Home Immunity In Denmark, Omicron reinfections reveal ineffective post-COVID-19 immunity

In Denmark, Omicron reinfections reveal ineffective post-COVID-19 immunity


In a recent study published on medRxiv* preprint server, researchers investigated cases of SARS-CoV-2 variant reinfection in Denmark.

The evolution of SARS-CoV-2 has led to the emergence of multiple variants of concern (VOCs) with increased transmissibility and immune evasion such as Omicron, leading to an upsurge in SARS-CoV-2 reinfections and difficulties mitigation of COVID-19 globally. Studies have investigated frequencies of SARS-CoV-2 reinfection, but have been limited to reverse transcriptase polymer chain reaction (RT-PCR) analysis data, in which SARS-CoV- 2 Pango are not specified. Additionally, a genomic sequencing report is usually categorized as an initial infection or a reinfection, but next-generation sequencing (NGS) data for an initial infection and a reinfection are rarely reported together for an individual.

Study: Rise in cases of SARS-CoV-2 Omicron reinfection reveals ineffective post-COVID-19 immunity in Denmark and points to the need for next-generation sequencing. Image Credit: Noiel/Shutterstock

About the study

In the present study, researchers characterized SARS-CoV-2 reinfections by variant based on the integration of RT-PCR analysis and next genome sequencing (NGS) analysis of the obtained sequences. from Danish SARS-CoV-2 positive individuals and GISAID (Global Initiative on Avian Influenza Data Sharing).

NGS data and clinical metadata of primary SARS-CoV-2 infections and reinfections from a single individual residing in Denmark were analyzed. A total of 21,708 reinfection entries were available between March 1, 2020 and August 28, 2022, with data on sample collection dates on primary infections and SARS-CoV-2 reinfections.

The team documented SARS-CoV-2 clinical metadata as time points of initial and subsequent SARS-CoV-2 infections to measure the duration between initial and subsequent SARS-CoV-2 infections. Additionally, RT-PCR scan results and NGS scan results were available for primary infections and reinfections, respectively.

The team excluded 70 case entries (

Reinfection cases were stratified by variant and subvariant. The GISAID database was searched for two files: one set of files including current metadata for >12 million SARS-CoV-2 sequences and the second set including filtered metadata files from only residents of Denmark with two infections associated with SARS-CoV-2 documented.


Primary infection and re-infection with Omicron (i.e. Omicron-Omicron infections) have been reported within a shorter period (even within three weeks, an average of 22 weeks) than non-Omicron-Omicron infections . Reinfections with Omicron within ten weeks of initial Omicron infection have been widely reported due to BA.1 followed by BA.2.

The frequency of reinfection was significantly higher with Omicron (25%, N=1875) after primary infections with any VOC. No cases of Alpha VOC-induced reinfection were reported, while Delta VOC caused reinfections in 2.3% (n=169) of cases. Pre-Omicron estimates of infection-induced natural immunity were above 90%, which fell to less than 10% within three to four months.

Among those with Delta-induced primary infections, reinfections due to Delta variant

Reinfections with Omicron were reported among 62% (n=211), 20%, (n=68) and 30% (n=102) of cases in which primary SARS-CoV-2 infections were caused by Omicron BA.1, BA.2, and BA.5, respectively, individuals with primary BA.2 infections demonstrated high reinfection frequencies (38%, n=129) with the Omicron BA.5 subvariant (26 %, n = 89). The results indicated that although the SARS-CoV-2 spike (S) protein of the three Omicron subvariants is similar, the differences in the Omicron subvariants were sufficient to prevent the nAbs induced by the infection Omicron BA.1/2 (neutralizing antibodies) to bind with Omicron BA.5 S.


Study findings showed that most cases of SARS-CoV-2 reinfection occurred due to Omicron. Reinfections with Omicron among people with primary Omicron infection have occurred in a short time, less than three weeks. The results indicated that primary infections with non-Omicron VOCs were insufficient to provide immune protection to prevent reinfections with Omicron.

Additionally, the results highlight Omicron’s transmissibility and immune evasion and the need for updated SARS-CoV-2 vaccines, continued SARS-CoV-2 surveillance, and evolving SARS assessments. -CoV-2 to guide policymaking for better public health around the world. Additionally, the analysis underscores the need to analyze NGS data at the individual level to provide accurate estimates of the risks of reinfection with SARS-CoV-2.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.