Survey on chest CT findings in COVID-19 patients in Okinawa, Japan: differences between the delta and omicron variants

On Nov 21, 2023

This research was conducted in compliance with the “Declaration of Helsinki (revised in October 2013)” and the “Ethical Guidelines for Life Science and Medical Research Involving Human Subjects (Ministry of Education, Culture, Sports, Science and Technology, Ministry of Health, Labour and Welfare, Ministry of Economy, Trade and Industry Notification No. 1 in Reiwa 3)”.We performed a survey of chest CT examinations of patients with COVID-19 at 13 hospitals located in the central and southern areas of Okinawa Prefecture. These hospitals have board-certified radiologists and provide inpatient treatment for patients with COVID-19 pneumonia. Hospitals in the outlying islands (Miyako and Yaeyama Islands) and northern districts were excluded because of differences between the duration of waves of the COVID-19 pandemic in the islands and northern districts versus the waves in the central and southern areas of Okinawa Prefecture. We summarized the responses of 11 hospitals with complete answers.

Definition of pandemic wave

We defined the fifth wave of the pandemic as a period when there were 100 or more new infections per day (July 20 to September 25, 2021). For the sixth wave, the survey was conducted for the same number of days as the fifth wave (January 3 to March 10, 2022), starting from the date when the number of new infections exceeded 100 per day (Fig. 1). The total population of Okinawa Prefecture was 1,459,214 in July 2021. According to the reports of the Okinawa Prefecture COVID-19 Control, Epidemiology, and Statistical Analysis Committee, the proportion of Delta variant-caused infections was 78.9% during the week starting July 27, 2021, and the proportion of Omicron variant-caused infections was 87.2% during the week starting January 3, 2022. Therefore, the Delta variant was predominant during the fifth wave, and the Omicron variant was predominant in the sixth wave in Okinawa, Japan. Table 1 shows the data from the Okinawa Prefecture COVID-19 Control, Epidemiology, and Statistical Analysis Committee⁵.

Table 1 Data from the Okinawa Prefecture COVID-19 control, epidemiology, and statistical analysis committee⁵.

Survey

The list of survey questions was as follows:

Question 1: Chest CT criteria for COVID-19 patients.

The 3 criteria of patients with COVID-19 for undergoing chest CT were as follows: (1) Positive result on a reverse-transcriptase polymerase chain reaction [RT‒PCR] assay for SARS-CoV-2; (2) high-risk or hospitalized patients with moderate to severe COVID-19; and (3) determination by physicians.

Question 2: Number of patients with COVID-19 (confirmed by an RT‒PCR assay for SARS-CoV-2, genetic testing results were not required) who underwent chest CT during each wave (repeated exams for the same patient were not counted; only the initial CT assessment was counted).

Question 3-1 (for the cohort of Question 2): Characteristics of patients with COVID-19 who underwent CT:

Age (mean, median, minimum, maximum), percentage of male patients, number of patients needing tracheal intubation, number of patients who died, and number of patients with late-onset pneumonia who were classified as COVID-19 Reporting and Data System(CO-RADS) 1 at the initial assessment and had pneumonia on a repeated CT examination.

Question 3-2 (for the cohort of Question 2): CT image classification.

Number of patients in each COVID-19 Reporting and Data System CT image category (CO-RADS 0–5, Table 2).

Table 2 Chest CT findings of CO-RADS classification.

CT image classification

Chest CT images were classified into 5 categories according to CO-RADS (Table 2). The categories express the level of suspicion for COVID-19 pneumonia⁶ as follows: CO-RADS category 0, not interpretable (scan technically insufficient for assigning a score); CO-RADS category 1, very low (normal or noninfectious); CO-RADS category 2, low (typical for other infection but not COVID-19); CO-RADS category 3, equivocal/unsure (features compatible with COVID-19 pneumonia but also other diseases); CO-RADS category 4, high (suspicious for COVID-19 pneumonia); and CO-RADS category 5, very high (typical for COVID-19 pneumonia). CT classifications were performed by experienced radiologists at each institution.

Statistical analysis

Patients’ ages are expressed as the means and were compared by the paired t test. Categorical variables are expressed as numbers and percentages, and the variables were compared between the 2 waves by the chi-squared test. The CT implementation rate, which was the number of patients with COVID-19 undergoing CT examinations divided by the total number of COVID-19 cases in Okinawa Prefecture during the waves, was calculated. JMP 11 (SAS Institute Japan, Tokyo, Japan) was used for statistical analysis. A p value less than 0.05 was considered statistically significant.

Ethics approval

This survey was approved by the Ethics Committee for Clinical Research of University of the Ryukyus (approved number 23-2197-00-00-00). Written informed consent was obtained from all the participating facilities.

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