ROCKVILLE, Md. (AP) — Novavax Inc. said Wednesday that the swine flu vaccine it’s developing prompted an immune system response in patients during a midstage study being conducted in Mexico. Novavax is seeking approval in Mexico for a vaccine that fights the 2009 swine flu strain and said the study results should help in that effort. Novavax will move forward with a second stage in the study. Novavax says its vaccine can be grown in weeks instead of the months it takes to make a traditional vaccine. Most flu vaccines are grown inside chicken eggs, but Novavax uses genetically engineered virus-like particles, which resemble a virus but do not have the genetic material a virus needs to reproduce itself. The company does not expect U.S. approval of its vaccines for at least a few years.