Abstract
WHAT IS ALREADY KNOWN ABOUT THIS TOPIC? The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Delta variant has proved to have increased transmissibility, and mutations that can cause partial immune escape, which makes its transmission more insidious. WHAT IS ADDED BY THIS REPORT? This study showed that probable cases who had negative results in nucleic acid testing but had positive IgM test result and/or IgG test value of over 20 S/CO in antibodies testing, might serve as bridges in the Delta variant's transmission chain. WHAT ARE THE IMPLICATIONS FOR PUBLIC HEALTH PRACTICE? In border inspection and quarantine, tests for SARS-CoV-2 IgM and IgG antibodies should be strengthened alongside nucleic acid tests to prevent probable cases with transmission potential from crossing the land border into China. In contact tracing investigations, the bridging role of probable cases should be considered to reconstruct the transmission chain.