Merck’s Gardasil vaccine for cervical cancer isn’t cheap — it’s a three-shot series that costs about $360. Whether it’s worth the money depends on several variables, suggests an analysis in this week’s New England Journal of Medicine.

Assuming the vaccine provides lifelong immunity, vaccinating girls at age 12 costs $43,600 per quality-adjusted life-year (QALY) gained — a figure the authors, from Harvard’s School of Public Health, call “economically attractive.”

But the cost per QALY goes up significantly for vaccinating older girls and women, who are more likely to be infected already with HPV, the sexually transmitted virus that can cause cervical cancer.

The vaccine was introduced only two years ago; a temporary catch-up program to vaccinate girls up to age 18 would cost $97,300 per QALY, and extending vaccination to age 26 costs $152,700 per QALY, the analysis found. A Merck researcher told Dow Jones Newswires that the company’s estimates put the cost at less than $50,000 per QALY for women up to age 26.

(QALY, a standard metric used to analyze cost-effectiveness of medical interventions, is explained here.)

Likewise, the vaccine becomes less cost-effective if vaccine-induced immunity wears off after, say, 10 years.