This study aimed to evaluate the COVID-19 hospitalizations in a tertiary hospital by age group and month, considering the introduction and the advance of the vaccination against the disease. The laboratory-confirmed COVID-19-associated hospitalizations among people aged 20 years or older, that occurred between March 2020 and June 2021, were distributed by month of symptom onset and age group. The proportion of hospitalizations by age group was calculated for the year 2021. The proportions were compared using the chi-square test for trends. The marks of vaccination advances among different age groups were taken from the official website LocalizaSUS. In 2020, hospitalizations among people aged 60-80 years old were the most frequent (39.1%). From January-June 2021, when the vaccination commenced, while hospitalizations of patients aged 20 to < 40 and 40 to 60 years old showed an increasing trend, the older age groups and those with vaccination recommendations (from 60 to < 80 and from 80 or over) showed a downward trend. As of June 2021, with widespread vaccination, a drop in hospitalizations was observed in > 60 years old. At 20 to <40 and 40 to < 60, an increase in hospitalizations was observed. It demonstrates the important role of vaccination in combating the COVID-19 pandemic.