SARS-CoV-2 vaccine breakthrough infections are asymptomatic or mildly symptomatic and are infrequently transmitted.

Rovida, F.; Cassaniti, I.; Paolucci, S.; Percivalle, E.; Sarasini, A.; Piralla, A.; Giardina, F.; Sammartino, J. C.; Ferrari, A.; Bergami, F.; Muzzi, A.; Novelli, V.; Meloni, A.; Grugnetti, A. M.; Grugnetti, G.; Rona, C.; Daglio, M.; Marena, C.; Triarico, A.; Lilleri, D.; Baldanti, F.

Vaccine breakthrough SARS-CoV-2 infection was monitored in 3694 healthcare workers receiving 2 doses of BNT162b2. SARS-CoV2 infection was detected in 33 subjects, with a 3-months cumulative incidence of 0.90% and 0.42% in SARS-CoV-2-naive and experienced subjects, respectively. Vaccine protection was 87% in naive and 94% in experienced subjects when compared with a pre-vaccination control group. The infection was mildly symptomatic in 16 (48%) and asymptomatic in 17 (52%) subjects. Virus isolation was positive in 7/13 (54%) symptomatic and 4/8 (50%) asymptomatic subjects tested, and B.1.1.7 lineage was detected in all subjects. Antibody and T-cell responses were not reduced in subjects with breakthrough infection. Evidence of virus transmission, determined by contact tracing, was observed in two (6.1%) cases. This real-world data confirm the protective effect of the BNT162b2 vaccine. A triple antigenic exposure, as occurring in experienced subjects, may confer higher protection. Virus transmission from vaccinated subjects is infrequent.