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Objective:
The primary purpose of the study is to identify the roles of the chest CT imaging and vaccination in the COVID-19 pandemic. This retrospective study also aimed to establish the association between the chest CT total severity scores (CT-TSS) of adult COVID-19 positive patients admitted at Philippine General Hospital (PGH) and their vaccination status; compare the different COVID-19 vaccine brands administered to the patients based on their chest CT-TSS; and determine the incidence and characteristics of chest CT scan findings of COVID-19 vaccine breakthrough cases.

Materials and Methods:
We reviewed demographic data, COVID-19 vaccination records and chest CT scans of adult patients (> 18 years of age) who were confirmed to have COVID-19 via RT-PCR and were admitted at PGH from March 2021 to December 2022 was done. The chest CT-TSS by Kunwei et. al was used, due to its high specificity, to assess the degree of affectation of the lung parenchyma due to COVID-19. The TSS lung involvement was categorized as severe disease if the total score was more than or equal to 7.5.

Results:
Results showed that chest CT severity scores and severity score categories have a strong and significant association with the vaccination status of COVID-19 patients (<em>p</em> < 0.001). Based on the median score of the patient’s chest CT-TSS and the incidence of severe COVID-19 infections, Sinovac (whole inactivated virus vaccine) showed the highest chest CT-TSS score (10) and incidence of severe COVID-19 infections (52.3%) among the study population while Moderna (messenger RNA vaccine) showed the least number (median CT-TSS of 4 and 4.65% severe disease). Results showed that there is a moderately significant association between the first COVID-19 vaccination brands and the severity score category of the patients (<em>p</em> = 0.048). COVID-19 breakthrough infections (104/341 patients) were also identified in this study and most of them received the primary series of Sinovac (51.33%). The three most predominant chest CT scan findings among the breakthrough cases, in decreasing order, are ground glass densities (25.82%), consolidation (18.96%), and crazy paving pattern (17.31%).

Conclusion:
Completion of the primary vaccine series and administration of booster shots are relevant actions for the management and control of the COVID-19 infection. In light of the different COVID-19 vaccine brands, the efficacy of the each vaccine should also be considered. This study has provided significant evidence of vaccine efficacy in reducing disease severity in patients with COVID-19 based on their chest CT-TSS. Furthermore, breakthrough infections still emerge despite the efficacy of the COVID-19 vaccinations. But the majority of these patients present with a milder form of disease compared to unvaccinated patients. It is crucial to comprehend vaccination breakthrough infections to effectively combat continuing COVID-19 infections and prevent possible future outbreaks.