On 11 March 2020, the World Health Organization (WHO) declared the current coronavirus disease (COVID-19) outbreak, caused by the SARS-CoV-2 virus, to be a pandemic . In the first half of 2020, many governments worldwide implemented control measures including physical distancing, hygiene measures, and lockdowns to contain their epidemics. Although such measures were effective, they had a tremendous economic, societal and health impact. Within a year after SARS-CoV-2 emerged, the first vaccines were developed and approved after registration trials showing high efficacy against symptomatic SARS-CoV-2 infection [2–5].
In the Netherlands, the COVID-19 vaccination program started on 6 January 2021. As of 18 April 2021, four different COVID-19 vaccines have been approved and used in the Netherlands: Comirnaty (BNT162b2; BioNTech/Pfizer, Mainz, Germany/New York, United States (US)), Spikevax (mRNA-1273, Moderna, Cambridge, US), Vaxzevria (ChAdOx1-S; AstraZeneca, Cambridge, United Kingdom), and Janssen (Ad26.COV2-S (recombinant), Janssen-Cilag International NV, Beerse, Belgium). Hospital employees in direct COVID-19 care, general practitioners, residents and staff of long-term care facilities, and adult persons living in an institution and their caregivers, were the first groups to receive vaccination. The subsequent vaccine roll-out in the non-institutionalized adult population followed an age-specific approach, starting with the elderly. Also people below 60 years of age with a medical risk condition such as obesity, pulmonary or cardiac disease, diabetes and immune deficiency were prioritized . From July 2021 onwards, children of 12 years and older were also invited for vaccination. As of October 2021, persons with a severe immune deficiency were offered a third dose of Comirnaty or Spikevax as part of their primary series. The first booster campaign was initiated on 18 November 2021, using the same two vaccines, prioritizing health care workers and adults above 60 years of age. By 4 January 2022, all persons of 18 years and older had been invited for the first booster vaccination. Booster vaccination was also available for children of 12 years and older from 7 March 2022 onwards, children aged between 5 and 11 years with medical risk conditions from 18 December 2022 onwards, and those without a medical risk condition from 17 January 2022 onwards. Vaccinations are registered in the national COVID-19 vaccination Information and Monitoring System (CIMS) of the Dutch National Institute of Public Health and the Environment (RIVM) when the individual provided informed consent for registration . By 28 March 2022, a primary series vaccination coverage of 86% and a booster vaccination coverage of 64% of the Dutch adult population had been reached . A primary vaccination series is defined in the Netherlands as either one Janssen vaccination, one vaccination following previous SARS-CoV-2 infection, or two vaccinations with any of the mRNA or virus vector vaccines.
After implementation of a vaccination program, it is important to continuously monitor the program with regards to safety, effectiveness, and epidemiological impact, in real world settings . Pre-registration vaccine trial participants are usually not representative of the target population and usually do not contain sufficient numbers of individuals of special interest (e.g. children, elderly, and individuals with comorbidities), and duration of follow-up prior to vaccine registration is usually short [10, 11]. Furthermore, VE estimates against SARS-CoV-2 infection, disease and transmission over time, and against new virus variants, are needed, overall and for subgroups in the population .
Real-world VE can be obtained using observational study designs. In the Netherlands, as in other countries, nationwide COVID-19 surveillance data and observational studies using hospital data are used to monitor and evaluate VE against SARS-CoV-2 infections and disease, including severe disease leading to hospitalization. The advantages of using national surveillance data are the large numbers and data availability in real-time, but disadvantages include the lack of detailed sociodemographic, behavioral, and clinical metadata and a lack of biological sampling. For example, the Dutch national surveillance database lacks information on previous SARS-CoV-2 infections, especially asymptomatic and mild infections, and comorbidities, and blood sampling for immunological assessments is not routinely done. These limitations can be addressed in a prospective cohort study design.
In the current paper, we present the design and baseline characteristics of the study population of a large population-based prospective cohort study in community-dwelling persons aged 18–85 years to monitor the real-world VE against symptomatic SARS-CoV-2 infection in the Netherlands, the VAccination Study COvid-19 (VASCO). Participants are followed for 5 years to assess VE by time since vaccination and by subpopulation. Extensive metadata are being collected from cohort participants using digital questionnaires and vaccination status from CIMS, and serum samples for immunological testing are collected at regular intervals. Results will be used to evaluate the Dutch COVID-19 vaccination strategy, to monitor immunity in the population obtained through vaccination and infection, and to inform future vaccination strategies. Furthermore, VASCO provides an infrastructure to conduct substudies in response to unforeseen developments within the COVID-19 pandemic.