Burden of long COVID and associated risk factors among Japanese adults: findings of the CARE Japan Study

Background We conducted a comprehensive assessment of long COVID and its associated risk factors, including pre-existing medical conditions, in a Japanese adult population. We aimed to inform public health recommendations, surveillance, and health care delivery for all patients with a history of COVID-19 infection. Methods We used prospective cohort data from the CARE Japan Study (https://japan.helpstopcovid19.com/), which collects data on participants’ demographics at baseline and information on diagnostic test results for COVID-19, vaccination status, and post-COVID-19 symptoms at baseline and at monthly follow-up surveys for up to 1 year after COVID-19 infection. The primary outcome was long COVID, defined as the presence of ≥ 1 symptom for ≥ 4 weeks after acute COVID-19 infection. We used Kaplan–Meier survival analysis to determine the median survival time of long COVID and a multivariable Cox proportional hazards model to yield hazard ratios (HRs) with 95% confidence interval (CIs) so as to explore the determinants of long COVID. Results The final analysis included 1497 respondents, 1149 patients with long COVID and 348 without long COVID. The period prevalence of long COVID was 63.9%, 68.2%, and 56.8% during the 1st quartile, 2nd and 3rd quartiles, and 4th quartile, respectively. During the 2nd and 3rd quartiles, runny nose (32%) and fatigue (31%) had the highest period prevalence, followed by headache (24%) and nasal congestion (21%). Compared with female participants, their male counterparts were marginally less likely (HR: 0.87, 95% CI: 0.77–0.99) to report long COVID symptoms. Age showed an inverse association with long COVID. Participants with pre-existing psychological disorders (HR: 1.45, 95% CI: 1.24–1.69), lung diseases (HR: 1.46, 95% CI: 1.10–1.92), or seasonal allergies (HR: 1.32, 95% CI: 1.17–1.49) had considerably higher risks of developing long COVID, compared with participants who had no pre-existing medical conditions. Conclusions More than half of participants experienced long COVID symptoms, even in the 4th quartile of the study period. Fatigue, runny nose, headache, and cough were the most frequently reported long COVID symptoms. Younger age, female sex, and having pre-existing medical conditions were associated with a greater risk of developing long COVID symptoms. Clinical Trial Number: Not applicable.