One hundred percent effective is
a lofty claim. But a new study has shown that Merck's
human papillomavirus (HPV) vaccine, Gardasil, is just
that. HPV is a common sexually transmitted infection
that accounts for the majority of cases of cervical
cancer. This marks the first time a vaccine has been
successfully designed primarily for cancer prevention.
"I think this vaccine could have
a huge impact on public health," says Dr Allan Covens,
head of the Gynecological Oncology Tumour Site Group
at Sunnybrook and director of the U of T Obstetrics
and Gynecology Fellowship Program. Roughly 20% of women
are infected with HPV at some point in their lives,
notes Dr Covens. In most cases, the condition is asymptomatic
and cleared up by the immune system, but some types
can persist.
THE
PROOF
The study, presented on October 7 at a meeting of the
Infectious Diseases Society of America (IDSA), is part
of the ongoing Phase III program for Gardasil. The recombinant
vaccine was designed to target HPV types 16 and 18,
which together account for 70% of cervical cancers,
and HPV types 6 and 11, which cause 90% of cases of
genital warts.
More than 12,000 HPV-free women
aged 16 to 26 years were enrolled in the prospective,
double-blind trial. Participants received a three-dose
regimen of either Gardasil or placebo. In the group
who received the vaccine, not a single patient developed
precancerous cells or early cervical cancers associated
with HPV types 16 and 18, compared to 21 cases in the
placebo group.
Merck expects to file an application
with the FDA by the end of the year and the vaccine
should be available sometime in 2006. There's no word
yet on how long it will take before it's available in
Canada. In the meantime, the company is conducting a
four-year followup study to determine if the effectiveness
of the shots wanes over time.
FORGET
FALSE POSITIVES
Cervical cancer is the second most common cancer in
women worldwide. In Canada, 1,350 women are expected
to receive a diagnosis for the disease in 2005, while
approximately 400 will die from it.
Pap tests are currently used to
screen for the disease and pick up on any abnormal cell
growth. "The majority of women with abnormal pap smears
don't have cervical cancer and will never get it," stresses
Dr Covens. "But we don't want people to slip through
the cracks, so there's a huge over-investigation."
This means that patients often
needlessly suffer the stress and fear that comes with
abnormal test results. By preventing infection with
the most common strains of HPV, Gardasil should also
prevent a lot of unnecessary worry. "We'll still do
Pap smears like we do now," Dr Covens points out, "but
the proportion of abnormalities should decrease."
PREVENT
IT IN THE YOUNG
Once the vaccine is made available, the next question
will be who should get the jab — and when. Dr Covens
recommends the family doctor be brought on board. "If
you're going to target women, you've got to do it before
they become sexually active — so 11 or 12 years
old," he says emphatically. "You could do it at 14,
but unfortunately, you're going to miss a proportion
of girls."
Some conservative watchdogs and
religious groups have already begun sounding the alarm
that STI vaccines like this will encourage promiscuous
behaviour. While he's sensitive to their concern, Dr
Covens just doesn't buy it. "If you say: 'This is a
vaccine against a virus which is known to cause cervical
cancer,' that's not a licence to go out and have sex."
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