The COVID-19 pandemic, which began in 2019, continues to mutate as it spreads further and infects more people. Vaccination, earlier looked upon with scepticism, has now become the only saviour from the never-ending cycle of the COVID-19 virus. A recent study has said that COVID-19 is less severe in fully vaccinated patients.

According to the multicentre study, published in the journal Radiology, the clinical and imaging characteristics of COVID-19 breakthrough infections in fully vaccinated patients tend to be milder than those of partially vaccinated or unvaccinated patients. The number of confirmed COVID-19 cases worldwide now exceeds 270 million with an overall mortality rate of approximately 2 per cent. COVID-19 vaccines are effective and critical tools for bringing the pandemic under control.

However, vaccines are not 100 per cent effective at preventing illness. Breakthrough infections are defined as the detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) ribonucleic acid (RNA) or antigen in a respiratory specimen collected from a person 14 days or more after receiving all recommended doses of COVID-19 vaccines.

Breakthrough cases are on the rise with the highly transmissible Omicron variant. Therefore, it became important to know how vaccination impacted not only COVID-19 disease severity but also clinical data and medical imaging results.

"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," said the study's senior author, Yeon Joo Jeong, M.D., PhD, from the Department of Radiology and Biomedical Research Institute at Pusan National University Hospital in Busan, South Korea.

"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." In this retrospective multicentre cohort study, Dr Jeong and colleagues analysed data from adult patients registered in an open data repository for COVID-19 -- Korean Imaging Cohort for COVID-19 (KICC-19) -- between June and August 2021. Hospitalised patients with baseline chest X-rays were divided into three groups, according to their vaccination status.

The researchers evaluated differences between clinical and imaging features and analysed associations between clinical factors -- including vaccination status -- and clinical outcomes. Of the 761 hospitalised patients with COVID-19, the mean age was 47 years, and 385 (51 per cent) were women. Forty-seven patients (6.2 per cent) were fully vaccinated (breakthrough infection), 127 were partially vaccinated (17 per cent), and 587 (77 per cent) were unvaccinated. Chest CT scans were performed on 412 (54 per cent) of the patients during hospitalisation.

Of patients undergoing CT, the proportion of CT scans without pneumonia was 22 per cent (71/326) of unvaccinated patients, 30 per cent (19/64) of partially vaccinated patients, and 59 per cent (13/22) of fully vaccinated patients. Fully vaccinated status was associated with a lower risk of requiring supplemental oxygen than unvaccinated status, as well as a lower risk of intensive care unit (ICU) admission.

The results also showed associations between the risk of severe disease and clinical characteristics such as higher age, history of diabetes, lymphocytopenia, thrombocytopenia, elevated LDH (lactate dehydrogenase), and elevated CRP (C-reactive protein). Notably, age was also found to be an important predictor of more severe disease in COVID-19 patients, even in those with a breakthrough infection.

The researchers noted that observed differences in clinical characteristics may reflect differences in vaccination priorities based on underlying comorbidities. During the study period, high-risk groups, such as individuals over 65 years old, health care workers and people with disabilities were priority targets for COVID-19 vaccination. Therefore, elderly patients and patients with at least one comorbidity were more common in the vaccinated group than in the unvaccinated group in the study.

"Despite these differences, mechanical ventilation and in-hospital death occurred only in the unvaccinated group," Dr Jeong said. "Furthermore, after adjusting for baseline clinical characteristics, the analysis showed that fully vaccinated patients were at significantly lower risk of requiring supplemental oxygen and of ICU admission than unvaccinated patients."

Although additional research will be needed as different variants emerge, this study sheds light on the clinical effectiveness of COVID-19 vaccination in the context of breakthrough infections.