LAUSR

As the coronavirus disease 2019 (COVID‐19) pandemic is still ongoing and severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) variants are circulating worldwide, an increasing number of breakthrough infections are being detected despite the good efficacy of COVID‐19 vaccines. Data on 88 COVID‐19 breakthrough cases (breakthrough infections group) and 41 unvaccinated cases (unvaccinated group) from June 1 to August 22, 2021, were extracted from a cloud database established at Beijing Ditan Hospital to evaluate the clinical, immunological, and genomic characteristics of COVID‐19 breakthrough infections. Among these 129 COVID‐19 cases, 33 whole genomes were successfully sequenced, of which 23 were Delta variants, including 15 from the breakthrough infections group. Asymptomatic and mild cases predominated in both groups, but two patients developed severe disease in the unvaccinated group. The median time of viral shedding in the breakthrough infections group was significantly lower than that in the unvaccinated group (p = 0.003). In the breakthrough infections group, the IgG titers showed a significantly increasing trend (p = 0.007), and the CD4 + T lymphocyte count was significantly elevated (p = 0.018). For people infected with the Delta variant in the two groups, no significant difference was observed in either the quantitative reverse‐transcription polymerase chain reaction results or viral shedding time. In conclusion, among vaccinated patients, the cases of COVID‐19 vaccine breakthrough infections were mainly asymptomatic and mild, IgG titers were significantly increased and rose rapidly, and the viral shedding time was shorter.