Lead author Professor Tri Phan said children have two types of immunity.
The first is innate (natural).
This first line of defence, comprising physical barriers such as skin and mucosal surfaces, is usually 'very strong' in children and produces chemicals called interferon to kill bacteria and viruses.
Prof Phan said it is one of the reasons most children with no underlying medical conditions report mild symptoms and recover faster from COVID-19 infection than adults.
He said innate immunity, however, wanes with age.
Professor Tri Phan. Credit: Garvan Institute of Medical Research
B cells produce antibodies, and T cells kill virus-infected cells prohibiting the virus from replicating inside the human body.
"The innate immune system in kids is so fast and powerful that it gets rid of the virus quickly. It doesn't allow the B and T cells the time to develop adaptive memory," Prof Phan told SBS.
When children are next exposed to COVID, their bodies don't remember the virus and treat it as a new threatLead author Professor Tri Phan
"Children are at risk of getting sick when they become reinfected. This is why we think it's important to vaccinate children," Prof Phan said.
Reinfection risk
Professor Brendan McMullan, a paediatric infectious diseases specialist and microbiologist, said newer variants can also reinfect children.
"Vaccination is available for everyone aged five and older, as well as for children aged six months to less than five years who are at greater risk of severe disease," Prof McMullan said.
Department of Health data showed more than 2.2 million Australian children, including nearly half a million aged five and under, reported coronavirus infection, and 24 died since the pandemic started.
However, the Australian government doesn't compile reinfection data.
But it said the risk of reinfection depends on several factors such as age, previous infection, variant, an individual's immunity and vaccination.
"Some studies have shown that children have a lower risk of SARS-CoV-2 reinfection than adults."
"However, further research is needed on COVID-19 reinfection in different age groups, including determining the impact of waning immunity and the effect of different circulating strains/waves of transmission."
A new study published in JAMA Network Open showed COVID-19 was the top cause of death in children from an infectious disease between August 2021 and July 2022 in the US.
"COVID-19 was the underlying cause of death for more than 940,000 people in the US, including over 1,300 deaths among children and young people aged 0–19 years," said.
Complacency
Prof Phan says the notion that children get mild symptoms and recover faster from COVID-19 has made parents complacent.
The latest data from the department showed only 51 per cent of the children aged five to 15 have received two vaccine doses.
Prof Phan said vaccinating children is a good idea as studies show hybrid immunity (from infection and vaccination) provides better protection from severe disease and hospitalisation against COVID.
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