After the invasion of Poland by the Soviet Union in 1939, Weigl continued his research and work at an institution in Lemberg. There, he was able to increase production of his typhus vaccine. Weigl spent the next four years in Lemberg focusing his research on developing a vaccine for the spotted fever. He lead and directed the "Institute for Typhus and Virus Research" based in Lemberg. Weigl was able to create a vaccine for the spotted fever however, it did not offer immunity against the disease. Instead, it reduced the symptoms, allowing for a much milder infection in humans.