COVID-19 infections appear to be less severe among fully vaccinated patients, according to a new multi-center analysis of CT images published Tuesday in Radiology.

Breakthrough cases—occurring two weeks after the final dose—trended upward following the highly contagious omicron variant’s arrival. South Korean scientists recently set out to explore the imaging implications among such patients, reviewing data from 761 individuals hospitalized with COVID-19.

They found that fully and even partially vaccinated individuals had a significantly lower risk of requiring supplemental oxygen or being admitted to the intensive care unit. About 59% of those who received all doses had no evidence of COVID pneumonia on CT scans, versus 22% for unvaccinated patients.

“Although the risk of infection is much lower among vaccinated individuals, and vaccination reduces the severity of illness, clinical and imaging data of COVID-19 breakthrough infections have not been reported in detail,” study author Yeon Joo Jeong, MD, PhD, with the department of radiology at Pusan National University Hospital in Busan, South Korea, said in a statement. “The purpose of this study was to document the clinical and imaging features of COVID-19 breakthrough infections and compare them with those of infections in unvaccinated patients.”

For the retrospective study, Jeong et al. analyzed info across three centers, gathered through the Korean Imaging Cohort for COVID-19 open data repository. Adults in the study were an average age of 47 and treated between June and August of last year. Researchers separated patients who had received a baseline initial chest radiograph into three groups—fully (6%), partially (17%) and not vaccinated (77%). About 54% of patients in the study received a CT scan during their hospital stay, or more than 400 individuals. Out of that group, scans showed zero signs of pneumonia for 13/22 of the fully vaccinated compared to 71/326 for the unvaxxed cohort.

In a corresponding editorial, experts said the results “add up” and serve as powerful visual evidence for the field.

“The adage of ‘a picture is worth a thousand words’ holds true. [Study author Jong Eun Lee, MD] et al. have shown that burden of COVID pneumonia is much less in those who have been vaccinated,” Mark Schiebler, MD, and David Bluemke, MD, PhD, both with the University of Wisconsin–Madison School of Medicine, wrote Feb. 1. “The authors have furthered our understanding of vaccine effectiveness. If seeing is believing, the visual evidence provided by Lee et al. might even help to strengthen the hand of public health officials still working to overcome the problem of vaccine hesitancy. We can only hope.”

Read the full study in the Radiological Society of North America’s flagship journal here.