A new bivalent oral vaccine that targets two deadly strains of polio -- types 1 and 3 -- triggers a stronger immune response than the existing trivalent vaccine and an immune response that's similar to monovalent vaccines, a new study shows.

Use of trivalent and monovalent vaccines has reduced the number of countries where polio is endemic from more than 125 in 1988 to just four today. Transmission of polio virus types 1 and 3 persists in parts of Afghanistan, Pakistan, India and Nigeria.

The bivalent oral vaccine is already widely used in efforts to halt transmission of polio virus types 1 and 3, but this study set out to examine whether it triggers as much immunity as the other vaccines.

Between August and December 2008, researchers gave 830 newborns in India either the monovalent, bivalent or trivalent vaccines in two doses, including one at birth and one 30 days later. Antibody levels were measured in blood samples before vaccination and after the first and second doses of the vaccines.

Overall, the immune response induced by the bivalent vaccine was significantly higher than the trivalent vaccine and similar to the monovalent vaccine, the investigators found.

The study was released online Oct. 25 in advance of publication in an upcoming print issue of The Lancet.

"The major advantages of the bivalent vaccine ... is that it will enhance individual and population immunity simultaneously for both poliovirus types 1 and 3, without any serious loss in immunogenicity compared with the [monovalent vaccines]," the researchers said in a news release from the publisher.