Fig. 4

Distribution of 28 day ventilator-free days categorized according to vaccine status. The primary series (− ) group included patients who did not complete the primary vaccine series (0 or 1 dose), the primary series ( +) group included patients who completed the primary vaccine series (2 doses), and the booster group included patients who received the booster vaccine (3 or more doses). The bar chart in the figure indicates the median number of 28 day ventilator-free days. The p-value was determined by the Kruskal–Wallis H test