Differences in symptoms and clinical outcomes of COVID-19 infection across vaccination status and age groups during the national pandemic from December 2022 to February 2023 in China: a cross-sectional study.

BACKGROUND: The end of the 'dynamic zero-COVID' policy on 7 December 2022 caused an immediate Omicron outbreak in China, which is the first nationwide COVID-19 wave in China. We aimed to assess the difference in COVID-related symptoms and clinical outcomes across vaccination status and age groups. METHODS: This was a cross-sectional study. Data were collected via online survey or telephone interview. The target population was adults who had been infected during this wave since December 2022 and were not hospitalized due to COVID-19 infection at the survey time. Information on COVID-19 infection status, COVID-related symptoms, COVID-related pneumonia, clinic visits, hospital admissions, and dose of vaccines received was collected. Data were analyzed by log-binomial regression and negative binomial regression. RESULTS: Of 2350 participants aged 18-98 years, 82% were COVID-infected or probably infected, 81% of the participants had received at least one dose of a vaccine and 79% had received two or more doses. The percentage of vaccination decreased with age. Compared to the unvaccinated group, receiving three or more doses of vaccine was associated with a lower risk of pneumonia (prevalence ratio, PR: 0.550, 95% CI: 0.377 to 0.805), clinic visits (PR: 0.629, 95% CI: 0.488 to 0.810), and hospital admission (PR: 0.293, 95% CI: 0.162 to 0.530). Compared to young adults (aged below 36 years), older adults (aged above 75 years) were more likely to have fatigue (PR: 1.224, 95% CI 1.078 to 1.389), had a longer length of time to symptom disappearance (odds ratio 3.784, 95% CI 2.657 to 5.387). Compared to adults aged below 56 years, those aged above 75 years were associated with a higher risk of having COVID-19 related pneumonia (PR 3.669, 95% CI 2.190 to 6.148), clinic visits (PR: 1.727, 95%: 1.237 to 2.412), and hospitalization (PR 6.656, 95% CI 2.609 to 16.980). After further adjustment for multimorbidity, the effect size of the association between age groups and clinical outcomes was reduced. CONCLUSIONS: Older adults are at higher risk of severe COVID-related outcomes, and vaccination seems to protect against severe outcomes. Ongoing vaccination is necessary.