Antibody level dynamics until after the third dose of COVID-19 vaccination

Abstract

The antibody titers of volunteers, including elderly people, were investigated after the second dose of the BNT162b2 vaccine (Pfizer-BioNTech) as an mRNA vaccine against the coronavirus disease 2019 (COVID-19). Serum samples were collected from 105 volunteers (44 healthcare workers and 61 elderly people) 7-14 days after the second vaccine dose, and antibody titers were measured. The antibody titers of study participants in their 20s were significantly higher than those of other age groups. Furthermore, the antibody titers of participants aged <60 years were significantly higher than those of participants aged ≥60 years. Serum samples were repeatedly collected from 44 healthcare workers until after the third vaccine dose. Eight months after the second round of vaccination, the antibody titer levels decreased to the same level as that before the second vaccine dose. After the third booster vaccination, the antibody titer recovered to the same level as that after the second dose. Neutralizing activities were also investigated at four time points before and after the second vaccine dose. The antibody titers and neutralizing activity were positively correlated. Therefore, neutralizing activity can be predicted by measuring the antibody titer. In conclusion, the antibody titers in the elderly population were significantly lower than those in the younger population. Although the antibody titers increased following vaccination, their levels showed a decline after several months, returning to the same level as that after a single dose of mRNA vaccination. The antibody titer levels recovered after the third dose of vaccination, which had already been administered in Japan. Routine administration of vaccine should be considered in the future.

Keywords: Antibody titer; COVID-19; Neutralizing activity; mRNA vaccine.

Fig. 1

The antibody titers of people in the younger population were significantly higher than those in the elderly population. Serum specimens were collected from 105 volunteers who received the BNT162b2 vaccine (Pfizer-BioNTech) against SARS-CoV-2. The antibody titers were measured in sera 7–14 days after the second vaccination. Dot plots show the antibody titers and mean values of the age groups. A significant difference in the antibody titers was observed between people in their 20s and other age groups (a). The antibody titers in volunteers aged <60 years were significantly higher than those in volunteers aged ≥60 years (b). There were no significant differences in the antibody titers between male and female (c). *p < 0.01. **p < 0.05.

Fig. 2

The antibody titers after the third dose returned to levels after the second vaccination. The antibody titers of 44 healthcare workers were measured at nine time-points. The antibody titers decreased approximately eight months after the second vaccination (p < 0.001), but recovered after the third vaccination to the level after the second vaccination (p > 0.9); *p < 0.01.

Fig. 3

Neutralizing activities were measured before and after the second vaccination. Neutralizing activities of 44 healthcare workers were measured at four time points: just before the second dose, a week, three weeks and nine weeks after the second vaccination; *p < 0.01.

Fig. 4

Correlation between the antibody titers and neutralizing activity. The relationship between the antibody titer and neutralizing activity was analyzed at four time points: immediately before the second dose (a), a week (b), three weeks (c) and nine weeks (d) after the second vaccination. The correlation coefficients (r) and p-value were determined. An inhibition rate of >50% was considered positive.