Transmission of severe acute respiratory coronavirus virus 2 (SARS-CoV-2), delta variant, between two fully vaccinated healthcare personnel

To the Editor—Breakthrough severe acute respiratory coronavirus virus 2 (SARS-CoV-2) infection causing coronavirus disease 2019 (COVID-19) in fully vaccinated individuals occurs, and the frequency is increasing since the SARS-CoV-2 delta variant virus began circulating widely.1 COVID-19 vaccines are highly effective at reducing SARS-CoV-2 shedding and transmission.2 The question of whether fully vaccinated people with breakthrough COVID-19 can transmit the SARS-CoV-2 virus to others is central to the debate around the need for mitigation efforts including masking and physical distancing for fully vaccinated individuals. Discussion Recent CDC guidance says that fully vaccinated individuals may not need to wear masks indoors or practice physical distancing due to vaccine effectiveness and the low likelihood of a fully vaccinated person transmitting the virus to others.4 The genetic and epidemiological data from our investigation of 2 HCW with breakthrough SARS-CoV-2 infection strongly suggest transmission of the SARS-CoV-2 virus delta variant from one fully vaccinated individual to another in the setting of unmasked close contact. SARS-CoV-2 variants, such as the delta variant, can have higher viral loads, potentially increasing transmissibility and requiring enhanced public health measures.5 This apparent transmission of SARS-CoV-2 from one fully vaccinated person to another demonstrates that masking and physical distancing remain vital infection prevention measures for fully vaccinated people while the SARS-CoV-2 virus is still evolving and circulating.