NEW YORK (AP) — Three experimental Zika vaccines protected monkeys against infection from the virus, an encouraging sign as research moves into studies in people.

The experiment involved a traditional vaccine and two more cutting-edge ones. The traditional vaccine is expected to enter preliminary human studies this year. At least two other experimental Zika vaccines are in human studies already.

Researchers reported the success in monkey studies Thursday in a paper released by the journal Science. The traditional vaccine used a dead virus, while the other two used a single gene of the Zika virus to prime the monkeys’ immune systems to fight off the germ.

Efforts to develop a vaccine began after a massive Zika outbreak last year in Brazil.