General information

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.

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).