Changes in SARS-CoV-2 antibody titers after COVID-19 vaccination

In a recent study posted to the medRxiv* preprint server, researchers assess longitudinal alterations in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) immunoglobulin G (IgG) antibody titers after SARS-CoV-2 vaccination.

Study: Longitudinal changes in IgG-type SARS-CoV-2 antibody titers after COVID-19 vaccination and, prominent increase in antibody titers when infected after vaccination. Image Credit: /

Study: Longitudinal changes in IgG-type SARS-CoV-2 antibody titers after COVID-19 vaccination and, prominent increase in antibody titers when infected after vaccination. Image Credit: /

COVID-19 vaccination and antibody production

The extent and duration of SARS-CoV-2 antibody titers are regarded as crucial indications of the efficiency of novel coronavirus disease 2019 (COVID-19) vaccines. However, several studies have confirmed that viral antibody titers decline several months following SARS-CoV-2 immunization.

In 2022, Japan experienced its sixth wave of COVID-19 cases which was primarily driven by the rapid spread of the SARS-CoV-2 Omicron strain. In fact, the magnitude of patients who reported infection after the sixth wave was vastly greater than the total number of patients prior to the sixth wave; thus, it can be assumed that COVID-19 breakthrough infection after two or more immunizations is prevalent. However, only a few studies have studied SARS-CoV-2 antibody titers after spontaneous infection and immunization over time.

About the study

In the present study, researchers explore the alterations in antibody titers following the second and third doses of the SARS-CoV-2 vaccine and determine the antibody titers in spontaneous COVID-19 infections that occurred after immunization.

This longitudinal observational study involved the recruitment of both outpatients and medical personnel. Between June 2021 and February 2023, a total of 127 participants, comprising 74 outpatients with 53 medical staff members, submitted their serum samples for assessment of SARS-CoV-2 and receptor-binding domain (RBD) IgG antibody titers.

The association between antibody titers, participant age, and time interval following the second vaccine in 56 participants who did not have a prior history of SARS-CoV-2 infection and had a known vaccination date was assessed. After the third vaccination, the same assessment was also performed on 23 participants with a history of COVID-19 and a precise vaccination date.

The link between antibody titers before and after vaccination was evaluated in 13 individuals whose antibody titers were determined prior to and following receipt of the third vaccine dose. Antibody titers after infection were also assessed in patients who had received two or more COVID-19 vaccine doses.

Study findings

Antibody titers were evaluated in 56 participants after they had received the second vaccine dose. At the time of antibody titer assessment, no history of COVID-19 was reported in these participants.

With an average antibody titer of about 4,807 AU/mL, a trend toward decreasing antibody titers was noted. Furthermore, the correlation between antibody titers and the time interval post-second vaccination dose was negative. A weak and non-significant association between antibody titer and age was also noted.

Eight patients who had their antibody titers evaluated several times following receipt of the second vaccine dose exhibited longitudinal alterations in their antibody levels over time. Within one month after the second vaccination dose, the first antibody titer was estimated; however, the titers varied widely between people.

By three months, antibody titers declined by half to one-third. By six months, antibody titers dropped to a few hundred AU/mL.

Before receiving the third vaccine dose, antibody titers were significantly reduced for these eight patients. When the difference between the minimum and maximum titers was divided by the time span between the two serum samples, the researchers observed that the greater the maximum value, the faster the antibody titer decreased each day.

After administration of the third vaccine dose, 23 patients were assessed for their antibody titers. At this point, mean antibody titers were 12,617 AU/mL, which was significantly higher than that after the second vaccination.

The time interval between the third vaccination and the measurement of antibody titers correlated negatively with the antibody titer value, while a negative association was also observed between the time interval and titers following the second vaccine dose. Similar to the second vaccination dose, there could be a minor association between age and vaccine efficacy; however, this was not statistically significant.


The increase in anti-SARS-CoV-2 antibody titers and the timespan for which effective levels were maintained were considered to be accurate indicators of the effectiveness of novel COVID-19 vaccines.

Rather than indiscriminately prescribing booster immunization for the entire population, the researchers believe it would be preferable to evaluate antibody titers to prioritize individuals with low titers for booster vaccination. Nevertheless, future longitudinal studies of antibody titers on a wider scale are needed.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

#SARSCoV2 #antibody #titers #COVID19 #vaccination
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