Inexpensive SARS-CoV-2 Antibody Avidity Test Shows Immunity Months After Infection in All Ages

Scientist using microscope conducting genetic research of blood sample in laboratory.
The researchers’ main goal was to assess whether the marked increase in avidity of antibodies noted 7 to 8 months after an infection was also substantial in older adults.

The immunoglobulin (Ig) G avidity assay that uses a modified urea process was found to be an inexpensive alternative to IgG neutralizing assays. These findings were published in The Journal of Infectious Diseases.

Plasma samples (N=1527) were collected for a large longitudinal seroprevalence study conducted in Austria. A subsample (n=217) was assessed at baseline and follow-up (7 to 8 months) by the anti-SARS-CoV-2 enzyme-linked immunosorbent assay (ELISA) antibody test with a 10-minute 5.5 M urea treatment modification. Exposure to urea promotes detachment of low-avidity antibodies from the antigen. Neutralization capacity on the basis of patient age was assessed.

Plasma samples were provided by study participants; 58.5% were women with a mean age of 47.7 (SD, 16.3) years and 41.5% were men with a mean age of 46.7 (SD, 16.3) years. A total of 6 donors (men: n=4) had been hospitalized for their SARS-CoV-2 symptoms.

Avidity increased from 18% at baseline to 42% at follow-up. These trends were not associated with patient age at baseline (r, -0.0864) or follow-up (r, -0.0987).

Stratified by ages 18 to 40, 41 to 60, and 61 years and older, follow-up avidity was higher among the youngest group compared with the middle-aged group (P =.019) but not compared with the oldest individuals (P =.345). No significant difference in follow-up avidity was observed between the middle-aged and older groups (P =.224).

Stratified by 50% neutralization titer, no significant correlation was observed in avidity at baseline (b, 1.986). At follow-up, avidity was positively correlated with 50% neutralization titer (b, 0.455), in which 97.2% of samples indicated neutralization capacity against SARS-CoV-2 (³1:16) remained.

Comparing age with 50% neutralization titer revealed a more positive relationship at baseline (b, 0.014) than at follow-up (b, 0.021). Every 1-year increase in age was associated with a 1.01-fold (95% CI, 1.00-1.02) increase in neutralization capacity at baseline and 1.01-fold (95% CI, 1.01-1.02) increase at follow-up.

This study was limited by not having many samples from individuals older than 80 years; however, the high death toll for symptomatic infections among older individuals made their recruitment difficult.

These data indicated the 5.5 M urea treatment of serum samples before antibody testing was effective at detecting neutralizing capacity, even in samples from older individuals. These findings indicated this inexpensive test was effective and may indicate most individuals had long-lasting protection against reinfection with COVID-19.

An additional follow-up is planned to test for longer lasting antibody protection.


Pichler D, Baumgartner M, Kimpel J, et al. Marked increase in avidity of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) antibodies 7-8 months after infection is not diminished in old age. J Infect Dis. Published online June 4, 2021. doi:10.1093/infdis/jiab300