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Lene Wulff Krogsgaard,1 Laura Espenhain,1 Siri Tribler,1 Charlotte Sværke Jørgensen,2 Christian Holm Hansen,1 Frederik Trier Møller,1 Ida Glode Helmuth,1 Ute Wolff Sönksen,3,4 Anne-Marie Vangsted,4 Henrik Ullum,5 Steen Ethelberg1,6 1Department of Infectious Disease Epidemiology and Prevention, Statens Serum Institut, Copenhagen, Denmark; 2Department of Virus & Microbiological Special Diagnostics, Statens Serum Institut, Copenhagen, Denmark; 3Department of Bacteria, Parasites and Fungi, Statens Serum Institut, Copenhagen, Denmark; 4TestCentre Denmark, Statens Serum Institut, Copenhagen, Denmark; 5Division of Infectious Disease Preparedness, Statens Serum Institut, Copenhagen, Denmark; 6Department of Public Health, University of Copenhagen, Copenhagen, DenmarkCorrespondence: Steen Ethelberg, Department of Infectious Disease Epidemiology and Prevention, Statens Serum Institut, 5 Artillerivej, 2300 Copenhagen, Denmark, Tel +45 3268 3545, Email set@ssi.dkBackground: Seroprevalence studies can be used to measure the progression of national COVID-19 epidemics. The Danish National Seroprevalence Survey of SARS-CoV-2 infections (DSS) was conducted as five separate surveys between May 2020 and May 2021. Here, we present results from the two last surveys conducted in February and May 2021.Methods: Persons aged 12 or older were randomly selected from the Danish Population Register and those having received COVID-19 vaccination subsequently excluded. Invitations to have blood drawn in local test centers were sent by mail. Samples were analyzed for whole Immunoglobulin by ELISA. Seroprevalence was estimated by sex, age and geography. Comparisons to vaccination uptake and RT-PCR test results were made.Results: In February 2021, we found detectable antibodies in 7.2% (95% CI: 6.3– 7.9%) of the invited participants (participation rate 25%) and in May 2021 in 8.6% (95% CI: 7.6– 9.5%) of the invited (participation rate: 14%). Seroprevalence did not differ by sex, but by age group, generally being higher among the < 50 than 50+ year-olds. In May 2021, levels of seroprevalence varied from an estimated 13% (95% CI: 12– 15%) in the capital to 5.2% (95% CI: 3.4– 7.4%) in rural areas. Combining seroprevalence results with vaccine coverage, estimates of protection against infection in May 2021 varied from 95% among 65+ year-olds down to 10– 20% among 12– 40 year-olds. In March–May 2021, an estimated 80% of all community SARS-CoV-2 infections were diagnosed by RT-PCR and captured by surveillance.Conclusion: Seroprevalence estimates doubled during the 2020– 21 winter wave of SARS-CoV-2 infections and then stabilized as vaccinations were rolled out. The epidemic affected large cities and younger people the most. Denmark saw comparatively low infections rates, but high test coverage; an estimated four out of five infections were detected by RT-PCR in March–May 2021.Keywords: seroepidemiological studies, COVID-19 serological testing, SARS-CoV-2, population register, questionnaire, ELISA

Katrin Gaardbo Kuhn,1 Eva Møller Nielsen,2 Kåre Mølbak,1,3 Steen Ethelberg1 1Infectious Disease Epidemiology & Prevention, Statens Serum Institut, Copenhagen, Denmark; 2Bacteria, Parasites & Fungi, Statens Serum Institut, Copenhagen, Denmark; 3Institute of Veterinary and Animal Sciences, University of Copenhagen, Copenhagen, Denmark Background: Each year more than 4,000 cases of campylobacteriosis are reported in Denmark, making it the most common bacterial gastrointestinal infection. Here we describe a case-control study to identify sources of infection with a focus on environmental factors.Methods: From January to December 2016, we conducted a prospective case-control study among Danish persons aged 1–30 years. Participants were invited by letter to complete an online questionnaire. Crude and adjusted ORs were calculated and final parsimonious multivariate models developed using logistic regression.Results: The study recruited 1366 cases and 4,418 controls, of whom 65% and 66%, respectively, completed the questionnaire. A multivariate model for domestically acquired cases showed, among others, increased risk of infection with bathing in fresh water (OR=5.1), contact to beach sand (OR=1.8), owning a pet dog with diarrhea (OR=4.6), and eating minced beef (OR=2.6) or chicken (OR=2.5). The model for children highlighted similar risk factors but also included bathing in a paddling pool (OR=13.6) and eating fresh strawberries (OR=5.3). A separate analysis for persons reporting foreign travel showed increased infection risk when traveling to Asia, Africa, or Turkey and that eating from street kitchens and having contact to water during traveling were also risk factors.Conclusion: Environmental factors and animal contact account for a sizeable proportion of domestic Campylobacter infections in the age group studied. The study also re-confirmed handling/consumption of chicken as an important risk factor while highlighting minced beef as a potential new risk factor. Overall, these results contribute to a better understanding of the transmission dynamics of Campylobacter and will be used to improve national guidelines for prevention of infection. Keywords: Campylobacter, determinants, case-control, environment, food

Summary: Campylobacter is the most frequently occurring cause of bacterial gastroenteritis in Europe. Unlike other zoonotic diseases, European‐wide incidences of Campylobacter infections have increased during the past decade, resulting in a significant disease burden. In Denmark, campylobacteriosis is notifiable by laboratory and a unique registration system of electronic transfer and storage of notified Campylobacter cases linked to the national person register of age, gender and geographical location allows collection of comprehensive case data. Using national surveillance data, we describe Campylobacter infections in Denmark from 2000 to 2015, focusing on age‐specific incidences, geography, seasonality and outbreaks. During the observed period, a total of 60,725 Campylobacter infections were registered with a mean annual incidence of 69.3 cases/100,000 population. From 2000 to 2014, the incidence of campylobacteriosis decreased by 20%, followed by an apparent increase of 20% from 2014 to 2015. Approximately one‐third of cases were travel‐related. Incidences were highest in males, young adults aged 20–29 years and children under 5 years of age. Generally, children under 10 years of age living in rural areas were at higher risk of infection. Infection patterns were seasonal with an increase from May to October, peaking in August. Outbreaks were identified each year, including four large waterborne outbreaks which all occurred following heavy rainfall events. For the most part, patterns of Campylobacter infection in Denmark during 2000 to 2015 remained remarkably constant and followed what is known about the disease with respect to demographic, temporal and spatial characteristics. To establish better targeted prevention and control measures, the current knowledge gaps regarding both Campylobacter microbiology (degree of clonal diversity and clustering) and the importance of different risk factors (food versus environment/climate) need to be filled. [ABSTRACT FROM AUTHOR]