Home' Trinidad and Tobago Guardian : November 11th 2013 Contents B20
body & soul
Guardian www.guardian.co.tt Monday, November 11, 2013
A single dose of the human papillomavirus (HPV)
vaccine may be enough to protect women against
infection with the virus over the long-term, a new
study from Costa Rica suggests.
In the study, women who received one, two, or
the standard three doses of the HPV vaccine all pro-
duced antibodies against the virus that remained at
stable levels in their bodies for four years after vac-
In addition, women who received one dose of the
vaccine had an immune response that was five to
nine times stronger than that seen in women who
were infected with HPV naturally.
However, women who received only one vaccine
dose produced antibodies at levels lower than those
of women given two or three doses. Still, researchers
don t yet know what level would be "good enough"
for protection. It could be that the immune response
generated after two or three doses is much more than
a person needs to be protected.
In fact, a previous study by the same researchers
suggested just that: women in that study who received
all three HPV doses were protected against infection
no better than those who received one or two.
"We don t know what the minimum required might
be for protection," said study researcher Mahboobeh
Safaeian, of the National Cancer Institute. But in the
new study, "the fact that [antibody levels] remained
higher than natural infection, and remained stable,
is a promising finding," Safaeian said.
More research is needed to validate the findings
in other populations, and over longer periods. Until
then, people in the US should get the recommended
three doses, Safaeian said.
The HPV vaccine is currently licensed for people
between ages nine and 26, and it works best if given
before people become sexually active. It is used to
protect against cervical cancer, which is often caused
by HPV infections.
If the number of recommended doses could be
lowered from three to just two or one, it would
simplify the vaccination schedule, Safaeian said.
Studies show that in the US, only about a third of
teen girls get all three doses. Fewer doses would also
reduce the cost of vaccination, which has been a
barrier to administering the vaccine to people in
developing countries, Safaeian said.
The new study is part of a larger trial in which
more than 3,500 women ages 18 to 25 were assigned
to receive three doses of the Cervarix HPV vaccine,
a vaccine that protects against the cancer-causing
strains HPV 16 and HPV 18. However, about 20 per
cent of participants did not end up receiving all three
The researchers analysed blood samples taken from
78 women who received one dose, 192 women who
received two doses, and 120 who received three doses
of the HPV vaccine, as well as 113 women who did
not receive the HPV vaccine, but had previously been
infected with HPV.
Four years after vaccination, all of these women
had antibodies against HPV 16 and HPV 18 in their
blood. Those who received two doses six months
apart had about the same levels of antibodies as
those who received all three doses.
Those who received one or two doses had antibody
levels that were five to 24 times higher than the
women who had been naturally infected.
"Because of the challenges associated with giving
three doses [of HPV vaccine] I think it s very welcomed
to see that there s a possibility that the vaccine may
not need to be given in a three-dose schedule," said
Dr Mike Brady, a paediatric infectious disease specialist
at Nationwide Children s Hospital in Columbus, Ohio.
But before doctors would feel comfortable giving
fewer than three doses, researchers will need to show
that vaccination with one or two doses also reduces
the risk of developing the early signs of cervical
cancer, Brady said. Currently, there s lots of
evidence that three doses protects against
cervical cancer, but the same cannot be said
for fewer doses.
When researchers initially developed the
HPV vaccine, which is a synthetic vaccine,
they didn t know exactly how many doses
would be needed for protection, Brady said.
So they modelled the dosing schedule after
that of other synthetic vaccines such as Hep-
atitis B, which require more than one dose.
However, the HPV vaccine is not like other
synthetic vaccines, it is made from so-called
virus-like particles, and it may turn out that
fewer doses than initially thought are needed,
Because the study evaluated only the Cer-
varix vaccine, it s not clear whether the find-
ings apply to the Gardasil vaccine, another
HPV vaccine that protects against four strains
of HPV, and is more widely used in the US.
The study is published in the November
issue of the journal Cancer Prevention
HPV vaccine: one dose may be enough
YOUR DAILY HEALTH
News and Advice
A single dose
of HPV vaccine
may be enough
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