The most recent SARS-CoV-2 variant of concern to emerge has been named omicron. 1 Its immune evasion potential was predicted by genomic data and has been preliminarily confirmed by observations of an increased incidence of reinfections and breakthrough infections. 2 This has triggered calls to intensify vaccination programmes including provision of vaccine booster doses. 3

A group of German visitors who had received three doses of SARS-CoV-2 vaccines, including at least two doses of an mRNA vaccine, experienced breakthrough infections with omicron between late November and early December, 2021, while in Cape Town, South Africa. The group consisted of five White women and two White men with an average age of 27· 7 years (range 25–39) and a mean body-mass index of 22· 2 kg/m2 (range 17· 9–29· 4), with no relevant medical history. Four of the individuals were participating in clinical elective training at different hospitals in Cape Town, whereas the others were on vacation. The individuals were members of two unlinked social groups and participated in regular social life in Cape Town, in compliance with applicable COVID-19 protocols. Upon arrival during the first half of November, 2021, each individual tested negative for SARS-CoV-2 by PCR and provided records of complete vaccination, including booster or third, doses administered via intramuscular injection using homologous (n= 5) and heterologous (n= 2) vaccination courses (appendix p 3). 4