Pfizer, BioNTech and University of Texas Medical Branch scientists tracked the immune response of participants in clinical trials for the vaccine. They found that neutralizing antibody levels, the key protective immunity, dropped significantly over seven to nine months after the two-dose vaccination. This drop of neutralizing antibody levels correlates to the observation of breakthrough infections in vaccinated individuals. After a third dose of the Pfizer-BioNTech vaccine, the neutralizing antibody levels increased many-fold higher than that after the second dose. More importantly, the third dose increased the antibody's ability to inhibit variants more efficiently, including Beta and Delta variants.