Even the sickest COVID-19 patients produce T cells to battle the virus, a new study reveals.

The research, published in Science Immunology, provides more proof that any vaccine for the deadly virus will have to bring T cells along to work with antibodies.

"This is key to understanding how the immune response fights the virus," said Alessandro Sette, a professor at La Jolla Institute for Immunology, who co-led the study, in a statement.

By closely examining their immune responses, it was clear that all 10 patients produced T cells targeting SARS-CoV-2, the virus that causes COVID-19.

"Activating these cells appears to be at least as important as the production of antibodies," said Erasmus MC virologist Rory de Vries, who co-led the study.