First part of the study: 55 families
The study is conducted in families in which one family member has tested positive for COVID-19. These families participate in the study in cooperation with the Municipal Public Health Service (GGD) in Utrecht. For the first part of the study, a total of 55 households took part, involving 242 participants, including 187 family members of an infected person. In total, the study involved 126 adults, 46 children between 12 and 18 years old, and 70 children between the ages of 1 and 11 years old. The first family was visited on 23 March, and the last family was visited on 24 May. When the first part of the study started, testing primarily involved adults who were tested via the GGD test lanes. People who tested positive for COVID-19 were contacted by the GGD to take part in the study. After that, their family members were also tested.
The testing policy has been changed since then. All children are now advised to get tested in a GGD test lane. In the second part of the study, the plan is to contact families in which a child who is under 18 years old was the first person who tested positive for COVID-19. After that, the family members will also be tested.
Structure of the study
Once there is confirmation that someone is infected, a nurse visits the family as soon as possible. The nurse then collects nose, throat, saliva and blood samples. A questionnaire is filled in, together with the family. This questionnaire includes questions about symptoms, any travel to other countries, and any underlying health conditions. The family then keeps track of their symptoms and collects stool samples for a month. If a family member becomes ill, we take nose and throat samples again and check if this person also has COVID-19. We take samples from the whole family two more times: 2-3 weeks and 4-6 weeks after the first home visit.
Processing the results
A research period lasts about 6 weeks for each family. The family members keep track of their symptoms for a month, and we collect samples from them several times over the course of 6 weeks. For that reason, the results will be processed six weeks after all the families have had their first home visit.
Looking at the seasonal flu (influenza), we often see younger children easily passing the virus to each other or to adults. This appears to occur far less frequently for the coronavirus SARS-CoV-2; younger children seem to pass the virus on less easily to other children and to adults, compared to the transmission rate for older children and adults. In order to learn more about this, RIVM will be conducting a second family study in cooperation with UMC Utrecht, looking at families in which a child was the first infected person in the family. New families will be participating in the second part of this study. It is not possible to sign up for the study yourself. More information about the second part will be provided shortly.
The findings from the study are in line with the results of research being done in other countries, including studies in China and Australia. Following peer review, RIVM published the results in the Nederlands Tijdschrift voor Geneeskunde (NTVG), the leading medical journal in the Netherlands. Read the article in NTVG: 'De rol van kinderen in de transmissie van SARS-CoV-2' (in Dutch).
All data from the first part of the study has been compiled by now, and has been subjected to more extensive analysis. These findings have also been made available in English. The English article is currently undergoing peer review, but is already available online. Read this preprint here: ‘High infection attack rates of SARS-CoV-2 in Dutch households revealed by dense sampling’.