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New research has shown children have a strong initial immune reaction to COVID-19 but are vulnerable to reinfection, highlighting the importance of vaccination.

The study, conducted by the with input from Sydney Children’s Hospitals ÐÇ¿ÕÌåÓý (SCHN), found that while children’s immune systems are primed to defeat the virus quickly, this does not transfer into long-lasting memory T cells.

So when children are next exposed to SARS-CoV-2, their body still treats it as a new threat.

 Associate Professor Philip Britton, Infectious Diseases Paediatrician at SCHN, was the clinical lead in the study. 

He said infants� immune systems are still naïve due to lack of exposure to viruses but as they move into adulthood, their cells develop an adaptive memory to protect them against those they’ve previously encountered.

“Over time as you get infections, your immune system becomes more ‘educated,� allowing you to make a faster immune response that’s tightly matched to the viruses that have infected you before,� A/Prof Britton said.

“Children’s immune systems move from relying mostly in the innate system (physical barriers that block viruses from entering the body) to needing the adaptive system (comprising of T and B cells) as a backup as they grow older and are unable to clear viruses as rapidly.�

The study examined T cells and cellular immune responses of children and their household family contacts who contracted COVID-19 infection, with asymptomatic or mild disease.

This meant researchers could control for the impact of genetic or environmental influences on the immune reaction, as well as compare the same severity of disease.

They then sequenced the white blood cell samples to analyse T cells in the cohort at the time of acute infection and one month later.

The results showed that children had various naïve T cells to fight the infection and weren’t generating sufficient T cell memory after they had recovered, meaning they are at risk of reinfection.

“The price children pay for being so good at getting rid of the virus in the first place is that they don’t have the opportunity to develop ‘adaptive� memory to protect them the second time they are exposed to the virus,� Professor Tri Phan, lead author and Co-Lead of the Precision Immunology Program at Garvan, said.

“[As] their immune system is still ‘naïve' and they don’t develop memory T cells, they are at risk of getting sick when they become reinfected. With each new infectious episode as they get older, there is a risk of their T cells becoming ‘exhausted� and ineffective, like the T cells in older people.

This is why we think it’s important to vaccinate children."

Adults, on the other hand, had few naïve T cells to fight the infection but developed better memory T cell responses. This suggests why older adults can have an immune over-reaction to COVID-19.

“When adults are infected for the first time with SARS-CoV-2, their memory T cells recognise only what they’ve seen before � like a familiar part of the coronavirus that is shared with the common cold coronaviruses,� Prof Phan said.

“This may lock the immune system in to a misdirected response that is not specific to SARS-CoV-2. It provides an opportunity for the virus to escape and multiply unchecked to cause more severe symptoms as the immune system ramps up to try and fix the problem.�

The was published in the January 2023 issue of the Clinical Immunology journal.