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Protection may last even longer, scientists reported. The finding may be a boon to low-income countries, where cervical cancer takes an enormous toll.
A single dose of the human papillomavirus vaccine is highly effective at preventing infections over three years, most likely lowering rates of cervical cancer and other diseases linked to the virus, according to a new study in Kenya.
A single-dose strategy would dramatically extend supplies of the vaccine, lower costs and simplify distribution, which would make vaccination a more viable option in countries with limited resources, experts said.
HPV is a sexually transmitted infection linked to cervical cancer and other malignancies. Health officials in many countries, including the United States, recommend two doses of the vaccine for adolescent girls younger than 15, and three doses for those who are older.
But observational data has long suggested that a single dose offers effective protection against HPV for at least a decade. The new results are the first confirmation from a gold-standard clinical trial that a single dose may be as effective as two or three doses, at least over three years.
Results of a direct comparison of one- and two-dose regimens will not be available until 2025.
At least 24 countries, including Mexico, Tonga and Guyana, have shifted to the one-dose approach, according to the World Health Organization.
The new evidence may convince more countries to adopt the strategy.
“What we had predicted was that this would be most interesting for the low- or middle-income countries,” said Paul Bloem, a senior adviser on HPV vaccination programs at the W.H.O. But high-income countries like Britain and Australia were among the first to change their policies, he noted.
The W.H.O. estimates that if widely deployed, a single-dose strategy could prevent 60 million cervical cancer cases 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 the W.H.O. 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 lower-income nations.
“It is also a disease that really kills women in the prime of their lives,” he added, “and does it in a really horrible way.”
More than 95 percent of cervical cancer is caused by sexually transmitted HPV. Multiple strains of the virus are prevalent, but subtypes 16 and 18 are responsible for 70 percent of cervical cancers.
The HPV vaccine debuted in 2006 and is a “near-perfect prevention intervention for cervical cancer and other HPV-associated cancers,” said 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 viral strains that cause cancers have dropped by more than 80 percent in the country, according to the Centers for Disease Control and Prevention.
Still, about 13,000 Americans are diagnosed with cervical cancer each year; about 4,000 women die of the disease annually.
HPV’s toll is much higher in low- and middle-income countries, where women have limited access to screening for cervical cancer or treatments for the disease. Roughly 90 percent of the deaths from cervical cancer in 2020 were among women who lived in resource-poor countries.
In Kenya, the vaccine is currently given in two doses. But only 33 percent of girls ages 9 to 14 receive the first dose, and only 16 percent return for the second. By contrast, more than 78 percent of adolescent girls in the United States received at least one dose of the vaccine in 2021.
A single-dose vaccination regimen is much easier to implement on a large scale and opens up more channels of delivery, such as village-wide campaigns and mobile clinics.
“It allows an opportunity for creativity of the delivery machinery,” 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 ages 15 to 20 to receive a single dose of the HPV vaccine targeting subtypes 16 and 18; an HPV vaccine targeting 16, 18 and seven other subtypes; or the meningococcal vaccine, as a control.
The scientists collected cervical and vaginal swabs from the women every six months and looked for persistent HPV infection up to 36 months.
The vaccine had an efficacy of 98 percent against the 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.
Earlier results from the study, published last year, showed that a single dose of both vaccines was highly effective for 18 months.
Based in part on that evidence, last year the W.H.O. changed its recommendation to one or two doses for girls and young women ages 9 to 20 years, and two doses with a six-month interval for women over 21.
Programs funded by Gavi have so far reached only about one-third of their goal, in part because of a shortage in vaccine supply. Roughly 20 million doses were available in 2022, Dr. Berkley said, but that number is expected to more than triple by 2025.