That is why it is important to note that the vaccine has not been tested for efficacy (effectiveness) in younger girls, she said. Instead, the effectiveness was "bridged" from the older girls to the younger ones - meaning that Merck assumed that because it proved effective in the older girls, it also would be effective in the younger ones. The actual tests on the younger girls, ages 9 to 15, were only for safety and immune response, Harper said, and then only as a shot by itself, or in combination with only one other vaccine, Hepa is B. It has not been tested in conjunction with any other shots a girl receives at about age 11, Harper said.