Sounds like they don't know how the memory in the cell will react yet.

"Jin Dong-Yan, a virology professor at the University of Hong Kong who was not part of the research group, said the study does not negate the possibility that other parts of the immune system could offer protection.

Some cells memorize how to cope with a virus when first infected and can muster effective protection if there is a second round of infection, he said. Scientists are still investigating whether this mechanism works for the new coronavirus."