Unlike traditional vaccines, which typically deliver an inactivated or weakened version of a virus to stimulate a person’s immune response, mRNA vaccines deliver genetic instructions that create a bit of a virus inside the individual’s cells. “Everyone is very familiar with mRNA vaccines from the pandemic,” said Sidi Chen, associate professor of genetics and neurosurgery at Yale School of Medicine, who served as the study’s senior author. Antigens are the substances that the immune system recognizes as foreign or possibly harmful, which then triggers an immune response. But if the body doesn’t recognize an antigen, it can’t mount a good immune response. The problem, Chen explained, is that some antigens created by mRNA vaccine are unable to make it to surface.