COVID-19 vaccines do a great job preventing severe disease in most people, but they don’t work well for those with a severely compromised immune system.

In healthy people, the current vaccines work by triggering the production of antibodies that bind to the SARS-CoV-2 virus that causes COVID-19, preventing it from infecting healthy cells. But for people with fewer antibodies—including patients with blood cancers or taking medications that suppress their immune response—the body’s response is less effective.

Now, he and his colleagues have developed a vaccine that specifically activates T cells, which they are testing in clinical trials in Germany.

Their hope, Heitmann says, is “to protect those who cannot benefit from currently approved vaccines.”

Another advantage of T cell vaccines is that, because they