Background We aimed to detect SARS-CoV-2 serum antibodies in the general population of the Netherlands and identify risk factors for seropositivity amidst the first COVID-19 epidemic wave. Methods Participants (n=3207, aged 2–90 years), enrolled from a previously established nationwide serosurveillance study, provided a self-collected fingerstick blood sample and completed a questionnaire (median inclusion date 3 April 2020). IgG antibodies targeted against the spike S1-protein of SARS-CoV-2 were quantified using a validated multiplex-immunoassay. Seroprevalence was estimated controlling for survey design, individual pre-pandemic concentration, and test performance. Random-effects logistic regression identified risk factors for seropositivity. Results Overall seroprevalence in the Netherlands was 2.8% (95% CI 2.1 to 3.7), with no differences between sexes or ethnic background, and regionally ranging between 1.3 and 4.0%. Estimates were highest among 18–39 year-olds (4.9%), and lowest in children 2–17 years (1.7%). Multivariable analysis revealed that persons taking immunosuppressants and those from the Orthodox-Reformed Protestant community had over four times higher odds of being seropositive compared to others. Anosmia/ageusia was the most discriminative symptom between seropositive (53%) and seronegative persons (4%, p<0.0001). Antibody concentrations in seropositive persons were significantly higher in those with fever or dyspnoea in contrast to those without (p=0.01 and p=0.04, respectively). Conclusions In the midst of the first epidemic wave, 2.8% of the Dutch population was estimated to be infected with SARS-CoV-2, that is, 30 times higher than reported. This study identified independent groups with increased odds for seropositivity that may require specific surveillance measures to guide future protective interventions internationally, including vaccination once available.

This paper outlines the methodology, study population and response rate of a third large Dutch population-based cross-sectional serosurvey carried-out in 2016/2017, primarily aiming to obtain insight into age-specific seroprevalence of vaccine-preventable diseases to evaluate the National Immunization Programme (NIP). In addition, Caribbean Netherlands (CN) was included, which enables additional research into tropical pathogens. A two-stage cluster sampling technique was used to draw a sample of Dutch residents (0–89 years) (NS), including an oversampling of non-Western migrants, persons living in low vaccination coverage (LVC) areas, and an extra sample of persons born in Suriname, Aruba and the former Dutch Antilles (SAN). A separate sample was drawn for each Caribbean island. At the consultation hours, questionnaires, blood samples, oro- and nasopharyngeal swabs, faeces, − and only in the Netherlands (NL) saliva and a diary about contact patterns – were obtained from participants. Vaccination- and medical history was retrieved, and in CN anthropometric measurements were taken. In total, blood samples and questionnaires were collected from 9415 persons: 5745 (14.4%) in the NS (including the non-Western migrants), 1354 (19.8%) in LVC areas, 501 (6.9%) SAN, and 1815 (23.4%) in CN. This study will give insight into protection of the population against infectious diseases included in the NIP. Research based on this large biobank will contribute to public health (policy) in NL and CN, e.g., regarding outbreak management and emerging pathogens. Further, we will be able to extend our knowledge on infectious diseases and its changing dynamics by linking serological data to results from additional materials collected, environmental- and pharmacological data.

This large, nationwide, population-based, seroepidemiological study provides evidence of the effectiveness of physical distancing (>1.5 m) and indoor group size reductions in reducing severe acute respiratory syndrome coronavirus 2 infection. Additionally, young adults may play an important role in viral spread, contrary to children up until age 12 years with whom close contact is permitted. Clinical Trials Registration: NTR8473.

Background: To inform future response planning we aimed to assess SARS‐CoV‐2 trends in infection‐ and/or vaccine‐induced immunity, including breakthrough infections, among (sub)groups, professions and regions in the Dutch population during the Variant of Concern (VOC)‐era.Methods: In this prospective population‐based cohort, randomly selected participants (n = 9985) aged 1–92 years (recruited early‐2020) donated home‐collected fingerstick‐blood samples at six timepoints in 2021/2022, covering waves dominated by Alpha, Delta, and multiple Omicron (sub‐)variants. IgG antibody assessment against Spike‐S1 and Nucleoprotein was combined with vaccination‐ and testing data to estimate infection‐induced (inf) and total (infection‐ and vaccination‐induced) seroprevalence.Results: Nationwide inf‐seroprevalence rose modestly from 12% (95% CI 11–13) since Alpha to 26% (95% CI 24–28) amidst Delta, while total seroprevalence increased rapidly to 87% (95% CI 85–88), particularly in elderly and those with comorbidities (i.e., vulnerable groups). Interestingly, highest infection rates were noticeable among low/middle educated elderly, non‐Western, those in contact professions, adolescents and young adults, and in low‐vaccination coverage regions. Following Omicron emergence, inf‐seroprevalence elevated sharply to 62% (95% CI 59–65) and further to 86% (95% CI 83–90) in late‐2022, with frequent breakthrough infections and decreasing seroprevalence dissimilarities between most groups. Whereas > 90% of < 60‐year‐olds had been infected at least once, 30% of vaccinated vulnerable individuals had still not acquired hybrid immunity.Conclusions: Groups identified to have been infected disproportionally during the acute phase of the pandemic require specific attention in evaluation of control measures and future response planning worldwide. Furthermore, ongoing tailored vaccination efforts and (sero‐)monitoring of vulnerable groups may remain important.

The National Immunization Program (NIP) on Bonaire, St. Eustatius and Saba (i.e., Caribbean Netherlands (CN)) includes the measles-mumps-rubella (MMR) vaccine since 1988/89. Seroepidemiological data is an important tool to evaluate the NIP, hence a cross-sectional representative population-based serosurveillance study was conducted for the first time in CN in mid-2017. Participants (n = 1829, aged 0–90 years) donated a blood sample and completed a health-related questionnaire. MMR-specific IgG antibodies were determined using a bead-based multiplex immunoassay and risk factors were analyzed using logistic regression models. Overall seroprevalence was high for measles (94%), but lower for mumps and rubella (both 85%). In NIP eligibles, including women of childbearing age, rubella seroprevalence (88%) exceeded the threshold for protection (85%); however, for measles (89%) this protective level (95%) was not met. MMR seropositivity was lowest in children who became CN resident at 11–17 years of age (especially for measles (72%)), mostly originating from Latin America and other non-Western countries. Interestingly, rubella seroprevalence was lowest in non-NIP eligible adults from Dutch overseas territories and Suriname (75%). Taken together, MMR immunity is generally good in CN, nonetheless some risk groups were identified. Additionally, we found evidence for a unique island epidemiology. In light of recent regional measles outbreaks, disease monitoring remains of utmost importance.

Background: The first wave of the COVID-19 pandemic in 2020 was largely mitigated by limiting contacts in the general population. In early 2022, most contact-reducing measures were lifted. Aim: To assess whether the population has reverted to pre-pandemic contact behaviour and how this would affect transmission potential of a newly emerging pathogen. Methods: We compared two studies on contact behaviour in the Netherlands: the PIENTER Corona study, conducted during and after the pandemic (held every 2–6 months from April 2020) and the PIENTER3 study (2016–17, as pre-pandemic baseline). In both, participants (ages 1–85 years) reported number and age group of all face-to-face persons contacted on the previous day in a survey. Transmission potential was examined using the next-generation matrix approach. Results: We found an average of 15.4 (95% CI: 14.3–16.4) community contacts per person per day after the pandemic in May 2023, 13% lower than baseline (17.8; 95% CI: 17.0–18.5). Among all ages, children (5–9 years) had the highest number of contacts, both pre- and post-pandemic. Mainly adults aged 20–59 years had not reverted to pre-pandemic behaviours, possibly because they more often work from home. Although the number of contacts is lower compared to the pre-pandemic period, the effect on transmission potential of a newly emerging respiratory pathogen is limited if all age groups were equally susceptible. Conclusion: Continuous monitoring of contacts can signal changes in contact patterns and can define a ‘new normal’ baseline. Both aspects are needed to prepare for a future pandemic.