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years who were BCG-vaccinated in childhood as compared to non-BCGvaccinated. Data on the effect of BCG vaccination on COVID-19 disease severity are unavailable (Hamiel 2020). In the adaptive response to any virus, cytotoxic T cells play an important role in regulating responses to viral infections and control of viral replication. Children could benefit from the fact that the cytotoxic effector function of CD8 T cells in viral infection in children may be less detrimental compared to adults. Immune dysregulation with exhaustion of T cells has been reported in adults with COVID-19 infection. Regarding humoral immunity, IgG maternal antibodies are actively transferred to the child via placenta and/or IgA via breast milk. They may not include anti CoV-2 antibodies, if the mother is naïve to CoV-2 or infected late in pregnancy. In mothers with COVID-19 pneumonia serum and throat swabs of their newborns were negative for CoV-2 but virus-specific IgG antibodies were detected (Zeng H 2020). Thus, neonates may benefit from placental transmission of virus-specific antibodies from pre-exposed mothers. As shown in SARS CoV-1 it is likely that in SARS-CoV-2 a newly infected child will mount a significant humoral response with neutralizing IgM (within days) and IgG antibodies (within 1-3 weeks) to one of the immunodominant epitopes, e.g. the crown-like spike proteins giving the coronaviruses their name. Infections with non-SARS COV are very common in children (see above); however, to what extent previous infections with nonSARS coronaviruses may have led to protective cross-reactive antibodies is unclear. Data regarding IgG and IgM seroprevalence and quality of the immune response in children are lacking. No human re-infections with CoV-2 have been demonstrated yet but overall it is not clear whether children mount a durable memory immune response to CoV-2. In summary, differences in the immune system such as more efficient innate and adaptive immunity to COV-2 (associated with better thymic function), cross-reactive immunity to common cold coronaviruses and differences in the ACE2 receptor expression as well as better overall health may be factors leading to a better COVID-19 outcome in children (Consiglio 2020).
Transmission Studies on the risk of acquiring SARS-CoV-2 infection in children in comparison to adults have shown contradicting results (Mehta 2020, Gudbjartsson 2020, Bi 2020). The exact role that children play in the transmission of SARSCoV-2 is not yet fully understood. Population based studies performed so far indicate that children might not play a major factor in the spreading of COVID-19 (Gudbjartsson 2020). Kamps – Hoffmann
