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During the pandemic, doctors and researchers noticed that children and teenagers who were infected with Covid-19 got sick less than adults.

Scientists estimate that children already had a previous level of immunity against Covid-19, provided by memory T-cells generated by common colds.

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Researchers from Sweden’s Karolinska Institutet and the Universities of Bern in Switzerland, Oslo in Norway and Linkoping in Sweden studied blood samples from children taken before the pandemic and identified memory T cells that react to cells infected with SARS-CoV- 2, the virus that causes Covid-19.

The study was based on 48 blood samples from two- and six-year-old children and 94 samples from adults aged 26-83. Blood samples from 58 people who had recently recovered from Covid-19 were also analyzed.

One possible explanation for this immunity in the children is that they had contracted colds caused by one of the four coronaviruses, which cause seasonal symptoms of the common cold. This could stimulate an immune response with T cells, capable of also reacting to cells infected with SARS-CoV-2.

The results also show that memory T cell responses to coronaviruses develop as early as two years of age. “These reactions are particularly strong early in life and become much weaker as we grow older,” explains the study’s lead author, Annika Karlsson, head of the research group at the Department of Laboratory Medicine at the Karolinska Institutet.

“Our findings show how the T cell response develops and changes over time and may guide future monitoring and vaccine development,” he adds.