Among people who were infected with SARS-CoV-2, avidity and neutralization data among antibody-positive people confirmed a strong immune response 7 to 8 months after infection, which was not reduced in those of older age, according to study results published in The Journal of Infectious Diseases. The study results also suggested that avidity assays are easy and inexpensive to perform, and their accuracy may compensate for expensive neutralization assays.

A team of investigators in Austria conducted a large longitudinal study to assess whether the antibody avidity observed at 7 to 8 months after SARS-CoV-2 infection was also robust in those of older age. The researchers also aimed to determine whether avidity was associated with sex and if high avidity was linked to a high neutralization capacity, which could highlight the importance of avidity tests for the assessment of potential protection against future SARS-CoV-2 infection without the need for neutralization assays.

Study participants included 217 adults (58.5% women) who were positive for anti-S1 immunoglobulin G (IgG) at the start of the study in April 2020 and at follow-up in November 2020. Blood samples were collected at both baseline and follow-up. At baseline, 6 patients reported having been hospitalized due to SARS-CoV-2 infection, and 11 patients presented with negligible antibody titers at follow-up.

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Study participants were divided into 3 age groups: 18 to 40 years, 41 to 60 years, and 61 years and older. During an observation period of 6.5 months, the avidity observed in all 217 samples increased from 18% to 42%, which was independent of age and not impaired in older adults. One outlier had a slight tendency toward low avidity with increased age, which may have been due to a limitation of having a smaller amount of data in the very old age group.

A significant increase in avidity at 6.5 months postinfection was noted among all 3 age groups (P =.0001), with no significant difference in avidity among older age groups. No significant difference in avidity was noted between men and women either at baseline (P >.1) or follow-up (P >.6).

Despite a decrease in neutralization capacity at 6.5 months, 97.2% of all plasma samples had neutralizing capacity. Although no relation between neutralization capacity and avidity was observed at baseline, univariate regression analyses found a 2-fold increase in neutralization capacity at follow-up; this was significantly linked to a 1.99 percentage point increase in relative avidity index.

The neutralization capacity was only slightly different at baseline between men and women; however, no differences were observed at follow-up. Six samples with a negative neutralization titer still displayed avidity that did not differ from samples that were positive for neutralizing antibodies.

At baseline and follow-up, there was a similar and significant effect of age on neutralization capacity; a 1-year increase in age was linked to a 1% increase in neutralization capacity at baseline and follow-up, suggesting that neutralization capacity did not decrease but may even increase in older age groups.

“[It] is very interesting that older people can mount a similar avidity [as] the young and that the assessment of avidity may actually represent an excellent surrogate marker for a sustained immunity, replacing the laborious and expensive tests neutralization assays,” the authors noted. “A follow up study over an even longer time period may confirm our findings, in particular that high avidity is a better and more universal marker indicating protection from infection,” concluded the investigators.


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