Neutralization Titers in Vaccinated Patients with SARS-CoV-2 Delta Breakthrough Infections

The continuous emergence of SARS-CoV-2 variants with increased transmission and immune evasion has caused breakthrough infections in the vaccinated population. It is important to determine the threshold of neutralizing antibody titers (NT(50)) that permit breakthrough infections in humans. Here, we tested the neutralization titers of vaccinated patients who contracted Delta variant. All 64 patients with Delta breakthrough infections exhibited NT(50) of less than 70. When the breakthrough sera were tested against USA-WA1/2020 (a strain isolated in late January 2020), 82.8%, 15.6%, and 1.6% of them had the NT(50) ranges of <20, 20 to 50, and 50 to 69, respectively. When the same breakthrough sera were tested against Delta-spike SARS-CoV-2, 68.7%, 26.6%, and 4.7% of them had the NT(50) ranges of <20, 20 to 50, and 50 to 69, respectively. Overall, the results suggest NT(50) of 70 as a potential neutralizing threshold required to prevent Delta breakthrough infections. These clinical laboratory results have implications in vaccine strategy and public health policy.