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A single dose of the human papillomavirus vaccine is highly effective in preventing infection for three years, possibly reducing rates of cervical cancer and other diseases linked to the virus, according to a new study in Kenya.
A single-dose strategy would dramatically increase vaccine supply, lower costs and simplify distribution, which would make vaccination a more viable option in countries with limited resources, experts say.
HPV is a sexually transmitted infection that is linked to cervical cancer and other malignancies. Health officials in many countries, including the United States, recommend two doses of the vaccine for 15-year-old girls, and three doses for older adults.
But observational data have long suggested that a single dose provides effective protection against HPV for at least ten years. The new results are the first confirmation from a gold standard clinical trial that a single dose can be as effective as two or three doses, for at least three years.
Results of a direct comparison of the single- and two-dose regimens will not be available until 2025.
At least 24 countries, including Mexico, Tonga and Guyana, have moved to a single-dose approach, according to the World Health Organization.
New evidence may convince more countries to adopt the strategy.
“What we have predicted is that it will be most interesting for low- or middle-income countries,” said Paul Bloem, senior adviser on the HPV vaccination program at the WHO. he notes.
The WHO estimates that if widely distributed, the single-dose strategy could prevent 60 million cases of cervical cancer and 45 million deaths worldwide over the next 100 years.
Cervical cancer is the fourth most common type of cancer in women worldwide, with an estimated 604,000 new cases in 2020, according to WHO The disease killed an estimated 342,000 women in 2020, more than the number who died during pregnancy or childbirth .
“This is a real killer of women,” said Dr. Seth Berkley, chief executive of Gavi, which funds immunization programs in low-income countries.
“It’s also a disease that kills women as they age,” he said, “and it does so in horrific ways.”
More than 95 percent of cervical cancers are caused by sexually transmitted HPV. Many strains of the virus are common, but subtypes 16 and 18 are responsible for 70 percent of cervical cancers.
The HPV vaccine debuted in 2006 and is “an almost perfect preventive intervention for cervical cancer and HPV-related cancers,” says Dr. Ruanne Barnabas, chief of the division of infectious diseases at Massachusetts General Hospital, who led the new study.
The Food and Drug Administration approved the vaccine that year in the United States, and since then infections with the cancer-causing strain of the virus have dropped more than 80 percent in the country, according to the Centers for Disease Control and Prevention.
Still, approximately 13,000 Americans are diagnosed with cervical cancer each year; approximately 4,000 women die each year from the disease.
The number of HPV victims is higher in low- and middle-income countries, where women have limited access to cervical cancer screening or treatment for the disease. About 90 percent of deaths from cervical cancer in 2020 will be among women living in resource-poor countries.
In Kenya, the vaccine is currently given in two doses. But only 33 percent of girls ages 9 to 14 received the first dose, and only 16 percent returned for the second. In contrast, more than 78 percent of adolescent girls in the United States will receive at least one dose of the vaccine by 2021.
Single-dose vaccination regimens are easier to implement on a large scale and open up more delivery channels, such as village-wide campaigns and mobile clinics.
“This allows opportunities for the creativity of delivery engines,” said Dr. Peter Dull, who leads HPV vaccine development at the Bill & Melinda Gates Foundation, which funded the study.
In the KEN SHE study, researchers randomly assigned 2,275 Kenyan women aged 15 to 20 to receive a single dose of the HPV vaccine targeting subtypes 16 and 18; the HPV vaccine targets 16, 18 and seven other subtypes; or meningococcal vaccine, as a control.
The scientists collected cervical and vaginal swabs from the women every six months and looked for persistent HPV infection for up to 36 months.
The vaccine was 98 percent effective against virus subtypes 16 and 18 over three years, and 96 percent against all cancer-causing strains, the study found. No serious side effects were reported.
The results of a previous study, published last year, showed that a single dose of the vaccine was very effective for 18 months.
Based in part on this evidence, last year the WHO changed the recommendation to one or two doses for girls and young women aged 9 to 20 years, and two doses at six-month intervals for women over 21 years.
The Gavi-funded program has so far only reached a third of its goal, partly due to a lack of vaccine supplies. About 20 million doses will be available by 2022, said Dr. Berkley, but that number is expected to more than triple by 2025.
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