Objective  To investigate the characteristics and prognosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine breakthrough infections for providing a reference for coronavirus disease 19 (COVID-19) epidemic prevention and control.

  Methods  We collected the information on 38 SARS-CoV-2 infections laboratory-confirmed among Chinese travelers entering China via Chengdu city, Sichuan province during the period from December 10, 2020 to July 7, 2021 and divided the infections into a vaccine breakthrough group (14 male cases ever receiving domestic inactivated COVID-19 vaccine and with confirmed genotype of SARS-CoV-2 strains inducing the infections) and a natural infection group (18 male and 6 female cases reporting no COVID-19 vaccination history). Comparison analyses were conducted to examine differences between the two groups in epidemiological characteristics, clinical/laboratory indicators and treatment outcomes.

  Results  The participants arrived at Chengdu from 15 foreign countries during December 10, 2020 – July 7, 2021 and were diagnosed with SARS-CoV-2 infection within the period of December 11, 2020 – July 12, 2021; all the participants reported no history of SARS-CoV-2 infection/nucleic acid positive before entering China. In the vaccine breakthrough group, 9 participants had two doses of COVID-19 vaccine by separate inoculations with the interval of 56 days, 3 had two doses in one inoculation and only 2 had one dose; the interval from the last vaccination to the confirmation of SARS-CoV-2 infection ranged 8 – 173 days, with a median of 100 days. No statistically significant between-group differences were observed in the distribution of age (35.0 ± 7.7 vs. 35.8 ± 10.9 years), Pangolin-based genotyping of SARS-CoV-2 strains isolated, and the time of nucleic acid negative conversion from the first positive detection (t = 1.106, P > 0.05). Compared to the participants of natural infection, the participants with vaccine breakthrough infection had following significantly higher indicators: Ct value for N gene (26.8 ± 4.1 vs. 22.4 ± 7.4) and ORF1ab gene (29.4 ± 4.5 vs. 24.8 ± 6.9) of SARS-CoV-2 (t = 2.376 and t = 2.228, both P < 0.05); 50th percentile (P₅₀), (25th percentile P₂₅, 75th percentile P₇₅) of immunoglobulin M (IgM) antibody titer (0.9 0.3, 11.6 vs. 0.040 0.027, 0.096); P₅₀ (P₂₅, P₇₅) of immunoglobulin G (IgG) antibody titer (4.5 1.7, 7.6 vs. 0.006 0.003, 0.052); total antibody titer (31.9 4.6, 916.4 vs. 0.015 0.010, 0.038) (P < 0.05 for all); and the counts of lymphocyte, CD3⁺ T lymphocyte, CD3⁺/CD4⁺ T lymphocyte, CD3⁺/CD8⁺T lymphocyte detected at the time of hospital admission (all P < 0.05). While the participants of vaccine breakthrough infection had significantly lower peripheral blood eosinophil count and neutrophil percentage than those of the participants with natural infection (both P < 0.05). Although there was no significant between-group difference in the duration of hospitalization (t = 1.889, P > 0.05), the proportion of the participants being hospitalized less than 20 days was significantly higher for the vaccine breakthrough group than that for the natural infection group.

  Conclusion  For COVID-19 vaccine breakthrough infections, the vaccination helps to reduce viral load and may enhance the vaccinees′ T-cell immune response and exert a positive impact on vaccinees′ outcome of the infection treatment.