"A breakthrough infection is a case of illness in which a vaccinated individual becomes sick from the same illness that the vaccine is meant to prevent. Simply,

"A breakthrough infection is a case of illness in which a vaccinated individual becomes sick from the same illness that the vaccine is meant to prevent. Simply, they occur when vaccines fail to provide immunity against the pathogen they are designed to target." via https://en.wikipedia.org/wiki/Breakthrough_infection https://en.wikipedia.org/wiki/Breakthrough_infection As far as "breakthrough infection increased risk of death", that's explained a little further down on the page in the article linked here on HN: Risk of death after SARS-CoV-2 infection was highest in unvaccinated Veterans regardless of age and comorbidity (Fig. 3). However, breakthrough infections were not benign, as shown by the higher risk of death in fully vaccinated Veterans who became infected compared to vaccinated Veterans who remained infection-free. This is a pretty long-winded way of saying that some people who were vaccinated still became infected, and some of those infected people did die. The importance of a study like this is to understand how the efficacy of the vaccine holds up over time, which can help inform public health recommendations and policy-making decisions.