evidence: Because long COVID is a new entity (it has been in existence for less than 5 years), longitudinal studies to characterize the long-term health trajectories of people with long COVID—up to 10 years, 20 years and 30 years—are needed, to understand rates and predictors of recovery and relapse of the various manifestations. These long-term studies will also help identify any latent consequences of the disease (that is, impacts that have not yet been realized) and secondary consequences (for example, the downstream health effects that emanate from long COVID). For example, understanding whether people with cognitive dysfunction (or brain fog) are at a higher risk of developing neurodegenerative diseases later in life is critical. [...] Various interim definitions of long COVID exist39,278,279,280,281,282,283, but there is not yet a universal consensus on the most optimal definition—which must be sufficiently nuanced to capture the complexity of the condition and its various manifestations. It is unlikely that a single definition will fit all needs. Consensus definitions that are optimized and empirically tested for various applications, including clinical care, epidemiological surveillance, and research, should be developed. Definitions must necessarily evolve to incorporate new understanding as the evidence base for long COVID grows.