Use of the particle agglutination/particle agglutination inhibition test for antigenic analysis of SARS‐CoV‐2

BACKGROUND: The antigenicity of SARS‐CoV‐2 is a critical issue for the effectiveness of the vaccine, and thus, it should be phenotypically evaluated by serological assays as new field isolates emerge. The hemagglutination/hemagglutination inhibition (HA/HI) tests are well known as a representative method for antigenic analysis of influenza viruses, but SARS‐CoV‐2 does not agglutinate human or guinea pig red blood cells. Therefore, the antigenic analysis requires complicated cell‐based assays using special equipment such as plate reader or ELISPOT analyzer. METHODS: Based on the HA/HI tests for influenza viruses, we developed the particle agglutination/particle agglutination inhibition (PA/PAI) test to easily and rapidly quantify the virus and antibody using human angiotensin‐converting enzyme 2 (hACE2)‐bound latex beads. The virus titers were determined by mixing the beads and the virus from culture supernatant, settling it overnight, and then observing the sedimentation/agglutination pattern (PA test). The neutralization antibody titers were determined by mixing virus‐infected hamster antisera in addition to the beads and virus (PAI test). RESULTS: The PA titer was positively correlated with the plaque‐forming units. The PAI titer using the hamster antisera clearly revealed the antigenic difference between the omicron and previous variants. The antigenic differences were supported by the results shown in other methods. CONCLUSIONS: The PAI test is an easy and rapid method to analyze the antigenicity of SARS‐CoV‐2.