Serological response to a third booster dose of BNT162b2 COVID‐19 vaccine among seronegative cancer patients

BACKGROUND AND AIM: The BNT162b2 COVID‐19 vaccine (Pfizer/BioNTech), given as a two‐dose series, 3 weeks apart, elicits a serological response in 84–98% of patients with cancer, even if administered while undergoing anticancer treatments. Herein, we report the impact of a third (booster) dose of BNT162b2, delivered 6 months following the second vaccine dose. METHODS: This pilot study included four patients with cancer who were seronegative after two vaccine doses, and received a third (booster) dose of BNT162b2 at 6 months following the second vaccine dose. The four patients received the three vaccine doses between December 2020 and July 2021. Samples were evaluated with an enzyme‐linked immunosorbent assay (ELISA) that detects IgG (Immunoglobulin G) antibodies against the RBD (receptor‐binding domain) of SARS‐CoV‐2. RESULTS: At a mean time of 19 days (ranges 7–28) after the second vaccination, all four patients were seronegative for RBD‐IgG. However, at a mean time of 21 days (ranges 20–22) after the third dose, three out of the four patients (75%) were now seropositive. Mean RBD‐IgG titers were increased after the third vaccine dose from 0.37 to 2.81 (Student's t‐test, p = 0.05, two‐sided). CONCLUSIONS: Although limited by the small sample size, our findings suggest that a third (booster) dose administered to patients with cancer, who remain seronegative despite two doses of BNT162b2, may be efficacious in eliciting an antibody response.