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The typical chest CT patterns seen in COVID-19 patients were far less frequent in individuals infected with Omicron and in the vaccinated, a retrospective multicenter study from France showed.

In the analysis of nearly 4,000 chest CTs of patients with confirmed SARS-CoV-2, multivariable analysis found that individuals infected during the period of Omicron predominance were 54% less likely to display typical CT findings (OR 0.46 vs Delta, 95% CI 0.37-0.57), Guillaume Gorincour, MD, PhD, of Centre Hospitalier Universitaire Édouard Herriot in Lyon, France, and colleagues reported.

These typical CT patterns of COVID -- ground glass opacities, intralobular reticulation, fibrotic bands, consolidations -- were also less frequent among individuals who received two (OR 0.32, 95% CI 0.25-0.41) or three (OR 0.20, 95% CI 0.15-0.27) vaccine doses, according to the findings in .

Furthermore, high severity scores per the French Society of Radiology–Society of Thoracic Imaging (SFR-SIT) were 53-67% less likely in patients who had received two (OR 0.47, 95% CI 0.36-0.60) or three (OR 0.33, 95% CI 0.24-0.46) doses of vaccine, regardless of age, sex, comorbidities, and SARS-CoV-2 variant.

"On a clinical point of view, those findings emphasize the need for good vaccine coverage and for the booster dose even with the Omicron variant," wrote Gorincour and colleagues.

"Our results confirm that both the Omicron variant and vaccination were associated with lower frequency of typical chest CT for COVID-19 and lesser extent of disease, with higher number of vaccine doses showing an increasing protective impact against high severity score," the group concluded.

In a linked , Soon Ho Yoon, MD, PhD, and Jin Mo Goo, MD, PhD, both of Seoul National University Hospital in South Korea, noted that "Omicron has been found to replicate more efficiently in the bronchi than in the lung parenchyma compared with other variants, and vaccination enhances immunity, inhibiting viral replication."

"As a result," they said, "both factors can lead to less frequent typical CT findings."

But while the study found a lower likelihood of typical COVID-19 patterns on CT among Omicron-infected patients, high severity scores on SFR-SIT were similar between Omicron versus Delta (OR 0.91, 95% CI 0.75-1.10).

"The limited impact of Omicron on CT severity may appear to contrast with its less severe clinical manifestations than other variants," wrote Yoon and Goo. "However, it should be noted that clinical studies on vaccine effectiveness tracked all vaccinated individuals, whereas radiologic studies evaluated individuals who underwent CT, and the indications for CT were often unidentified in those studies."

The analysis from Gorincour's group involved 3,876 patients at 93 emergency departments in France who had a positive SARS-CoV-2 test from July 2021 to March 2022 and who underwent chest CTs.

Patients had an average age of 66 years, and 43% were women. About 60% of the study population were unvaccinated, while 41% had received at least one dose, mostly with mRNA vaccines. Among the vaccinated subset, 52% had received two doses and 32% had received three doses.

CT scans of unvaccinated patients more often showed ground glass opacities (87% vs 63% for the vaccinated patients, P<0.001), intralobular reticulation (38% vs 25%, P<0.001), fibrotic bands (19% vs 15%, P=0.02), and consolidations (57% vs 35%, P<0.001). A high severity SFR-SIT score was observed in 25% of vaccinated patients versus 43% of the unvaccinated patients.

Most of the infections, 42%, occurred during the Omicron-predominant period, while 28% occurred during the Delta period and the rest during the transition between the two variants.

When looking at CT scans by predominant variant, those of patients with Delta infections more often contained ground glass opacities (85% vs 66% for Omicron patients, P<0.001), intralobular reticulation (35% vs 28%, P<0.001), fibrotic bands (17% vs 14%, P=0.03), and consolidations (54% vs 39%, P<0.001).

Patients with findings typical of non-SARS-CoV-2 lung infections (i.e., acute lobar pneumonia or bronchiolitis) were excluded from the analysis of severity, "because of the risk of confusion between features truly related to COVID-19 and features related to other infections," the study authors noted.

Limitations included that data were collected in real-life emergency departments, which "may have been imperfectly reported by patients and clinicians due to their workload," said Gorincour and colleagues. In addition, since reports were based on the SFR-SIT template, bronchial wall thickening and peribronchovascular predilection were not assessed, although they were noted to be more frequent in Omicron infections. Moreover, radiologists were aware of vaccine status during chest CT assessment, which may have biased their interpretation, and chest CTs were not necessarily ordered for COVID-19.

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