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Virological characteristics of SARS-CoV-2 vaccine breakthrough infections in health care workers

October 1, 2021

Research by Marc C. Shamier, Alma Tostmann, Susanne Bogers, Janet de Wilde, Jeroen IJpelaar, Willemijn A. van der Kleij, Herbert de Jager, Bart L. Haagmans, Richard Molenkamp, Bas. B. Oude Munnink, Carsten van Rossum, Janette Rahamat-Langendoen, Nannet van der Geest, Chantal P. Bleeker-Rovers, Heiman Wertheim, Marion P.G. Koopmans & Corine H. GeurtsvanKessel


Background SARS-CoV-2 vaccines are highly effective at preventing COVID-19-related morbidity and mortality. As no vaccine is 100% effective, breakthrough infections are expected to occur.

Methods We analyzed the virological characteristics of 161 vaccine breakthrough infections in a population of 24,706 vaccinated healthcare workers (HCWs), using RT-PCR and virus culture.

Results The delta variant (B.1.617.2) was identified in the majority of cases. Despite similar Ct-values, we demonstrate lower probability of infectious virus detection in respiratory samples of vaccinated HCWs with breakthrough infections compared to unvaccinated HCWs with primary SARS-CoV-2 infections. Nevertheless, infectious virus was found in 68.6% of breakthrough infections and Ct-values decreased throughout the first 3 days of illness.

Conclusions We conclude that rare vaccine breakthrough infections occur, but infectious virus shedding is reduced in these cases.

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