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Two doses of the vaccine that protects against human papillomavirus (HPV) may be just as effective as the three-dose regimen that is currently recommended, according to the latest research from the National Cancer Institute (NCI).

All girls and young women aged 9 to 26 are recommended to receive the three-shot inoculation, according to the Centers for Disease Control and Prevention (CDC). But vaccination rates remain low: the CDC reports that one-third of eligible teenagers in the U.S. start but do not finish the full three-dose regimen, which is given over six months. There are a variety of reasons for the failure, including scheduling conflicts and financial difficulties. At $400 for three doses, price can clearly be a problem for the uninsured and underinsured.

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Now the new NCI study, published in the Journal of the National Cancer Institute, finds that two doses of the vaccine may be just as effective as three. Even one dose may offer a high level of protection against HPV.

The study involved 7,466 women enrolled in the Costa Rica Vaccine Trial who were given either the HPV vaccine Cervarix or a vaccine for hepatitis A. Although all women receiving Cervarix were supposed to get the full three doses, about 20% did not for a variety of reasons.

Four years after vaccination, the researchers found that women who got two doses had similar levels of protection against the virus as those who got three. Even those who got a single dose of vaccine showed a high level of protection, but the number of women in that group was relatively small.

One important limitation of the finding is that researchers have thus far only been able to follow women over the short-term. The authors say that at least a decade of follow-up is needed to see how long protection may last. Also, the current study participants were between the ages of 18 and 25, while inoculation in the U.S. typically occurs at age 11 or 12. It’s not clear whether younger girls would have a similar response to fewer doses.

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For now, the current three-dose recommendation will remain intact. The vaccine protects against HPV strains 16 and 18, which are responsible for 75% of all cervical cancers.

Meredith Melnick is a reporter at TIME. Find her on Twitter at @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.