Attenuation of antibody titres during 3-6 months after the second dose of the BNT162b2 vaccine depends on sex, with age and smoking as risk factors for lower antibody titres at 6 months

ABSTRACT Objective We aimed to determine antibody titres at 6 months and their rate of change during 3-6 months after the second dose of the BNT162b2 coronavirus disease 2019 (COVID-19) mRNA vaccine (Pfizer/BioNTech) and to explore clinical variables associated with titres in Japan. Methods We enrolled 365 healthcare workers (250 women, 115 men) whose 3-month antibody titres were analyzed in our previous study and whose blood samples were collected 183 ± 15 days after the second dose. Participant characteristics collected previously were used. The relationships of these factors with antibody titres at 6 months and rates of change in antibody titres during 3-6 months were analyzed. Results Median age was 44 years. Median antibody titre at 6 months was 539 U/mL. Older participants had significantly lower antibody titres (20s, 752 U/mL; 60s–70s, 365 U/mL). In age-adjusted analysis, smoking was the only factor associated with lower antibody titres. Median rate of change in antibody titres during 3-6 months was −29.4%. The only factor significantly associated with the rate of change in Ab titres was not age or smoking, but sex (women, −31.6%; men, −25.1%). Conclusion The most important factors associated with lower antibody titres at 6 months were age and smoking, as at 3 months, probably reflecting their effect on peak antibody titres. However, antibody titres significantly attenuated during 3-6 months in women alone, which reduced the sex difference in antibody titres seen during the first 3 months. Antibody titres may be affected by different factors at different time points.