Trends in Antibody Titers after SARS-CoV-2 Vaccination - Insights from a Self-Paid Tests at a General Internal Medicine Clinic

Abstract Background The rise in antibody titers against the novel coronavirus (SARS-CoV-2), and its duration is considered an important indicator for confirming the efficacy of the COVID-19 vaccine, and self-paid tests of antibody titer are conducted at many facilities nationwide. Methods The relationship between the number of days after the second and third dose of vaccines, age, and antibody titer was determined from the medical records of general internal medicine clinics that conducted self-paid testing of SARS-CoV-2 antibody titer using Elecsys Anti-SARS-CoV-2 S (Roche Diagnostics), as well as the relationship between the number of days after two or more dose of vaccines and antibody titer. We also examined antibody titers in cases of spontaneous infection with SARS-CoV-2 after two or more doses of the vaccine. Results Age and antibody titer were negatively correlated in 86 subjects whose antibody titer was measured within 1 month after the second vaccination with the COVID-19 vaccine. In 31 subjects whose antibody titer was measured within one month after the third dose of vaccine, age and antibody titer showed a negative correlation (p<0.01). The mean antibody titer after the third vaccination was 20704.9±16820.7 U/mL, more than 10 times the mean antibody titer after the second dose of vaccine of 1601.9±1297.9 U/mL. There were also cases of infection after the third or fourth dose of vaccine, with antibody titers in the tens of thousands after infection, but they still received further booster vaccinations after infection. Discussion SARS-CoV-2 antibody titers measured within 1 month of the second or third dose of vaccine showed a negative correlation with age, and antibody titers also showed a negative correlation trend with the number of days after the second dose of vaccine. It is considered that many people in Japan received further booster vaccinations after spontaneous infection, even though they already had antibody titers in the tens of thousands U/mL by “hybrid immunity” after spontaneous infection following two or more dose of vaccine. Conclusion After measuring SARS-CoV-2 antibody titers, booster vaccination of those with low antibody titers should be done on a priority basis.