High levels of anti-SARS-CoV-2 neutralizing antibodies (nAb) have been shown to be protective against severe SARS-CoV-2 infection [1], but nAbs wane over time following vaccination and/or infection [2,3]. Neutralizing antibodies require significant expense and infrastructure to measure and are largely limited to research settings. Serum anti-SARS-CoV-2 IgG (IgG) testing is readily available in commercial settings at relatively low cost. IgG testing has been sporadically ordered in primary care settings, although there has been little to guide its clinical interpretation [4,5].

This application illustrates the relationship between serum anti-SARS-CoV-2 IgG and anti-SARS-CoV-2 neutralizing antibody IC50 titers in an adult vaccinated U.S. population and provides for adjustment of that relationship based on renal function and prior COVID infection, the only factors that we found could impact this correlation [6]. It also provides a comparative range of neutralizing antibody titers at different time points after vaccination and puts the measured IgG in that context [2].

How It Works:
The app converts the value of IgG provided from any of the given testing platforms into BAU/mL and then calculates the corresponding IC50 of neutralizing antibody based on our data. This calculated value is shown as a central black dot to represent the predicted log(IC50) value and an upper and lower dot to represent the confidence interval. The background colors of this spectrum represent the range of known log(IC50) values (neutralizing antibody values) after certain number of months from prior vaccination or after COVID-19.

If the calculated log(IC50) for the IgG value you enter falls on or above the gray band, it matches the log(IC50) value of those who received a vaccine dose or had COVID-19 wihin the last 3 months. This represents a high neutralizing capacity against the SARS-CoV-2 virus. log(IC50) range for gray band: Please fill out input values.

If the calculated value falls on the pink band, it matches the range of values of people who received a vaccine dose at least 6 months prior. This band represents waning level of neutralizing antibody. log(IC50) range for pink band: Please fill out input values.

If the calculated value falls below the pink band, your levels are equivalent to that of unvaccinated individuals. This indicates very low, if any, neutralizing capacity against the virus. This represents log(IC50) below Please fill out input values.

We hope this application may provide a clinical tool for physicians which will enable them to personalize discussions with their patients about vaccine booster timing. It will likely be most helpful for individuals who may be hesitant to get re-vaccinated because they have recently had COVID or may have had several other boosters and may not think they need another. This application is not intended to replace clinical judgement or offer conclusions about immune protection from COVID-19 or vaccine efficacy.

Neutralizing antibodies are only one component of the immune response to infection. We cannot extrapolate cellular immunity from these data. This application is based on a small sample of patients (127 sera samples in 100 unique subjects (age 20–93 years; mean 63.83; SD 15.63; 29% female; 67% White), collected from Veterans and healthcare workers at one clinical location in the United States). There are limited numbers of immunocompromised individuals within the sample so may have limited generalizability to those individuals [7].