Seroprevalence of SARS-CoV-2 Antibodies in Denmark: Results of Two Nationwide Population-Based Surveys, February and May 2021

Background

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.

Underreporting of the 5-year tetanus, diphtheria, pertussis and polio booster vaccination in the Danish Vaccination Register

Background

In Denmark, vaccination coverage is measured using the Danish Vaccination Register (DDV). In general, the vaccination coverage is high, but for some vaccinations, the coverage is suboptimal with geographical variation. This study aims to validate the vaccination coverage of the 5-year booster and identify overall reasons for non-vaccination in Copenhagen.

Methods

We validated the coverage of the 5-year tetanus, diphtheria, pertussis and polio booster for children born in 2010 and living in Copenhagen municipality in 2018, an area with low coverage (current estimate: 89%). We identified all children born in 2010 in the Civil Registration System and sent an electronic questionnaire to parents of children without a record of the 5-year booster in the DDV.

Results

Parents of 692 children were contacted and 49% participated. Of those, 186 (55%) reported that the child was vaccinated: 61% by their general practitioner and 34% abroad. The most common reason for non-vaccination was forgetfulness (31%), 26% did not want their child vaccinated and 17% had migrated from abroad and were not aware of the vaccination schedule. Considering only children with documentation for the vaccination, the corrected vaccination coverage was 91%.

Conclusions

We conclude that the coverage of the 5-year booster in Copenhagen is currently underestimated and should be adjusted by 2%. We recommend increased awareness from general practitioners and tailored communication about the vaccination programme targeting immigrants in Denmark.

Supplementary Information

Supplementary information accompanies this paper at 10.1186/s12889-020-09816-w.

Are unexplained adverse health events following HPV vaccination associated with infectious mononucleosis? - A Danish nationwide matched case-control study

BACKGROUND

In Denmark, the acceptance of the HPV vaccination program has been threatened by reports of suspected adverse events. Epstein Barr Virus (EBV) infection is associated with symptoms of long-lasting tiredness and may be misinterpreted as HPV vaccine adverse events. The main aim of this study was to examine if EBV infection around time of HPV vaccination was a risk factor for later suspected vaccine adverse events.

METHODS

The study was a nationwide register-based matched case-control study. Cases were females vaccinated against HPV in the period 2011 throughout 2017 with suspected adverse events. For each case, five HPV vaccinated females without suspected adverse events were selected. Information about EBV infection was obtained from the Danish Microbiology Database and assessed for three time periods: (1) before first HPV vaccination, (2) around time of HPV vaccination, and (3) any time during the study period 2010-2017. Multiple logistic regression was used to estimate the association between EBV and suspected adverse events.

RESULTS

We identified 1217 cases, matched to 6085 controls. A higher proportion of cases (38; 3.1%) than controls (31; 0.5%) were tested for EBV around time of HPV vaccination and cases had elevated odds for testing both EBV positive (OR 4.52, 95% CI 2.68-7.63) and EBV negative (OR 20.99, 95% CI 5.81-75.79). Only five females were classified with acute/recent EVB infection in this period.

CONCLUSION

Misinterpretation of EBV infection late symptoms is not a leading explanation for Danish females experiencing suspected adverse events after HPV vaccination. Although EBV cannot be excluded as an explanatory factor for a very small proportion of suspected adverse events, the findings are more likely explained by protopathic bias, i.e. the fact that a larger proportion of females suspecting adverse events are tested for EBV.

Infectious diseases detected by screening after arrival to Denmark in internationally adopted children

AIM

To show the prevalence of selected infectious diseases among internationally adopted children (IAC) in Denmark.

BACKGROUND

Each year approximately 200 IAC arrive in Denmark. These are at increased risk of infectious diseases rarely seen in Danish children. Studies from the 1990s showed that 60% of IAC had infectious diseases and that the majority of these were undetected without screening.

METHODS

The study is a prospective study of medical records from children seen in the adoption clinic at Copenhagen University Hospital in the period 2009-2013. Screening was done for hepatitis A (HAV), B (HBV) and C (HCV), syphilis, HIV, tuberculosis (TB) and intestinal parasites.

RESULTS

In 245 IAC tested, 2% had evidence of recent HAV infection, 3% with HBV and one child with HCV, and no cases of HIV were found. One child had antibodies against syphilis (anti-Trpa AB positivity), and 2% were latently infected with tuberculosis. We found 30% infected with pathogenic intestinal parasites. Only 46% had serologic evidence of immunisation against HBV.

CONCLUSION

The prevalence of infections in IAC was lower than previously reported but compared to the general population, a higher prevalence of intestinal parasites, hepatitis and tuberculosis was found. We recommend that IAC are offered screening shortly after arrival.

Post-varicella Arterial Ischemic Stroke in Denmark 2010 to 2016

BACKGROUND

Varicella, most often a benign disease of childhood, is associated with an increased risk of arterial ischemic stroke in children. The aim of the present study was to estimate the incidence of post-varicella arterial ischemic stroke in the Danish child population and describe clinical characteristics of children admitted with post-varicella arterial ischemic stroke.

METHODS

In the Danish National Patient Register, we identified inpatients 28 days to 16 years of age with a discharge diagnosis of stroke or cerebrovascular disease from 2010 to 2016. Medical files were reviewed, and children with arterial ischemic stroke and varicella infection less than 12 months before onset of symptoms were included.

RESULTS

We identified 15 children with arterial ischemic stroke and varicella less than 12 months before. In nine children, the diagnosis was confirmed by detection of varicella zoster virus DNA or varicella zoster virus immunoglobulin G in the cerebrospinal fluid. All children were previously healthy, the mean age was four years, and 67% were male. The median time from varicella rash to arterial ischemic stroke was 4.6 months. The most common location of arterial ischemic stroke was the basal ganglia, and affected vessels were most often in the anterior circulation. Fifty-three percent experienced neurological sequelae of varying degree.

CONCLUSIONS

In Denmark, where varicella vaccination is not part of the childhood vaccination program, the estimated risk of post-varicella arterial ischemic stroke was one case (including possible cases) per 26,000 children with varicella.

Varicella in Europe-A review of the epidemiology and experience with vaccination

There is no consensus as regards the European varicella immunisation policy; some countries have introduced varicella vaccination in their routine childhood immunisation programs whereas others have decided against or are debating. With the aim of providing an overview of the epidemiology of varicella in Europe and addressing the different strategies and the experiences so far, we performed a review of epidemiological studies done in Europe from 2004 to 2014. Varicella is mainly a disease of childhood, but sero-epidemiological studies show regional differences in the proportion of susceptible adults. Hospitalisation due to varicella is not common, but complications and hospitalisation mainly affect previously healthy children, which underlines the importance of not dismissing varicella as a disease of little importance. The experience with universal vaccination in Europe shows that vaccination leads to a rapid reduction of disease incidence. Vaccine effectiveness is high and a protective herd effect is obtained. Experience with vaccination in Europe has not been long enough, though, to draw conclusions on benefits and drawbacks with vaccination as well as the capacity for national programs in Europe to maintain a sufficiently high coverage to prevent a change in age group distribution to older children and young adults or on the impact that varicella immunisation may have on the epidemiology of shingles.