Medscape Medical News reports that Gardisil, the vaccine against 4 strains of Human Papilloma Virus that cause cervical cancer and warts, also causes the body to make antibodies against other strains of the virus. The information comes from data obtained in the last 7 years of research on the vaccine:
The first evidence of this cross-protection comes from the original and ongoing Merck trials of the vaccine, which protects against HPV types 6, 11, 16, and 18. These new data were presented here at the 47th Annual Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC).
The dominant HPV 16/18 strains account for about 70% of all cervical cancers; the vaccine protects against approximately 99% of infections and almost 100% against the development of lesions by those strains. Ten other strains of HPV (31, 33, 35, 39, 45, 51, 52, 56, 58, and 59) account for approximately an additional 20% of cervical cancers worldwide.
Combining analysis of all 10 strains showed cross-protection that reduced incidence of precursor lesions by about 38%. The effect was most dramatic with strains 31 and 45, where protection reached about 45%.
Lead investigator Darron Brown, MD, PhD, professor of medicine, microbiology, and immunology at the Indiana University School of Medicine, Indianapolis, said this analysis had been built into the initial study design. The foundation for the work “came from in vitro cross-neutralization studies that showed some suggestion that antibodies to closely related types may be able to cross-neutralize other close types.
“But I think that we all are pleasantly surprised with the high degree of cross-protection that was shown to occur, both in terms of resisting infection and, more importantly, disease,” he told Medscape Infectious Diseases.
Dr. Brown suspects that the different degrees of cross-protection the researchers observed are probably related to the level of antibody generated, and perhaps to memory B cells, both of which are likely have an individual genetic component to them.
Dr. Brown mentions a point that I believe is important: the different types of immunity to viruses. When we contact HPV through the skin or mucus membranes, our bodies fight the disease with T-cell immunity. Gardisil induces B-cell immunity. The latter lasts much longer, because of so-called “memory cells” that lie dormant in the body for many years. That is why infection with one virus may not give immunity to other strains or even that strain with future exposure.
There are more than 60 strains of the HPV virus. About 15 are thought to cause cervical cancer; Gardasil protects against 12 of those, plus two more that cause genital warts but not cancer.
Two strains cause 70 percent of cervical cancer. Merck studies following 17,600 young women for three years found the vaccine to be 99 percent effective in blocking those strains.
New analysis of that data shows that the vaccine reduced incidence of HPV-caused precancerous lesions by nearly two-thirds for the three next most common HPV strains in North America. While those three strains are less common elsewhere, together they cause about 11 percent of cervical cancer worldwide.