Data collection

The surveys are conducted by I&O Research and commissioned by the GOR Network. They contact their research panel members to solicit participants. The number of adults who take part in the survey is stated on the summary page for each research round.

Materials

Data collection took place online using a pre-programmed survey. The survey questionnaire was compiled by the GOR Network and consisted of various questions with a wide range of answer categories. The following topics were covered:

• socio-demographic features
• experiences during the COVID-19 period
• perceived impact of the coronavirus measuresgeneral health
• mental health
• physical health
• care and support needs
• delayed care

Representative sample and weighting

The sample was disproportional with respect to province, meaning that each province was sufficiently represented, regardless of the size of the province. We tried to find at least 200 participants per province. Following data collection, we corrected for deviations in representative sample by assigning weighting factors based on sex, age, region and level of education; we then compared the composition of the sample to that of the population of the Netherlands (derived from the ‘Golden Standard’ used by Statistics Netherlands – CBS).

Data analysis and presentation of the results

The website offers weighted cross-sectional figures. This helps us provide insight into the responses of Dutch adults in terms of mental and physical health, support needs and perceived impact of the measures.

In each round, we review which additional analyses may be needed. For example, in research round 3, exploratory research took place to identify potential risk factors and shielding factors for persistent impact from experienced events related to COVID-19. The in-depth analyses differ from one round to the next.

Chi-squared tests were used to determine which characteristics may affect reported mental and physical health, support needs and perceived impact of the measures. Due to the large sample size, we kept to a significance level of 1% (p ≤ .01). To map out regional differences, we looked at mental and physical health, support needs and perceived impact of the measures for each province. The comprehensive tables and results per province are provided in the factsheets that are published for each research round; see the Publications page for more details.

About surveillance for the Nivel Primary Care Database

The surveillance figures from the Nivel Primary Care Database are intended for use in detecting infectious diseases and other health conditions in the general population of the Netherlands, but can also be used to monitor other acute symptoms. This resource provides weekly figures on symptoms and conditions among the Dutch population based on anonymised data from electronic medical records kept by GP practices. The aggregated figures show how many people consulted their GP for specific symptoms or health conditions in that week, divided by the total number of patients registered in that practice (prevalence). The data comes from about 380 GP practices, offering healthcare services to roughly 1.6 million registered patients (9% of the Dutch population). Health data on these symptoms and conditions is routinely recorded, based on the International Classification of Primary Care (ICPC), version 1. This ICPC-1 system is used by all general practitioners in the Netherlands.

Acute health problems/conditions that may be related to the COVID-19 pandemic (whether directly or indirectly), as presented to GPs, have been mapped. In this study, we focus on a number of non-specific symptoms (designated as “not otherwise specified” or NOS) that could have a physical or mental cause. This includes both symptoms that are recorded in the context of episodic health conditions and symptoms that are not associated with a specific health condition. Some of these symptoms are listed by RIVM as possible persistent symptoms after an infection with the coronavirus SARS-CoV-2. However, it is not possible to identify any direct link, since SARS-CoV-2 infection is not recorded in the data for this study. See the table below for the symptoms included in the study and the corresponding ICPC-1 codes.

List of non-specific symptoms and corresponding ICPC-1 codes 

* Relevant for consideration of Long COVID, according to the RIVM list