Abstract The continuous emergence of SARS-CoV-2 variants with increased transmission and immune evasion has caused breakthrough infections in vaccinated population. It is important to determine the threshold of neutralizing antibody titers that permit breakthrough infections. Here we tested the neutralization titers of vaccinated patients who contracted Delta variant. All 75 patients with Delta breakthrough infections exhibited neutralization titers (NT 50 ) of less than 70. Among the breakthrough patients, 76%, 18.7%, and 5.3% of them had the NT 50 ranges of &lt;20, 20-50, and 50-69, respectively. These clinical laboratory results have implications in vaccine strategy and public health policy.

SARS-CoV-2 Delta breakthrough infections in vaccinated patients