Mixing vaccines — scientists call it “heterologous prime-boost” — is not a new idea, and researchers have experimented with it in fighting a handful of other diseases, like Ebola. Scientists have long theorized that giving people two slightly different vaccines might generate a stronger immune response, perhaps because the vaccines stimulate slightly different parts of the immune system or teach it to recognize different parts of an invading pathogen. “The argument is that one and one makes three,” said John Moore, a virologist at Weill Cornell Medicine. “How well that argument holds up in practice in the Covid area is going to need to be judged by the actual data.”