Factors associated with antibody titer levels among an occupational cohort of fully vaccinated individuals and subsequent risk of COVID‐19 infection: A cohort study

This study (1) determined the association of time since initial vaccine regimen, booster dose receipt, and COVID‐19 history with antibody titer, as well as change in titer levels over a defined period, and (2) determined risk of COVID‐19 associated with low titer levels. This observational study used data from staff participating in the National Football League COVID‐19 Monitoring Program. A cohort of staff consented to antibody‐focused sub‐study, during which detailed longitudinal data were collected. Among all staff in the program who received antibody testing, COVID‐19 incidence following antibody testing was determined. Five hundred eighty‐six sub‐study participants completed initial antibody testing; 80% (469) completed follow‐up testing 50−101 days later. Among 389 individuals who were not boosted at initial testing, the odds of titer < 1000 AU/mL (vs. ≥1000 AU/mL) increased 44% (odds ratio [OR] = 1.44, 95% confidence interval [CI]: 1.18−1.75) for every 30 days since final dose. Among 126 participants boosted before initial testing with no COVID‐19 history, 125 (99%) had a value > 2500 AU/ml; 86 (96%) of 90 tested at follow‐up and did not develop COVID‐19 in the interim remained at that value. One thousand fifty‐seven fully vaccinated (330 [29%] boosted at antibody test) individuals participating in the monitoring program were followed to determine COVID‐19 status. Individuals with titer value < 1000 AU/mL had twice the risk of COVID‐19 as those with >2500 AU/mL (HR = 2.02, 95% CI: 1.28−3.18). Antibody levels decrease postvaccination; boosting increases titer values. While antibody level is not a clear proxy for infection immunity, lower titer values are associated with higher COVID‐19 incidence, suggesting increased protection from boosters.