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How do we know how many infectious people there are?

After a person contracts coronavirus, there’s a period during which they can pass it on to others. It’s important to know how many people with the virus are infectious. In this article we explain how we estimate the number of infectious people in the Netherlands.

Woman sneezes into her elbow.

The article is an explanation of the page on Infectious people.

After a person contracts coronavirus, there’s a period during which they can pass it on to others. How long this infectious period is varies from person to person. Although we cannot measure exactly how many people are infectious at any given moment, we can provide a reliable estimate.

Estimating the number of infectious people

The National Institute for Public Health and the Environment (RIVM) determines the number of infectious people on the basis of hospital admissions and data from the Pienter Corona study.

The Pienter Corona study, which is conducted in rounds every two to three months, collects data from a representative sample of the Dutch population – people of all ages from urban and rural areas throughout the country. The test subjects complete a detailed questionnaire and submit a blood sample, which is tested for coronavirus antibodies.

The number of infectious people is estimated using the number of Pienter Corona test subjects in each age group who have antibodies against coronavirus and the number of patients with COVID 19 admitted to hospital up to that time. By combining these data RIVM can estimate, for each age group, the number of people who have been infected with coronavirus for every one COVID 19 patient admitted to hospital. By multiplying this by the average amount of time a person is infectious, RIVM then gets an estimate of the number of infectious people in the population.

Number  of infectious people in the Netherlands

Number of infectious people over time

This number is always a week or two old; more recent data is incomplete and would produce an estimate that is not reliable enough. The Dashboard receives data from RIVM twice a week: on Tuesday and on Friday. This includes figures for each day of the week because the calculations are performed daily.

The number of infectious people changes over time. This is clearly visible in the graph below (you can always view the graph with the most up-to-date figures on the Dashboard). The dark blue line in the graph shows that, for example, on 15 May 2021 there were an estimated 105,032 infectious people. The light blue shading shows the range of uncertainty – the actual number on that date could have been anywhere between 72,434 and 138,286.

Graph with number of infectious people over time

Number of infectious people and reproduction number

The number of infectious people and the reproduction number (R), the average number of people that a single person with the virus infects, are closely related. When R is greater than 1, the number of people getting infected is increasing and the virus is spreading faster. In other words, a high reproduction number drives up the number of infectious people, and vice versa.

For example, if the reproduction number is 1.5 and there are 100 infectious people, these 100 people will infect 150 other people. But 100,000 infectious people will infect 150,000 other people. The numbers add up very quickly.


Read the article ‘What is the reproduction number and why is it always two weeks old?’