In Denmark, Omicron reinfections reveal ineffective post-COVID-19 immunity Denmark immunity Omicron Coronavirus Disease COVID HerdImmunity medrxivpreprint cwru TAMU StanfordMed ColumbiaMed
The evolution of SARS-CoV-2 has led to the emergence of multiple variants of concern with increased transmissibility and immune-evasiveness such as Omicron, leading to an upsurge of SARS-CoV-2 reinfections and difficulties in COVID-19 mitigation globally. Studies have investigated SARS-CoV-2 reinfection frequencies but were restricted to reverse transcriptase polymer chain reaction analysis data, in which SARS-CoV-2 Pango lineages are not specified.
NGS data and clinical metadata of primary SARS-CoV-2 infections and reinfections from the same individual residing in Denmark were analyzed. In total, 21,708 entries of reinfections were available between 1 March 2020 and 28 August 2022, with data on the dates of sample collection about primary SARS-CoV-2 infections and reinfections.
Reinfection frequency was significantly higher with Omicron after primary infections with any VOC. No Alpha VOC-induced reinfection cases were reported, whereas Delta VOC caused reinfections in 2.3% of cases. Pre-Omicron estimates of natural infection-induced immunity were above 90%, which dropped to below 10% in three to four months.