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 7th round was in March and April 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 7 was about 6,900.

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 7 – March/April 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 a previous coronavirus infection in their blood. In autumn 2021 (round 6), nearly 87% of the population had antibodies against the coronavirus in their blood. 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 in their blood. At that time, about 60% of the population was found to have evidence of a previous coronavirus infection in their blood.

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, particularly involving the Delta and Omicron variants of the virus. In round 7, there were hardly any differences in antibodies 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. Since January 2021, people started getting vaccinated against SARS-CoV-2. Due to that fact, since the fourth round in February 2021, a distinction has been made between immunity resulting from infection or from vaccination. This is established 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 4–6, the antibody results from the seventh round (March/April 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. By the seventh round of the study, everyone aged 5 years and over had had the opportunity to be vaccinated against the coronavirus SARS-CoV-2. At that point, children had been eligible for the basic series of vaccinations, while adults were eligible for the booster vaccination. In general, vaccination coverage was 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 vaccination and/or infection, orange line in Figure 4) was 95% or higher.

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, reddish-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 percentage of people with antibodies from infection decreased by age and was still lowest in the oldest age groups, at 20-30%. The highest number of infections took place in people under 50; by now, it is estimated that at least 60% of people in each age group under 50 has had a SARS-CoV-2 infection. The percentage of infections rose particularly fast in the youngest age groups, with a peak of 85-90% among children and adolescents in secondary school in round 7. Compared to rounds 1 through 6, round 7 also showed a major increase in seroprevalence among children in primary school.

Figure 4: Percentage of people who have antibodies by age over time (rounds 2-7). 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 7 (March/April 2022). The orange line shows total seroprevalence after infection and vaccination in round 7 (March/April 2022).

People who have antibodies due to infection and/or vaccination by region

The study is also investigating the distribution of the virus throughout the Netherlands. Figure 5 shows the percentage of participants per GGD region that had the virus since the first wave. A lighter colour (light orange) means a lower percentage per region on average, while a darker colour (dark red) means a higher percentage per region on average.

During the first wave (round 2 – June/July 2020 and round 3 – September/October 2020), the prevalence of antibodies from infection was significantly lower in the northern provinces than in the central regions, and much lower compared to the south of the Netherlands. Although seroprevalence was still highest in the southern provinces after the second wave (round 4 – February/March 2021), it was clearly distributed more evenly across the entire country. A more comparable pattern was observed after the wave involving the Alpha variant (round 5 – June/July 2021), with the strongest increases in the GGD regions of Zuid-Limburg, Utrecht, Fryslân and Hollands-Noorden. During the wave involving the Delta variant (round 6 – November/December 2021), the occurrence of antibodies due to infection continued to differ between northern and southern regions. The GGD regions of Amsterdam, Zeeland, Zuid-Limburg, Hart voor Brabant and West-Brabant had the highest percentages.

After the end of the Delta wave in late 2021, the Netherlands faced the first wave of the Omicron variant. The highly contagious Delta and Omicron variants probably contributed to the high numbers of infections, as seen in round 7 of the study (March/April 2022). Minor differences were still visible between the northern and southern regions at this point. In round 7, the highest percentages of people with antibodies resulting from infection were observed in the GGD regions of Hart voor Brabant, West-Brabant and Flevoland (all above 70%), while the lowest were seen in Zeeland. Between rounds 6 and 7 the largest part of the number of infections in almost all GGD-regions have at least doubled (except for GGD Zeeland). Between rounds 6 and 7 the proportion of infections has at least doubled in almost all GGD-regions (except for GGD Zeeland). At that time, the biggest increases (three- to four) were observed in Friesland, Noord en Oost-Gelderland, Flevoland, Drenthe and IJsselland.

Figure 5: Percentage of people who have antibodies due to infection, over time (rounds 2 through 7 – June/July 2020 through March/April 2022), by GGD region.

Figure 6 illustrates the total percentage of people who have antibodies due to infection and vaccination in the seventh round (March/April 2022), by GGD region.  There are minimal regional differences in the total percentage of people with antibodies, ranging from 92% to 99%. Relatively speaking, the GGD regions of Gelderland-Midden, Fryslân, Twente and Utrecht had the lowest percentages (under 94%) and the GGD regions of Zeeland, Zuid-Limburg, Kennemerland, Zaanstreek/Waterland and Hollands-Midden had the highest percentages (over 97%).

Figure 6: Total percentage of people who have antibodies due to infection and/or vaccination in round 7 (March/April 2022) (scale 80-100%), by GGD region.

Antibodies after vaccination with various vaccines

The Netherlands started vaccinating against the coronavirus SARS-CoV-2 in January 2021. So far, four vaccines have mainly been used in the Netherlands: Pfizer/BioNTech (Comirnaty), Moderna (Spikevax), AstraZeneca/Oxford (Vaxzevria) and Janssen (Jcovden). This study examined antibody levels after vaccination, comparing the results for these four vaccines. We looked at antibodies after full vaccination, i.e. 14 days or more after the second dose of Pfizer, Moderna and AstraZeneca or 14 days or more after one dose of Janssen.

Nearly all adults aged 18-64 years had antibodies in their blood after full vaccination. That was the case in 99-100% of participants who received Pfizer, Moderna or AstraZeneca. The few who did not produce antibodies were vulnerable people with an underlying health condition (such as organ transplants or cardiovascular disease). Not everyone had antibodies in the blood 14 days after vaccination with Janssen (95%), but antibody levels increased over time and by 28 days had risen to 98%.

Figure 7 shows the levels of antibodies for each of the four vaccines at 14 days or more after 1 and 2 vaccinations in adults aged 18 to 64 years. Two groups are displayed here: people who were infected with the coronavirus before vaccination (orange clouds) and people who were not (purple clouds). In the group that had had a coronavirus infection before vaccination, a strong increase in antibodies was visible after only one COVID-19 vaccination. This was the case for all four vaccines. In people who had a previous infection, the levels of antibodies did not increase further after a second vaccination with Pfizer, Moderna or AstraZeneca (i.e. there is almost no difference between the orange clouds per type of vaccine).

Read more in the published scientific article describing these results and other conclusions

Figure 7: Levels of antibodies at 14 days or more after vaccination with Pfizer (Comirnaty), Moderna (Spikevax), AstraZeneca (Vaxzevria) and Janssen (Jcovden) in adults aged 18-64 years. A distinction is made between people who are infected with the coronavirus before vaccination (orange clouds) and people who are not (purple clouds). The dots above the dotted line indicate that the person has antibodies. The black lines in the clouds indicate the median level for that group: half of the people have a value above the black line, and the other half below it.

Duration of the presence of antibodies after infection and/or vaccination

It has become apparent from the PIENTER Corona Study that 90% of the people whose blood samples were found to contain SARS-CoV-2 antibodies as a result of infection with the virus still had antibodies in their blood nearly 18 months later. This involves IgG antibodies. These are the most important type of antibodies, since they provide long-term protection. The research results also show that the antibodies grew stronger over time. They were more effective in binding to the virus, so fewer antibodies were needed to achieve the same protection. Read more in the published scientific article describing these results for up to 7 months post infection and other conclusions(link is external).

Figure 8 shows persistent antibody levels over time in different age groups, up to 16 months after infection. Since this figure only looks at antibodies produced after infection, and since many people have been vaccinated by now, there were fewer people with antibodies after infection at the later dates on the chart. Antibody levels drop shortly after infection, but the decrease slows over time, so many people continue to have antibodies for a long time.

Figure 8: Antibody levels after infection, in people of various ages who have not received a vaccination. This looks at the levels of antibodies against the spike protein of the virus, which is also in the vaccine, and can be compared to the virus. 100 in this figure corresponds to 102 in Figures 7, 9 and 10, and 10,000 corresponds to 104.

Figure 9 shows the antibody levels over time in different age groups for three different vaccines (Pfizer, Moderna and AstraZeneca). For all vaccines, antibody levels decrease over time. This holds true for all age groups. It is interesting to note that younger people have more antibodies than older people after vaccination with the Pfizer vaccine.

Figure 9: Antibody levels over time, after two vaccinations, for three different vaccines in different age groups. Information is only displayed for age groups for which data has been recorded in sufficient numbers to show significant trends. Only people who have not been infected with the virus have been included in this figure.

Table 1 shows the percentage of people with antibodies after completing basic vaccination. Before booster vaccinations were available, the standard practice was to administer 2 vaccinations with Pfizer/BioNTech, Moderna or AstraZeneca, or 1 vaccination with Janssen. Nearly everyone still had antibodies 100-200 days after these vaccinations. The number of people with antibodies decreases gradually with age.

Table 1. The percentage of people with antibodies by age group, after completing basic vaccination with Pfizer, Moderna, AstraZeneca (2 vaccinations) or Jcovden (1 vaccination).

Only people who have not been infected with the SARS-CoV-2 virus have been included in this table

Figure 10 shows the antibody levels after vaccination with Pfizer, Moderna and AstraZeneca in people who had a SARS-CoV-2 infection before they were vaccinated. People who had first had an infection (Figure 10, red lines) had more antibodies after vaccination than people who had not been infected before vaccination (Figure 10, black lines). This holds true for all three vaccines, but antibody levels shortly after vaccination vary between vaccines. Also, people who had an infection before being vaccinated retain more antibodies over time (Figure 10, the red lines show less rapid decrease than the black lines). Future analyses will also look at the effect of booster vaccinations and the effect of infections after vaccination.

Figure 10. Antibody levels over time after vaccination with Pfizer, Moderna and AstraZeneca in people who did (red lines) or did not (black lines) have a SARS-CoV-2 infection before they were vaccinated.

Risk of infection

Data from the second round of the study (spring 2020) showed that social distancing is an important measure in stopping the spread of the virus. Participants who had followed the recommendation to stay 1.5 metres apart were less likely to have antibodies in their blood during the first wave, while antibodies were found in the blood of 5.5% of the participants who were less compliant with the distancing rule. And antibodies were only found in the blood of 4% of people who did keep their distance during contact with others. Participants who mostly had contact with children under the age of 10 were hardly ever infected during those encounters. Similarly, participants who indicated that they had an occupation involving physical contact with children were not more likely to be infected. However, the size of the group of people who spent time together (within 1.5 metres) proved to be an important predictor of infection. Antibodies were found in the blood of 6.2% of the participants who had attended a meeting or gathering (involving more than 20 people) in an indoor environment during the first wave. This was almost 1.5 times more often than people who had not attended meetings at all (4.2%). Other conclusions drawn from the analyses can be read in the scientific article(link is external).