Sero-epidemiology COVID-19 in Belgium 2020

version 26Mar2020

Goal of the study

Collecting residual serum samples to detect antibodies against COVID19, see how the proportion of susceptible people in the population evolves over time and how this is distributed over age groups (and gender and region).  In other words, to monitor how many people have been infected over time (cumulatively), and whether the evolution differs across age groups (and gender and region). This is very important information, both for the policy to control the current outbreak and for the prevention / treatment of a possible 2nd or 3rd wave.


2 laboratories that mainly serve outpatient practices, AML (Algemeen Medisch Laboratorium) and Laboratoire Luc Olivier, store residual serum from samples collected for diagnostic purposes (no extra sample is collected). For samples from patients under 20 years of age, we collaborate with partner laboratories (Bruyland, Anacura, Somedi, LBS, Baudouin, Synlab).

Sample collection has started as soon as approval of the ethics committee was obtained (March 30, 2020). Every 3 weeks samples are set aside for 1 week, at least until the end of June 2020. The total number of samples per collection period is, for all labs together, 4000 for the first collection and decreases over the weeks. Collection is stratified: 400 samples per age group of 10 years; the same number of men and women per age group; per age group half of Flanders and the other half of Wallonia and Brussels.

Exclusion of hospital samples and exclusion of duplicates takes place in the laboratories, which also store the samples temporarily until funding and a test are available to determine antibodies against SARS-CoV-2, on behalf of the CEV, UAntwerpen.

Samples are made available unlinked and anonymously, with the only data being: Unique code; Age in years; Collection date; Sex; Postal Code.

Scheduled analysis

Analysis of antibodies against SARS-CoV-2: proportions of seropostive, seronegative or equivalent according to the test used (IgG ELISA, Eurimmune)

Determining overall susceptibility and differences per age group, region and gender by means of suitable methods such as univariate analysis and logistic regression; comparison of these proportions between the different collection periods.


  1. First sample collection: Result based on 3910 samples: 3% of the samples were sero-positive. Seroprevalence was similar across age groups.
  2. Second sample collection: Results based on 3397 samples: 6,2% of the samples were sero-positive. The increase was more pronounced in the 20-29 and 80+ age group.
  3. Third sample collection: Results based on 3253 samples: 6,9% of the samples were sero-positive. This increase is minimal and can be attributed to coincidence variation.