debut of 15.7±1.6 for females and 15.6±1.6 for males. No statistically significant differences were found on considering gender and the city of residence. In recent years, lifestyle changes, especially in sexual behaviour, have been registered among adolescents in Europe [25,34]. Young people travel a lot and have more open and changeable interpersonal relationships. We noted a decrease in the mean age at first intercourse on comparing the various age-groups of the subjects (Table ​(Table2);2); indeed, in the 14–16 age-group, 47.5% of the sexually active females and 60.2% of the sexually active males reported having their first sexual intercourse before their 15th birthday, and a third of the interviewees aged 14–16years old declared that they already indulged in regular sexual activity.

Most of the females stated that their sexual debut was with a partner 2.4years older, while the males reported their first intercourse with a partner of the same age. As the sexual behaviour of the partner, particularly the number of previous sexual partners, is a risk factor for the transmission of mucosal HPV infection, having an older partner increases the probability of acquiring the virus. Consequently, females could have a higher risk than males at sexual debut.

The number lifetime sexual partner increases with age, and males report more multiple partnerships than females.

On the basis of age at sexual debut and the number of lifetime sexual partners, the respondents were grouped into different risk-group. Considering lifestyle-related risk factors, males appear to have a higher probability of acquiring HPV infection than females. Indeed, more males fell into the high-risk group (41.4%) than the low- (38.2%) or medium-risk groups (20.4%), while most females were in the low-risk group (55.0%). The exact probability of transmission through sexual contact or partnership is not precisely known. Barnabas et al. estimated a value of transmission probability using a mathematical modelling analysis. They considered that the transmission probability could varied between 0 to 1, and they found that the probability of transmission was approximately 0.4 for annual partner change rate [28]. Burchell et al. report a partnership transmission probability of 0.20; they observe little difference in the rate of male-to-female (3.5 transmissions per 100 person-months) versus female-to-male transmission (4.0 transmissions per 100 person-months) [35]. Other studies have found higher rates of female-to-male than male-to-female transmission [36-38].

Our data indicate a low use of condoms. With regard to regular condom use during the last year, no difference between the genders emerged. In the female group, however, we noted statistically significant differences (p<0.05) between age-groups. The percentage of females who regularly use condoms was seen to decline as age increased, in particular in the 20–24 age-group. This could be explained by the fact that older females reported more stable and durable relationships and a greater use of the pill. We surmise, also on the basis of literature data [39,40], that many young people use condoms to prevent unwanted pregnancies, but that they have scant knowledge or perception of the risk of sexually transmitted infections. In the male group, the percentage of students who regularly use condoms increased with increasing age.

Furthermore, it is essential to make every effort to improve the coverage of HPV vaccination. Indeed, data on coverage in Italy (updated as of 30^th June 2011) revealed a coverage rate of 65% for the primary target of vaccination (12-year-old girls); since the beginning of the immunization program, no improvement has yet been observed [43]. To this end, it is necessary to implement a range of broad-spectrum strategies, including communication campaigns to increase awareness of sexually transmitted infections and their prevention. Teenagers’ sexual behaviours have both short-term and long-term consequences, and interventions that focus on this target population may be the most effect-tive in helping to promote overall health among young adults.

Information campaigns can only be successful if they reach their target populations. In the present case, as our target population is that of young people, we should exploit the means of communication which they themselves use, such as social networks (facebook, twitter) and websites dedicated to the young (information online). To achieve high coverage rates of anti-HPV vaccination, these channels of communication have been utilised in the United Kingdom, where very good results have been obtained within a short time [44].