Does covid-19 have a lasting effect on the immune system?

T-cells (immune cells) remain active in multiple organs for at least two years after infection. A study involving 24 participants demonstrated this using a scan that visualises active immune cells. Notably, this applied not only to people with persistent symptoms, but also to people who felt fully recovered.

After recovering from covid-19, certain immune cells show long-lasting changes in their composition. In a cohort of 101 recovered patients in Tanzania, so-called B-cells (which produce antibodies) and NK-cells (which clear out damaged cells) were measurably different more than six months later. In people who had experienced severe illness, those changes were greater. Furthermore, the T-cells of severely ill individuals reacted more intensely to stimuli from an entirely different pathogen than the coronavirus, suggesting that the immune system has changed broadly, not just in its focus on the virus.

In Long Covid, even more disruptions have been identified. A cross-sectional study with 275 participants found higher antibodies against the Epstein-Barr virus in Long Covid patients compared with recovered controls. This may indicate that this dormant virus becomes active again after covid-19. Multiple reviews also describe exhausted immune cells, chronic low-grade inflammation, damage to blood vessel walls, and evidence of auto-immunity (in which the immune system attacks the body itself). In five Long Covid patients, viral RNA was found in intestinal tissue up to nearly two years after infection. The sample is too small for broad conclusions, but it fits the hypothesis that the virus hides in tissues and continues to activate the immune system.

The severity of the illness experienced plays a role. A small study (24 persons) found stronger immune memory responses after one year in people who had been more severely ill. People who had had almost no symptoms had weaker responses. Vaccination after a non-severe infection did noticeably raise antibody levels, as shown in the same Tanzanian cohort, although the numbers are limited.

Most findings come from observational research and reviews. Direct cause-and-effect relationships have not yet been fully established. How large the long-term clinical consequences are, and for whom the risk is greatest, remains an open question.