The PIENTER Corona Study was launched in April 2020. As shown in Figure 1 below, a new round of research took place every few months after that. The 8th round was in June and July 2022. The study group was expanded by adding new participants in round 2 and in round 6. Several thousand blood samples were examined in each round. The number of blood samples examined in round 8 was about 6,200.
In each round, slightly more than half of the blood samples generally came from women. Also, there are usually slightly more older participants than younger ones. This is taken into account in our analyses.
Figure 1: Timeline and (approximate) total number of blood samples per research round in rounds 1 through 8.
*New participants were invited in round 2 and round 6.
Number of participants by age and region
The average age of the participants in the PIENTER Corona Study is about 50 years old. The youngest participant is 1 year old and the oldest participant is 92 years old. The figure below shows the current age distribution of the participants for each region in the Netherlands. The age groups of 70-74 years and 75-79 years had the most participants, and the age groups are evenly distributed across the regions.
Figure 2: Age distribution of participants (round 8 – March/April June/July 2022). The regions shown consist of the following provinces: North = Groningen, Friesland, Drenthe and Overijssel; Midwest = North Holland and Flevoland; Midwest = Utrecht and Gelderland; Southwest = South Holland and Zeeland; Southeast = North Brabant and Limburg. LVC sample = Low Vaccination Coverage, consisting of a few municipalities in the Netherlands where general vaccination coverage (against all kinds of infectious diseases) is lower compared to the rest of the Netherlands.
The number of participants per municipality since round 6 (after the last expansion) is shown in Figure 3. The larger the blue dot, the more people from this municipality are participating. The distribution of blood samples for each municipality shows some correlation to the population distribution in the Netherlands: more people from the Randstad conurbation took part in the study. The research results are analysed with due consideration of differences in the participants’ gender, age, region and ethnic background.
Figure 3: Number of participants per municipality (after the last expansion in round 6 – November/December 2021).
People who have antibodies
The research results show what percentage of the general population in the Netherlands is estimated to have generated antibodies against the coronavirus (SARS-CoV-2) that causes COVID-19. This allows us to estimate the number of people who have built up immunity to the virus due to infection, and (since the vaccination campaign started in 2021) also due to vaccination. The percentage of people who have antibodies is called seroprevalence.
During the first round of research in the spring of 2020, this was just under 3%. In the second round in the summer, it rose to 4.5%, reaching approximately 5% during the third round of the study in autumn 2020. In February 2021, seroprevalence had risen to over 14%. At that time, about 2% of the participants had antibodies resulting from vaccination and about 12% had had a coronavirus infection. In the fifth round (summer 2021), nearly 65% of the population had antibodies against the coronavirus in their blood, and about 20% had evidence of previous infection. In autumn 2021 (round 6), nearly 87% of the population had antibodies. At that time, over 25% of the total population turned out to have evidence of a previous infection in their blood. In the seventh round (spring 2022), about 95% of the population had antibodies. At that time, about 60% of the population was found to have evidence of a previous infection in their blood. In the eighth round (summer 2022), the percentage of people with antibodies remained consistently high (>95%). Nearly three-quarters of the population had had at least one infection by that time.
The increase in the percentage of people with antibodies since previous rounds is mainly due to the start of the vaccination programme in early 2021, and partly due to an increase in infections, mainly involving the Delta and Omicron variants of the virus. In rounds 7 and 8, there were hardly any differences between men and women, or between people from different ethnic backgrounds.
Age distribution of people who have antibodies
Figure 4 shows the percentage of participants with antibodies distributed across the age groups in the population (from 1 up to 92 years old) since the first wave in 2020. The research results from the PIENTER Corona Study are then extrapolated to the general population. An estimate is provided for the percentage of the Dutch population that has been in contact with the coronavirus in each age group.
Vaccination against SARS-CoV-2 started in January 2021 in the Netherlands. Since the fourth round in February 2021, immunity resulting from infection and/or from vaccination has been investigated. This research is done by looking at different types of antibodies in combination with information from the questionnaire and information about infections from previous rounds. In addition, some people also become infected after vaccination.
As seen in rounds 4–7, the antibody results from the eighth round (June/July 2022) show a combination of vaccination and infections in the population (Figure 4, orange line). At the start of 2021, the older age groups and vulnerable people were the first to receive a COVID-19 vaccination. The younger age groups followed later. In general, vaccination coverage is high among adults and quite a bit lower among children under 12. In all age groups over 12 years, the percentage of people with antibodies (resulting from infection and/or vaccination, orange line in Figure 4) was 95% or higher in round 8.
From round 2 through round 6, there was a similar trend in the percentage of people who showed evidence of infection (Figure 4, from light green to dark green dotted line). In every round, young adults showed the highest percentage of infections, relatively speaking, followed by people aged 50-59 years; this percentage was lowest in the oldest age groups.
In the seventh round (Figure 4, brown dotted line), there was a major increase in the percentage of infections compared to the previous round, caused by the Delta and Omicron variants of the coronavirus SARS-CoV-2. The strongest increase occurred in people under 50, especially in children, and then gradually declined in older people. Round 8 shows a 10-15% increase in infections across all ages (reddish-brown dotted line). Overall, young people are still the age group with the highest percentage of infections (up to 95%).
Figure 4: Percentage of people who have antibodies by age over time (rounds 2-8). The dotted lines (light green to reddish-brown) show the percentage of people who have antibodies due to infection in round 2 (June/July 2020) up to and including round 8 (June/July 2022). The orange line shows total seroprevalence after infection and vaccination in round 8 (June/July 2022).
The results of the PIENTER Corona Study are presented in scientific articles for publication, so they can be read by everyone. As a result, other countries can also benefit from the research and use key findings in formulating their public health policies. When articles are published online, they will be posted on this page.
Analysis of SARS-CoV-2 seroprevalence and risk factors, as well as symptoms in relation to antibody levels after infection during the first wave (based on data from round 1):
Vos ERA, den Hartog G, Schepp RM, et al. Nationwide seroprevalence of SARS-CoV-2 and identification of risk factors in the general population of the Netherlands during the first epidemic wave. Journal of Epidemiology and Community Health. 2020 Nov 28;75(6):489–95.
The effect of social distancing on contact patterns in the population (based on data from rounds 1 and 2):
Backer JA, Mollema L, Vos ER, et al. Impact of physical distancing measures against COVID-19 on contacts and mixing patterns: repeated cross-sectional surveys, the Netherlands, 2016–17, April 2020 and June 2020. Eurosurveillance. 2021 Feb;26(8):2000994.
Estimated asymptomatic SARS-CoV-2 infections in the population (based on data from the PIENTER3 study, and from PIENTER Corona Study rounds 1 and 2):
McDonald SA, Miura F, Vos ERA, et al. Estimating the asymptomatic proportion of SARS-CoV-2 infection in the general population: Analysis of nationwide serosurvey data in the Netherlands. European Journal of Epidemiology. 2021 Jul;36(7):735-739.
The effect of social distancing measures on SARS-CoV-2 infection after the first wave (based on data from round 2):
Vos ERA, van Boven M, den Hartog G, et al. Associations between measures of social distancing and SARS-CoV-2 seropositivity: a nationwide population-based study in the Netherlands. Clinical Infectious Diseases. 2021 Dec 16;73(12):2318-2321.
Duration of immunity and binding strength of SARS-CoV-2 antibodies more than six months after infection (based on data from rounds 1-3):
den Hartog G, Vos ERA, van den Hoogen LL, et al. Persistence of antibodies to SARS-CoV-2 in relation to symptoms in a nationwide prospective study. Clinical Infectious Diseases. 2021 Dec 16;73(12):2155-2162.
Use of antibodies to identify breakthrough infections after vaccination (using data from round 4):
van den Hoogen LL, Smits G, van Hagen CCE, et al. Seropositivity to Nucleoprotein to detect mild and asymptomatic SARS-CoV-2 infections: A complementary tool to detect breakthrough infections after COVID-19 vaccination? Vaccine. 2022 Apr 1;40(15):2251-2257.
The antibody response after vaccination (using data from round 5):
van den Hoogen LL, Verheul MK, Vos ERA et al. SARS-CoV-2 Spike S1-specific IgG kinetic profiles following mRNA or vector-based vaccination in the general Dutch population show distinct kinetics. Scientific Reports. 2022 Apr 8;12(1):5935.
The burden of disease from acute COVID-19 (using data from round 3):
McDonald SA, Lagerweij GR, de Boer P, et al. The estimated disease burden of acute COVID-19 in the Netherlands in 2020, in disability-adjusted life-years. European Journal of Epidemiology. 2022 Aug 11;1-13.
Contact patterns in the population of the Netherlands (based on data from rounds 1, 2, 4 and 5):
Backer JA, Bogaardt L, Beutels P, et al. Dynamics of non-household contacts during the COVID-19 pandemic in 2020 and 2021 in the Netherlands. Scientific Reports 13, 5166 (2023).
Pre-print on the age-specific severity of SARS-CoV-2 infections over time (based on data from rounds 1–5):
de Boer PT, van de Kassteele J, Vos ERA, et al. Age-specific severity of SARS-CoV-2 in February 2020 – June 2021 in the Netherlands. medRxiv 2023.02.09.23285703.
Laboratory method for measuring SARS-CoV-2 antibodies:
den Hartog G, Schepp RM, Kuijer M, et al. SARS-CoV-2–Specific Antibody Detection for Seroepidemiology: A Multiplex Analysis Approach Accounting for Accurate Seroprevalence. The Journal of Infectious Diseases. 2020 Oct 1;222(9):1452-1461.
RIVM previously published an article on the scientific background of the PIENTER3 study:
Verberk JDM, Vos RA, Mollema L, et al. Third national biobank for population-based seroprevalence studies in the Netherlands, including the Caribbean Netherlands. BMC Infectious Diseases. 2019 May;19(1):470.