The number of head and neck cancers linked to a virus spread by oral sex is rising rapidly and suggests boys as well as girls should be offered protection through vaccination, doctors said Friday.

Despite an overall slight decline in head and neck cancers in recent years, cases of a particular form called oropharyngeal squamous cell carcinoma (OSCC) have increased sharply, particularly in the developed world.

This growth seems to be linked to cancers caused by the human papillomavirus (HPV), scientists said in a report in the British Medical Journal.

Two vaccines — Cervarix, made by GlaxoSmithKline, and Gardasil, made by Merck & Co — can prevent HPV, which causes virtually all cases of cervical cancer, the second most common cancer in women worldwide.

Many rich nations have launched HPV immunization programs for girls to try to protect them from the common sexually transmitted virus before they become sexually active.

The scientists, led by Hisham Mehanna of the Institute of Head and Neck Studies at University Hospital Coventry, said while including boys in immunization plans has been seen as too expensive, it may be time to look again.

"We need to look at the evidence again to re-evaluate the cost-effectiveness of male children in light of this new and rapidly rising incidence," he said in an interview.

Analyst Savvas Neophytou at Panmure Gordon in London said such studies would dispel worries about sales prospects for HPV vaccines, and repeated his 'buy' recommendation for Glaxo stock.

"The emergence of new data such as this may increase motivation amongst national vaccination authorities worldwide to re-double efforts to vaccinate children before they become sexually active," he said in an equity research note.

Glaxo shares were slightly lower, in line with a weaker European pharmaceutical sector and down 0.4 percent by 0905 GMT.

More than 500,000 cases of cervical cancer are diagnosed annually in women and it kills around 200,000 a year. Head and neck cancer is the sixth most common cancer among men and women, with about 640,000 new cases each year worldwide.

A recent study found the risk of developing oropharyngeal carcinoma was linked to a history of six or more lifetime sexual partners, four or more lifetime oral sex partners, and, for men, an earlier age at first sexual intercourse.

"Sexual transmission of HPV — primarily through orogenital intercourse — might be the reason for the increase in incidence of HPV related oropharyngeal carcinoma," wrote Mehanna.

The scientists pointed to recent studies which showed a 70 percent increase in the detection of HPV in biopsies taken to diagnose oropharyngeal carcinoma in Stockholm since the 1970s.

HPV-related cancer was also reported in 60-80 percent of recent biopsy samples in studies in the United States, compared with 40 percent in the previous decade, they wrote.

Mehanna said the findings had other important health implications. Patients with HPV-related head and neck cancers were typically younger and employed, he said, and because their tumors appeared to be less deadly than those caused by factors like smoking and drinking, patients may also live longer with the physical and psychological effects of treatment.

"This means they would need prolonged support from health, social, and other services, and may require help in returning to work," he said.