Dr. Jakob Cramer, the head of clinical development at CEPI, a vaccine development organization, said that vaccines using viral vectors were not the only kind that might benefit from mixing. In fact, certain combinations might provoke a different, more effective immune response than a single type of vaccine. “Immunologically, there are several arguments in favor of exploring heterologous priming,” Dr. Kramer said.

Another kind of Covid-19 vaccine being tested contains the actual spike protein, rather than genetic instructions for it. Some of the vaccines contain the entire protein; others contain just a fragment of it. Currently, there are 29 protein-based vaccines for Covid-19 in clinical trials, although none have been authorized yet.

Dr. Wheatley and his colleagues have been testing protein-based vaccines in mice. They injected the full spike protein into the animals as a first dose. For the second dose, they injected only the tip of the spike, a region known as the receptor-binding domain, or R.B.D.