AbbreviationsCOVID-19 (coronavirus disease 2019), CT (computed tomography), CXR (chest radiograph), RT-PCR (reverse transcriptase-polymerase chain reaction), ICU (intensive care unit), WBC (white blood cell), LDH (lactate dehydrogenase), CRP (C-reactive protein) SummaryVaccinated patients with COVID-19 breakthrough infections showed fewer chest CT findings of pneumonia compared to unvaccinated patients. Key Results1. Of 761 hospitalized patients with COVID-19 disease and chest radiographs, 77% (587/761) were in unvaccinated patients while breakthrough infection in fully vaccinated subjects occurred in 6.2% (47/761) patients.2. The initial chest x-ray showed no pneumonia in 75% of fully vaccinated patients with breakthrough infection and 63% of unvaccinated patients (p=.37).3. In 412 patients with chest CT during hospitalization, a CT showed no pneumonia in 59% of fully vaccinated patients with breakthrough infection and 27% of unvaccinated patients (p=.01).

COVID-19 has emerged as a pandemic leading to a global public health crisis of unprecedented morbidity. A comprehensive insight into the imaging of COVID-19 has enabled early diagnosis, stratification of disease severity, and identification of potential sequelae. The evolution of COVID-19 can be divided into early infectious, pulmonary, and hyperinflammatory phases. Clinical features, imaging features, and management are different among the three phases. In the early stage, peripheral ground-glass opacities are predominant CT findings, and therapy directly targeting SARS-CoV-2 is effective. In the later stage, organizing pneumonia or diffuse alveolar damage pattern are predominant CT findings and anti-inflammatory therapies are more beneficial. The risk of severe disease or hospitalization is lower in breakthrough or Omicron variant infection compared with nonimmunized or Delta variant infections. The protection rates of the fourth dose of mRNA vaccination were 34% and 67% against overall infection and hospitalizations for severe illness, respectively. After acute COVID-19 pneumonia, most residual CT abnormalities gradually decreased in extent, but they may remain as linear or multifocal reticular or cystic lesions. Advanced insights into the pathophysiologic and imaging features of COVID-19 along with vaccine benefits have improved patient care, but emerging knowledge of post–COVID-19 condition, or long COVID, also presents radiology with new challenges. © RSNA, 2023 Supplemental material is available for this article.

Background Few reports have evaluated the effect of the SARS-CoV-2 variant and vaccination on the clinical and imaging features of COVID-19. Purpose To evaluate and compare the effect of vaccination and variant prevalence on the clinical and imaging features of infections by the SARS-CoV-2. Materials and Methods Consecutive adults hospitalized for confirmed COVID-19 at three centers (two academic medical centers and one community hospital) and registered in a nationwide open data repository for COVID-19 between August 2021 and March 2022 were retrospectively included. All patients had available chest radiographs or CT. Patients were divided into two groups according to predominant variant type over the study period. Differences between clinical and imaging features were analyzed using Pearson χ 2 test, Fisher exact test, or the independent t-test. Multivariable logistic regression analyses were used to evaluate the effect of variant predominance and vaccination status on imaging features of pneumonia and clinical severity. Results Of the 2180 patients (mean age, 57 years ± 21, 1171 women), 1022 patients (46%) were treated during the Delta variant predominant period and 1158 (54%) during the Omicron period. The Omicron variant prevalence was associated with lower pneumonia severity based on CT scores (OR, 0.71 [95% CI: 0.51, 0.99; P = .04]) and lower clinical severity based on ICU admission or in-hospital death (OR 0.43, 95% CI: 0.24, 0.77, P = .004) than the Delta variant prevalence. Vaccination was associated with the lowest odds of severe pneumonia based on CT scores (OR 0.05, 95% CI:0.03, 0.13, P < .001) and clinical severity based on ICU admission or in-hospital death (OR 0.15, 95% CI: 0.07, 0.31, P < .001) relative to no vaccination. Conclusion The SARS-CoV-2 Omicron variant prevalence and vaccination were associated with better clinical outcomes and lower severe pneumonia risk relative to Delta variant prevalence.

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