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. Round 10 took place in April and May 2023. 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 10 was about 5,300.

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 now 2 years old and the oldest participant is 91 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 10 – April/May 2023). 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 infectious diseases covered by the National Immunisation Programme) 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

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 summer 2020, 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, the vaccination campaign had just started in the Netherlands and about 2% of the participants had antibodies resulting from vaccination, while about 12% had had a coronavirus infection. In summer 2021 (round 5), 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), more than 85% 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 spring 2022 (round 7), about 95% of the population had antibodies; this percentage has remained consistently high since then. At that time, slightly more than 60% of the population was found to have evidence of a previous infection in their blood. In summer 2022 (round 8), nearly 75% of the population had had at least one infection. By the end of 2022 (round 9), this percentage had increased to over 85% , rising to 95% in spring 2023 (round 10).

After the Omicron variant arrived, there were hardly any differences between men and women, or between people from different ethnic backgrounds. Previous regional differences have also disappeared.

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 of the Netherlands. 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. Therefore, 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, especially since the Omicron variant.

Figure 4: Percentage of people who have antibodies by age over time   (A. 2020/21: rounds 2–6 and B. 2022/23: rounds 7–10). The dotted lines show the percentage of people who have antibodies due to infection, while the solid line shows total seroprevalence after infection and vaccination.

 Continuing the pattern seen in rounds 4–9, the antibody results from the 10th round (April/May 2023) show a combination of vaccination and infections in the population (Figure 4A: 2020/21, Figure 4B: 2022/23). At the start of 2021, the older age groups and vulnerable people were the first to be invited for a COVID-19 vaccination. The younger age groups followed later. This is clearly visible from the dark green solid lines in Figure 4B, which rose higher according to age over the course of 2021. 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, dark green line in Figure 4A) was already 90% or higher starting in round 6 (autumn 2021), and rose to 100% in the following rounds (Figure 4B).

From round 2 through round 6, there was a similar trend in the percentage of people who showed evidence of infection (Figure 4A). In the 7th round (March 2022, light brown dotted line in Figure 4B), 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. Round 8 (June 2022) showed a 10-15% increase in infections across all ages (Figure 4B,  red dotted line). Overall, young people were the age group with the highest percentage of infections (up to 95%). In round 9 (November 2022), the percentage of people who showed evidence of one or more infections in their blood rose among all adults by about 15% (Figure 4B, dark red dotted line). Nearly everyone under 50 years old had been infected at least once by then. The oldest age groups still showed the lowest percentage, relatively speaking. Among 80-year-olds, for example, about two in three people had had a SARS-CoV-2 infection at some point in time.

Since round 10 (April 2023) (Figure 4B, very dark red dotted line), the percentage of infections has continued to increase across all age groups. At that time, about 90% of people in their 70s had been infected at least once, compared to about four in five people (approximately 80%) in the 80-89 age group.

Mucosal antibodies

The SARS-CoV-2 virus enters the body through the upper respiratory tract, including the nose. Antibodies in the blood offer effective protection against severe COVID-19, but are less effective against infection in the respiratory tract at the entry point for the virus. Antibodies in the respiratory system do help protect against infection. These are called mucosal antibodies. In round 8, a number of PIENTER Corona Study participants were also asked to provide a nasal swab (similar to taking a COVID-19 self-test). These samples taken from the nose were checked for mucosal antibodies against SARS-CoV-2.

The body produces various types of antibodies. These tests looked at the IgG type that is common in blood, and to a lesser extent also in the nose, as well as the IgA type that is important in mucous membranes such as the nose. Nasal IgG-type antibodies were found in about 93% of all participants who provided a nasal swab and about 98% of these participants aged 12 years and older (Figure 5A). People who had been vaccinated were just as likely to have nasal IgG antibodies as people who had been vaccinated and had also had a previous infection. About 24% of all participants had type IgA antibodies. That percentage rises to about 33% if people had had a SARS-CoV-2 infection (Figure 5B). Young children are less likely to have nasal antibodies than children aged 12 years and older. After vaccination, IgG antibodies are produced and can be found in the nose, but nasal IgA antibodies are not usually found after vaccination. After a SARS-CoV-2 infection, both IgG and IgA antibodies are found in about 33% of participants. Nasal IgG and IgA antibodies are both present in higher levels in people who have had a previous infection (Figure 5C-D). In addition, people with more antibodies were less likely to become infected with the virus in the next round. Mucosal antibodies make the virus less able to bind to human cells and help the body clear the virus.

Figure 5: Mucosal antibodies in nasal fluid from participants in round 8 – June/July 2022, distinguishing between people who had had a previous infection (red) and people who showed no evidence of previous infection (blue). BAU / AU are the units used to measure antibody concentrations.

(A) Percentage of participants with mucosal S1 IgG antibodies by age;
(B) Percentage of participants with mucosal S1 IgA antibodies by age;
(C) Mucosal S1 IgG antibody concentrations by age;
(D) Mucosal S1 IgA antibody concentrations by age;
(E) Total mucosal S1 IgG antibody concentrations;
(F) Total mucosal S1 IgA antibody concentrations.

Post-COVID

There are people who have long-term symptoms after a SARS-CoV-2 infection. These persistent symptoms are also known as post-COVID. Research on post-COVID is also being conducted in the context of the PIENTER Corona Study. To estimate the impact of post-COVID, the researchers include questions about quality of life in the survey. Results on this topic will be shared on this website in the near future.