Effect of COVID-19 vaccine in adults infected with the Delta variant of SARS-CoV-2: a retrospective cohort study

Background: Reducing mortality among those infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remains a critical challenge in clinic. The objective of this study was to analyze the effect of the coronavirus disease 2019 (COVID-19) vaccine on the prognosis of individuals infected with the Delta variant of SARS-CoV-2.

Methods: In a single-center, retrospective cohort study, all adult patients with COVID-19 from designated hospital in Xi'an, China, during the Delta outbreak from December 2021 to January 2022 were enrolled. The patients were divided into two groups according to whether they received the COVID-19 vaccine, and differences in clinical outcomes (pneumonia, oxygen therapy, severe disease, and mechanical ventilation or death), symptoms, and nucleic acid-negative time between the two groups were compared.

Results: A total of 651 adult patients with COVID-19 were included, among whom 578 were vaccinated and 73 were not vaccinated. Compared with the unvaccinated group, the vaccinated group had lower rates of pneumonia (49.8% vs. 67.1%; P=0.005), oxygen therapy (20.9% vs. 57.5%; P<0.001), severe illness (1.6% vs. 26.0%; P<0.001), and mechanical ventilation or mortality (0.3% vs. 13.7%; P<0.001). Multivariate logistic regression analysis showed that COVID-19 vaccination significantly reduced the risk of requiring oxygen therapy, severe illness, and mechanical ventilation or death. Compared with the unvaccinated group, the vaccinated group had a higher incidence of sore throat (31.8% vs. 17.8%; P=0.01) and a lower incidence of shortness of breath (3.1% vs. 20.5%; P<0.001), diarrhea (1.2% vs. 5.5%; P=0.03), and nausea or vomiting (1.4% vs. 6.8%; P=0.007). The median time of nucleic acid transition to negative was 14.0 [interquartile range (IQR), 10.0-17.0] and 15.0 (IQR, 11.0-18.0) days (P=0.18) in the vaccinated and unvaccinated groups, respectively.

Conclusions: Vaccination may reduce the risk of oxygen therapy, severe illness, and mechanical ventilation or death in patients with Delta variant COVID-19, as well as the incidence of pneumonia. Vaccinated patients had a higher incidence of sore throat and a lower incidence of shortness of breath, diarrhea, and nausea or vomiting compared to nonvaccinated patients. Vaccination did not shorten the time for the emergence of nucleic acid-negative status.