Measles is caused by measles virus (MeV). The disease is associated with lymphopenia and immune suppression, which is an important cause of measles-associated morbidity and mortality. Measles-induced immune suppression can last several years, whereas measles lymphopenia is usually resolved within two weeks. At the same time, measles induces lifelong immunity. This apparent contradiction, known as the 'measles paradox', was partially solved when investigators demonstrated that MeV infects and depletes pre-existing memory cells, thereby causing 'immune amnesia'. This model is supported by observations in animal models and clinical studies, but several questions remain to be addressed, like the duration of measles-induced amnesia and changes in the immune repertoire after measles. to address the immunological questions regarding MeV infection.

Measles is caused by measles virus (MeV). The disease is associated with lymphopenia and immune suppression, which is an important cause of measles-associated morbidity and mortality. Measles-induced immune suppression can last several years, whereas measles lymphopenia is usually resolved within two weeks. At the same time, measles induces lifelong immunity. This apparent contradiction, known as the 'measles paradox', was partially solved when investigators demonstrated that MeV infects and depletes pre-existing memory cells, thereby causing 'immune amnesia'. This model is supported by observations in animal models and clinical studies, but several questions remain to be addressed, like the duration of measles-induced amnesia and changes in the immune repertoire after measles. Recently, investigators have acquired permission to address these remaining questions in 18-25 years old adults (MEC-2024-0230). However, investigators have reservations about the feasibility of including enough participants between 18 and 25 years old that have not been vaccinated against or infected with MeV; it is possible that investigators will not reach sufficient inclusions to address the immunological questions regarding MeV infection in that protocol. Therefore, investigators propose to additionally study these questions in children in the age of 4 up to and including 17.

Inclusion Criteria:

Group A

• Aged 4 - 17 years old
• Susceptible to measles
• No pre-existing immunity against measles (vaccination or earlier infection)

Group B

• Aged 4 - 17 years old
• Protected against measles due to vaccination or earlier infection

Exclusion Criteria:

A potential subject who meets any of the following criteria will be excluded from participation in this study:

• Diagnosed chronic disease that lasted over 3 months
• Immune suppression (due to medication or underlying disease)
• Group A; Detectable MeV-antibodies in the T1 blood sample

Children in group A will be from 4 up to and including 17 years of age and include children who are not protected against measles. Families willing to participate will self-identify them to the researchers, with the help of schools, Municipal Health Services and regional general practitioners.

Children in group B will be from 4 up to and including 17 years of age and have received one or two MMR vaccinations, depending on their age. The children will be recruited from the same geographical regions with low vaccine coverage as the children in group A, for example classmates.