Neither vaccinations nor immunity from infections seem to thwart SARS-CoV-2 for long. The frequency of new infections within a few months of a previous bout or a shot is one of COVID-19’s most vexing puzzles. Now, scientists have learned that a little-known type of immune cell in the bone marrow may play a major role in this failure.

The study, which appeared last month in Nature Medicine, found that people who received repeated doses of vaccine, and in some cases also became infected with SARS-CoV-2, largely failed to make special antibody-producing cells called long-lived plasma cells (LLPCs).