| Summary: | Since its first description in November 2021, the SARS-CoV-2 variant of concern Omicron (B.1.1.529) has emerged as the dominant strain in the COVID-19 pandemic. To date, it remains unclear if boosted vaccination protects against transmission. Using data from the largest German Public Health Department, Cologne, we analyzed breakthrough infections in booster-vaccinated infected persons (IP; booster-vaccinated group (BVG); <i>n</i> = 202) and fully vaccinated, not boosted SARS-COV2-positive patients (>3 month after receiving the second dose; unboosted, fully vaccinated group (FVG); <i>n</i> = 202) to close contacts compared to an age- and sex-matched unvaccinated control group (UCG; <i>n</i> = 202). On average, IPs had 0.42 ± 0.52 infected contacts in relation to the total number of contacts in the BVG vs. 0.57 ± 0.44 in the FVG vs. 0.56 ± 0.43 in the UVG (<i>p</i> = 0.054). In the median test, pairwise comparison revealed a significant difference between the BVG and both other groups; no difference was found between the fully vaccinated and the unvaccinated control group. Now, these findings must be verified in larger samples, considering the role of Omicron subvariants and the vaccination status of the contact person. However, the importance of the booster vaccination in breaking possible chains of infection in the immune escape variant Omicron is obvious.
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