We conducted the study with 541 patients. The mean age was 70.2, and 52.1% of the patients were women. 73.6% of the patients were 65 years or older. The most common comorbidities were hypertension, diabetes mellitus, and COPD. The rate of the alpha variant was 54.3%, and the delta variant was 29.4%. The mortality rate was 45.8%, and the ICU admission rate was 55.3%. The day between the last dose of vaccine and hospitalization was 120± 56 days. Sixty-five years and older was higher ICU admission and mortality, and female patients were less admitted to the ICU. Except for diabetes, ICU admission and mortality were higher in the comorbidities. While the delta (B.1.617.2) variant had higher ICU admission, the alpha (B.1.1.7) variant had a higher mortality rate. There was no difference between the time between the last vaccination dose to hospitalization, ICU admission, and mortality. Patients vaccinated with two-dose Sinovac-CoronaVac had a higher mortality rate.
Consistent with the literature, advanced age, male gender, and comorbidities are associated with poor outcomes such as mortality and ICU admission in our study . Diabetes mellitus prepare the ground for the severe course of COVID-19 and is known to increase the risk of death . But we did not find this result in our study.
In our study, the in-hospital mortality rate was relatively high compared to the literature [8,9]. We think the mortality rate is high, as our research mainly included elderly patients with moderate and severe COVID-19.
There is a higher risk of hospital admission and death for COVID-19 patients infected with the delta variant than the alpha variant. The delta variant causes a more significant health burden than the alpha variant, especially in unvaccinated populations [10,11]. Our study included only vaccinated individuals, and individuals infected with the delta variant had higher ICU admission, but the alpha variant had a higher mortality rate. We think this difference maybe because most of the patients had the alpha variant in our study.
For approximately six months after two vaccine doses, protective antibodies decrease about five times in the BNT162b2 vaccine and six times in the CoronaVac vaccine [12,13]. In our study, the average time between hospitalization after the second dose was four months and was not associated with death or ICU admission. Our research results conclude that both vaccines effectively prevent death and ICU admission.
CoronaVac, produced by the Sinovac company, is the world's most widely used COVID-19 vaccine. The CoronaVac vaccine is an inactivated vaccine that uses the killed SARS-CoV-2 virus and is less effective because it triggers an immune response against many viral proteins. In contrast, mRNA vaccines start reacting to the spike protein the virus uses to enter human cells . The Chilean government has reported that, based on data from nearly two million people who received two doses of CoronaVac and the third dose of CoronaVac, Pfizer-BioNTech or Oxford-AstraZeneca vaccine, protection against COVID-19 increased from fifty percent to eighty percent after two vaccinations . In our study, patients were given a third or fourth dose of the Pfizer-BioNTech vaccine after two doses of the CoronaVac vaccine. In the light of this information, we suggest administering a third and fourth dose of mRNA vaccine to subjects vaccinated primarily with two doses of CoronaVac.
Our study has several limitations. The main limitations of our study include its retrospective design, short follow-up period, and assessment of hospitalized patients only. Another limitation of the study; we only examined moderate and severe patients.
In conclusion, our study found that patients vaccinated with only two-dose CoronaVac had a higher mortality rate. However, there was no difference in ICU hospitalization between the CoronoVac, BioNTech, and CoronaVac plus BioNTech groups. Although the patients were hospitalized four months after the vaccination, there was no significant difference between the post-vaccination time and ICU/ mortality. In our study, the delta variant had higher ICU admission, and the alpha variant had a higher mortality rate. Our study suggests a third and fourth dose of mRNA vaccine to subjects vaccinated primarily with two doses of CoronaVac.