Exposure to a common seasonal coronavirus stimulates a memory T cell response that protects children against COVID-19. However, this immune response peaks at age 6.
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A team of researchers from the Karolinska Institutet in Sweden wanted to determine whether contracting OC43, 1 of the 4 coronaviruses responsible for seasonal colds, stimulates a T cell response that protects children against COVID-19.
The investigators assessed humoral and cellular immune responses against OC43 and COVID-19 in children and adults who had never been exposed to COVID-19. They collected 48 blood samples from 2- and 6-year-old children, and 94 samples from adults aged 26-83 years. As a control, the study also included blood samples from 58 individuals who had recently recovered from COVID-19.
The investigators found preexisting COVID-19 reactive CD4+ T cell responses targeting spike, nucleocapsid, and membrane were closely linked to the frequency of OC43-specific memory CD4+ T cells in childhood.
This is evidence that functional cross-reactive memory CD4+ T cell immunity against COVID-19 is established during early childhood, similar to early seroconversion with seasonal human coronavirus OC43.
The study authors wrote that compared to other viruses, the high OC43 seroprevalence at age 2 suggests memory responses to coronaviruses develop at a young age. However, unlike other viruses, these OC43-specific antibodies did not increase further with age. “These reactions are especially strong early in life and grow much weaker as we get older,” said Annika Karlsson, a study author and research group leader at the Karolinska Institutet Department of Laboratory Medicine.
The robustness of the memory CD4+ T cell responses peaked at age 6 and then declined with age. The investigators concluded that these age-dependent qualitative differences in preexisting COVID-19-reactive T cell responses may reflect an individual’s ability to control coronavirus infections and mount an immune response to vaccination.
