The HPV Vaccine

Good comparative article on Merck's quadrivalent [6, 11, 16, 18] vaccine Gardasil and GlaxoSmithKline's bivalent [16, 18] vaccine Cervarix:

JACKSONVILLE, FLA. - Both vaccines to prevent human papillomavirus infection will be highly effective, Dr. Diane M. Harper said at a conference on STD prevention sponsored by the Centers for Disease Control and Prevention.

Approximately 600 women participated in efficacy studies for a quadrivalent vaccine (Gardasil, Merck), and another 1,100 participated in efficacy studies for a bivalent vaccine (Cervarix, GlaxoSmithKline).

All women were screened at baseline to ensure seronegativity for high-risk strains 16 and 18 of the human papillomavirus (HPV) as well as for strains 6 and 11, which are also included in the quadrivalent product.

"The response was 100% for the bivalent and 89% for the quadrivalent for persistent, vaccine-specific HPV types for those who got the vaccine on time," Dr. Harper said. The recommended regimen for both vaccines is a 0.5-cc injection at 0, 2, and 6 months. In the studies, 94% of participants received all three doses, although not all according to protocol. The off-schedule efficacy was 95% and 89%, respectively, but these differences were not statistically different. "It indicated these will work in a real-world setting," she added.

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Both vaccines target high-risk HPV type 16, the type primarily implicated in cervical cancer. Type 16, together with types 18 (also in both vaccines), 31, and 45, account for 81% of cervical cancers, Dr. Harper said. The bivalent vaccine, "although it was designed for 16 and 18, is just as efficacious for HPV 45 and half effective for HPV 31, so that is exciting," said Dr. Harper...

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"These vaccines are preventive; they are not therapeutic-that is important to know," Dr. Harper said. "These prevent possible infection by HPV; these are not vaccines that prevent cancer." Because it is important that prevention lasts a long time, the need for a booster shot is anticipated with the bivalent vaccine 7-10 years later, Dr. Harper said. "We don't know if the quadrivalent vaccine will require a booster."

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Adverse effects at local injection sites, including pain, erythema, and edema, were similar for both vaccines versus placebo. Other side effects such as headaches, gastrointestinal problems, and fatigue occurred at similar, "acceptable" rates in placebo and vaccine recipients, Dr. Harper said.

Other safety concerns with vaccines include new-onset autoimmune disease and musculoskeletal problems. "We can say no in both cases," she said.

Here's more on the important concept that HPV vaccines are preventive, not therapeutic (paraphrase/translation from medspeak mine):

Antibody production is the primary goal of the prophylactic HPV vaccines. In order to induce the immune system to produce HPV-specific antibodies, you need to stimulate it. [This is what happens with natural exposure to HPV--the virus enters the host's cells, stimulates an immune response, and the body produces antibodies which then clear the infection.] Since HPV, in order to cause infection, must infect actively proliferating and differentiating tissue, HPV cannot be routinely grown in the lab. The technology to produce a "live virus" or "live attenuated virus" vaccine is not available.

So, the next best thing is to use bits of HPV which are still able to stimulate the immune system to produce antibodies. [Hence the designation of HPV vaccines as subunit vaccines.] The HPV vaccines are based primarily on highly immune system-stimulating bits of the HPV outer coat protein [L1 major capsid protein]. The L1 protein self-assembles into virus-like particles (VLPs) that closely mimic the structure of the natural HPV. The VLPs are then able to stimulate the body to produce HPV antibodies.

One important point about VLPs--they do not contain viral DNA. This means VLPs are not infectious, and they have no cancer-producing potential.

Bottom line: HPV vaccines are created from noninfectious virus-like particles (VLPs) of the major capsid protein, L1, that closely mimic natural HPV virions. The vaccines are intended to prevent infection, not treat disease.