Kids’ immune systems a protective factor against severe COVID

Morgan Liotta

26/02/2021 2:37:24 PM

New research offers fresh insight into why most children with COVID-19 don’t get as sick adults.

Young children bouncing on trampoline
Various studies have examined the reason behind why children have reduced severity of COVID-19.

Children’s immune system attacks the SARS-CoV-2 virus in a way not seen in adults, according to new research from the Murdoch Children’s Research Institute (MCRI).
Infection in children is characterised by activation of the specialised white blood cell neutrophils that helps heal damaged tissues and resolves infections, rapidly targeting the virus.
This immune response was not replicated among adults in the study.
Lead author of the research paper Dr Melanie Neeland said the study offers new understanding as to why children have reduced severity of the disease compared to adults, and the immune mechanisms behind this protection.
‘Children are less likely to become infected with the virus and up to a third are asymptomatic, which is strikingly different to the higher prevalence and severity observed in children for most other respiratory viruses,’ she said.
‘Understanding the underlying age-related differences in the severity of COVID-19 will provide important insights and opportunities for prevention and treatment, both for COVID-19 and possible future pandemics.’
The study involved an analysis of blood samples from 48 children and 70 adults across 28 Melbourne households infected with, or exposed to, COVID-19.
Immune responses were monitored during the acute phase of infection and up to two months afterwards.
Speaking to newsGP, Dr Neeland said the study showed that children with COVID-19 had a ‘highly activated innate immune system’ during the acute phase of infection.
‘This was characterised by increased activation of inflammatory cells and reduced proportions of first-responder immune cells in the bloodstream, suggesting these cells are migrating to sites of infection to clear the virus before it can really take hold,’ she said.
‘Importantly, the quick and robust innate immune responses were not observed in adults with COVID-19.’
Children and adults who were exposed to, but tested negative for, COVID-19 also had altered immune responses through increased neutrophil numbers weeks after exposure, potentially providing a level of protection, the study found.
These findings build on previous MCRI research conducted on the immune response of a family after prolonged exposure to the virus. It showed that children who had been infected were able to build an immune response that was highly effective at stopping the virus from replicating, meaning they never returned a positive test.
Other studies demonstrate that children of all ages are less likely to become infected with the virus compared to adults, and up to one third of primary-school aged children can be asymptomatic.
A February 2020 New England Journal of Medicine study suggests the only age group spared by severe infection appears to be children aged under 14 years.
But some concerns remain about how high the risk is of children spreading the virus to adults, which Dr Neeland said will require further research.
‘Globally, studies have suggested that younger children may be less likely to transmit the virus in the household, although exact transmission dynamics in children of different ages remain unclear,’ she said.
In a December 2020 comprehensive review by Head of Infectious Diseases Research at MCRI Professor Nigel Curtis, age-related COVID severity factors were examined.
Key factors determining the differences in severity are those that put adults at higher risk and those that protect children.
newsGP previously reported on risk factors for adults, and Professor Curtis’ review found that for children, factors that might protect them include:

  • differences in innate and adaptive immunity
  • more frequent recurrent and concurrent infections
  • pre-existing immunity to coronaviruses
  • differences in microbiota
  • higher levels of melatonin
  • protective off-target effects of live vaccines
  • lower intensity of exposure to SARS-CoV-2.
The study adds that a number of factors have been suggested to explain the difference between COVID-19 severity in children and adults, and can be categorised into those that put adults at higher risk and those that protect children.
‘Children certainly can transmit the disease, but they are not the major drivers of transmission,’ Professor Curtis tweeted.
With most of Australia awaiting how the vaccine rollout will indicate immune response, clinical trials are currently underway assessing the efficacy of COVID-19 vaccines in children, Dr Neeland said.
‘Results from these trials are highly anticipated and will inform vaccine strategies for children,’ she said.
‘We are interested to explore whether the immune response we see in children during natural infection is also reflected in a vaccination setting.’
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