Two types, 16 and 18, are linked to 70% of cases of cervical cancer, diagnosed in 470,000 women worldwide each year but rare in the U.S., thanks to widespread screening. These types are also linked to less common cancers: About 80% of cancers of the anus or vulva are associated with HPV 16, and about half of penile cancers are linked to either 16 or 18, says Luisa Villa, lead study author and biologist with the Ludwig Institute for Cancer Research in Sao Paulo, Brazil.

No one who received the vaccine developed genital warts, but three in the placebo group did. And no one who received the vaccine developed pre-cancerous cervical lesions, but three who got the placebo did. Pain at the injection site was the only vaccine side effect.

[Eliav Barr] and Villa's paper suggests that universal HPV vaccination might be most effective in 10-to-13-year-olds, who are not likely to be infected with HPV. The large ongoing trial includes boys and girls as young as 9, Barr says.