Characteristics of hospitalized patients with COVID-19 during the first to fifth waves of infection: a report from the Japan COVID-19 Task Force

Abstract

Background: We aimed to elucidate differences in the characteristics of patients with coronavirus disease 2019 (COVID-19) requiring hospitalization in Japan, by COVID-19 waves, from conventional strains to the Delta variant.

Methods: We used secondary data from a database and performed a retrospective cohort study that included 3261 patients aged ≥ 18 years enrolled from 78 hospitals that participated in the Japan COVID-19 Task Force between February 2020 and September 2021.

Results: Patients hospitalized during the second (mean age, 53.2 years [standard deviation {SD}, ± 18.9]) and fifth (mean age, 50.7 years [SD ± 13.9]) COVID-19 waves had a lower mean age than those hospitalized during the other COVID-19 waves. Patients hospitalized during the first COVID-19 wave had a longer hospital stay (mean, 30.3 days [SD ± 21.5], p < 0.0001), and post-hospitalization complications, such as bacterial infections (21.3%, p < 0.0001), were also noticeable. In addition, there was an increase in the use of drugs such as remdesivir/baricitinib/tocilizumab/steroids during the latter COVID-19 waves. In the fifth COVID-19 wave, patients exhibited a greater number of presenting symptoms, and a higher percentage of patients required oxygen therapy at the time of admission. However, the percentage of patients requiring invasive mechanical ventilation was the highest in the first COVID-19 wave and the mortality rate was the highest in the third COVID-19 wave.

Conclusions: We identified differences in clinical characteristics of hospitalized patients with COVID-19 in each COVID-19 wave up to the fifth COVID-19 wave in Japan. The fifth COVID-19 wave was associated with greater disease severity on admission, the third COVID-19 wave had the highest mortality rate, and the first COVID-19 wave had the highest percentage of patients requiring mechanical ventilation.

Keywords: COVID-19; Hospitalization; Respiratory infection; Wave of infection.

Fig. 1

Number of newly diagnosed COVID-19 cases between January 2020 and September 2021 in Japan in each COVID-19 wave. The first case of COVID-19 in Japan was reported on January 16, 2020. Remdesivir was approved by Japan on May 7, 2020. Dexamethasone was first published in the Clinical Practice Guide on July 17, 2020. Baricitinib was approved for use on April 23, 2021

Fig. 2

Flowchart describing patient selection. All consecutive patients with COVID-19 aged ≥ 18 years who were hospitalized during the study period (between February 2020 and September 2021) and recruited through the Japan COVID-19 Task Force were included. After excluding 160 patients, 3261 patients were enrolled in this study

Fig. 3

Comparison of clinical symptoms. The total number of eight clinical symptoms per COVID-19 wave of SD is shown A. In addition to analysis of variance, all COVID-19 waves were compared using the Tukey–Kramer method (fifth COVID-19 wave vs. first, second, third, and fourth COVID-19 waves: p < 0.0001). A comparison using analysis of variance for each of the eight clinical symptoms (disturbance of consciousness, p = 0.0291; fever, p < 0.0001; malaise, p < 0.0001; cough/sputum, p < 0.0001; dyspnea, p < 0.0001; nasal discharge/pharyngeal pain, p = 0.0256; gastrointestinal symptoms, abdominal pain/diarrhea/nausea and vomiting, p < 0.0001; and taste and smell disturbances, p < 0.0001) are shown in B

Fig. 4

Comparison of medications prescribed per COVID-19 waves. Data are presented as n (%). p values were calculated using the χ² test

Fig. 5

Comparison of COVID-19 severity in each COVID-19 wave. The percentage of COVID-19 severity on admission and the worst condition during hospitalization: low-flow oxygen therapy, high-flow oxygen therapy (using high-flow nasal cannula oxygen therapy or non-invasive positive pressure ventilation) using invasive mechanical ventilation, and hospital death data are shown for each wave. The vertical axis of each wave shows rates of 0–100%