Societal COVID-19 epidemic counter measures and activities associated with SARS-CoV-2 infection in an adult unvaccinated population - a case-control study in Denmark, June 2021 (preprint)

Measures to restrict physical inter-personal contact in the community have been widely implemented during the COVID-19 pandemic. We studied determinants for infection with SARS-CoV-2 with the aim of testing the efficiency of such measures. We conducted a national matched case-control study among unvaccinated persons aged 18-49 years. Cases were selected among those testing positive for SARS-CoV-2 by RT-PCR over a five-day period in June 2021. Controls were selected from the national population register and were individually matched on age, sex and municipality of residence and had not previously tested positive. Cases and controls were interviewed via telephone about contact with other persons and exposures in the community. We included 500 cases and 529 controls and determined odds ratios (ORs) and 95% confidence intervals (95%CIs) by conditional logistical regression with adjustment for household size and immigration status. We found having had contact with another individual with a known infection as the main determinant for SARS-CoV-2 infection. Reporting close contact with an infected person who either had or did not have symptoms resulted in ORs of 20 (95%CI:9.8-39) and 8.5 (95%CI 4.5-16) respectively. In contrast, community exposures were generally not associated with disease; several exposures were negatively associated. Exceptions were: attending fitness centers, OR=1.4 (95%CI:1.0-2.0) and consumption of alcohol in restaurants or cafes, OR=2.3 (95%CI:1.3-4.2). For reference, we provide a timeline of non-pharmaceutical interventions in place in Denmark from February 2020 to March 2022. Fitness centers and alcohol consumption were mildly associated with infection, in agreement with findings of our similar study conducted six month earlier (Epidemiology & Infection 2021;150:e9.). Transmission of disease through involvement in community activities appeared to occur only rarely, suggesting that community restrictions in place were efficient. Instead, transmission appeared to primarily take place in a confined space via contact to known persons.

Occurrence and significance of Omicron BA.1 infection followed by BA.2 reinfection (preprint)

The newly found Omicron SARS-CoV-2 variant of concern has rapidly spread worldwide. Omicron carries numerous mutations in key regions and is associated with increased transmissibility and immune escape. The variant has recently been divided into four subvariants with substantial genomic differences, in particular between Omicron BA.1 and BA.2. With the surge of Omicron subvariants BA.1 and BA.2, a large number of reinfections from earlier cases has been observed, raising the question of whether BA.2 specifically can escape the natural immunity acquired shortly after a BA.1 infection. To investigate this, we selected a subset of samples from more than 1,8 million cases of infections in the period from November 22, 2021, until February 11, 2022. Here, individuals with two positive samples, more than 20 and less than 60 days apart, were selected. From a total of 187 reinfection cases, we identified 47 instances of BA.2 reinfections shortly after a BA.1 infection, mostly in young unvaccinated individuals with mild disease not resulting in hospitalization or death. In conclusion, we provide evidence that Omicron BA.2 reinfections do occur shortly after BA.1 infections but are rare.

Prevalence of SARS-CoV-2 antibodies in Denmark: Nationwide, population-based seroepidemiological surveys (preprint)

Background. Seroprevalence studies have proven an important tool to monitor the progression of the coronavirus disease 2019 (COVID-19) epidemic. We present results of consecutive population-based seroprevalence surveys performed in Denmark in 2020. Methods. Invitation letters including a questionnaire covering symptoms were sent to representatively drawn samples of the population in spring, late summer and autumn/winter of 2020. Blood samples from participants taken at public test-centers were analyzed for total Ig and seroprevalence estimates per population segment calculated and compared to other surveillance parameters. Results. From 34,081 participating individuals (response rate 33%), we obtained seroprevalence estimates increasing from 1.1% (95%CI: 0.7%-1.7) in May to 4.0 % (95%CI: 3.4%-4.7%) in December 2020. By December 2020, 1.5% of the population 12 years and older had tested positive by PCR. Seroprevalence estimates were roughly 3 times higher in those aged 12-29 compared to 65+ and higher in metropolitan municipalities. Among seropositives, loss of taste/smell were the more specific symptoms, 32%-56% did not report any symptoms. In half of seroconverted families, we did not see evidence of transmission between generations. Infected individuals in older age groups were hospitalized several fold more often than in younger. Conclusions. Seroprevalence increased during 2020; younger age groups were primarily infected in the autumn/winter surge. Approximately half were asymptomatically infected. Denmark has a high per capita test rate; roughly two undiagnosed infections of COVID-19 were estimated to occur for each diagnosed case. The epidemic appears to have progressed relatively modestly during 2020 in Denmark.