Reactive synovitis of the knee joint after COVID-19 vaccination: The first ultrastructural analysis of synovial fluid.

Background: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and the resulting coronavirus 2019 disease (COVID-19) have spread all around the world since 2019 and have affected millions of people. The development of COVID-19 vaccines helped to decelerate the spread of the virus. However, as in the case of vaccines against other infectious diseases, adverse events can also present with COVID-19 vaccines.

Case Presentation: We report here a rare case of a 53-year-old man with knee-joint synovitis, after the first dose of messenger RNA vaccine, with no fever and a negative COVID-19 reverse transcription polymerase chain reaction test. During a clinical examination the suspicion of pyogenic arthritis was excluded by blood tests and by a complex joint effusion examination, including a microbiological and cytological-energy analysis of the synovial fluid. The treatment received by our patient consisted of 3 doses of dexamethasone administered intravenously over a period of 3 days. All the symptoms improved after this therapy, and in the 3-week follow-up period we recorded full recovery with no consequences.

Conclusion: Case reports on patients undergoing COVID-19 vaccination should be examined in order to detect rare and long-term side-effects. This is the first report to present the outcomes of an ultrastructural analysis of post-vaccination synovitis.