Surrogate virus neutralization test to determine salivary neutralizing antibody inhibition of ACE2 and SARS-CoV-2 spike RBD complex

The viral pathogen SARS-CoV-2 caused a pandemic with detrimental effects across multiple domains. Mutations that gain transmission and infectability advantages have complicated virus prevention and mitigation. SARS-CoV-2 virus gains entry to human cells through the complex formed between the human Angiotensin Converting Enzyme 2 (hACE-2) and the viral spike protein. Neutralizing antibodies can block the binding of the spike protein to hACE-2, thereby neutralizing the virus. These antibodies are present in mucosal fluids and plasma. In this study, we demonstrate the deployment of a laboratory-developed surrogate virus neutralization test (sVNT) to assess the neutralizing activity of antibodies in saliva. Samples are collected from donors who completed the first two doses of the Pfizer vaccine, on average 36.6 weeks apart, and received the booster shot in September 2021. We analyzed the neutralizing activity of the saliva antibody against the spike receptor-binding domain (RBD) of the reference spike and the Omicron variant. The analytical sensitivity and specificity of the laboratory-developed sVNT are comparable to those of the FDA-approved sVNT. Vaccine-elicited saliva antibodies were able to block the binding of hACE-2 to the spike RBD of the reference strain. However, Omicron spike RBD abolished saliva neutralizing ability, implicating escape from mucosal immune defense. The experience we gained from assessing SARS-CoV-2 virus neutralization in saliva can be broadly applied to urgent needs against other pathogens, enabling us to quickly deploy a surrogate neutralization test to evaluate immune protection.