LONDON (Reuters) - Scientists have identified the eight human papillomavirus (HPV) types responsible for more than 90 percent of cervical cancer cases worldwide and say they should be the targets for the next generation of vaccines.

Drugmakers GlaxoSmithKline and Merck & Co. already make vaccines against HPV strains that cause many cases of cervical cancer, which is the second most common cancer in women worldwide and is expected to kill 328,000 this year.

In a large study looking at 60 years of data from 10,575 cases of invasive cervical cancer across 38 countries, an international team of scientists led by Silvia de Sanjose of the Catalan Institute of Oncology in Barcelona examined which types of HPV contributed most to the global incidence of the disease.

Their results, published in the Lancet medical journal on Monday, showed that eight HPV types -- types 16, 18, 45, 33, 31, 52, 58, and 35 in descending order of frequency -- were responsible for more than 90 percent of cases.

GSK’s vaccine Cervarix and Merck’s Gardasil protect against HPV types 16 and 18, and, through cross-protection, partially also against HPV types 31 and 45.

Many wealthy countries have started HPV immunization programs with these shots for girls before they become sexually active, but the vaccines are generally too expensive and inaccessible for most people living in poorer nations.

De Sanjose said the study’s findings reinforced “the rationale for prevention of cervical cancer through use of existing vaccines” and would aid the development of second-generation vaccines against multiple HPV types.

Around 80 percent of the world’s cervical cancer cases now occur in developing countries and the disease is caused by high-risk and sexually transmitted types of HPV.

Of the more than 118 different HPV types that have been identified, about 40 infect the genital tract and 12 are known to be cancer-causing, the scientists explained.

In their study, which involved cancer cases from Europe, North America, central South America, Africa, Asia, and Oceania, the researchers also identified several rare HPV types -- types 26, 30, 61, 67, 69, 82, and 91 -- which can also cause cervical cancer but account for only one percent of cases worldwide.

In a commentary about the study in the Lancet, Cosette Wheeler of the University of New Mexico Health Sciences Center in the United States described the research as “a Herculean effort that might be the benchmark for all time.”

The results “clearly support future directions for cervical screening and type-specific triage, establish the potential worldwide impact of current HPV vaccines, and set priorities for next-generation vaccines,” she wrote.