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Kids under five at lowest risk of getting Covid-19 from adults

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According to KKH study, adult-to-child transmission rate among 137 households was at 6.1%

Children under the age of five are at the lowest risk of being infected by Covid-19 from adults, as they may be more resistant to the virus, according to a household transmission study conducted by the KK Women's and Children's Hospital (KKH).

According to the study results released yesterday, the adult-to-child transmission rate among 137 households was at 6.1 per cent.

The risk of secondary infection in children was also higher if the index Covid-19 patient was the child's mother.

The youngest group of infected children - between zero and four years old - had the lowest adult-to-child transmission rate of 1.3 per cent, compared with 8.1 per cent for older children between five and nine years old, and 9.8 per cent for those between 10 and 16 years old.

Dr Yung Chee Fu, consultant at the KKH's Infectious Disease Service, said this trend could be because of the lower expression of the angiotensin-converting enzyme 2 (ACE2) receptors - the receptor that the Sars-CoV-2 virus uses for host entry - in the nasal epithelium of younger children, making them more resistant to the Sars-CoV-2 infection.

ACE2 receptors are found in various parts of the body including the respiratory airways, which are used as entry points by the Sars-CoV-2 virus.

Other possible reasons could include differences in children's immune responses against the virus compared with adults.

Around four in 10 children asymptomatic

A separate study of 39 children infected with Covid-19 between January and May found that 38.5 per cent of the children remained asymptomatic.

There was a larger proportion of younger children who were symptomatic compared with older ones, with 75 per cent of children from ages zero to four exhibiting symptoms compared with children aged five to 16.

Common symptoms included low-grade fever, runny nose, sore throat, diarrhoea, and loss of smell or taste.

None of the symptomatic children reported shortness of breath or developed signs similar to Kawasaki disease.

Higher viral loads were discovered in the nasopharynx of symptomatic children, indicating the possibility of higher transmissibility.

However, both asymptomatic and symptomatic children had experienced peak viral loads around day two to three of their diagnosis, suggesting viral shedding and transmission in the pre-symptomatic phase.

All 39 children surveyed stayed in the hospital for an average of 15 days.

They had a mild disease course and have already been discharged.

Young children may not be primary drivers of Covid-19 transmission in pre-schools

The screening of children from three potential transmissions of Covid-19 incidents in schools - a secondary school and two pre-schools - did not detect any evidence of Sars-CoV-2 transmission among children.

Close contacts of two Covid-19 cases - a five-year-old pre-school child and a 12-year-old secondary school student had tested negative for Covid-19.

In contrast, when an adult staff member tested positive for Covid-19 at the second pre-school, 16 staff members and 11 cases from their households tested positive.

None of the 77 children identified had tested positive.

Dr Yung noted the impact of Covid-19 on children ranged from "asymptomatic to moderate rather than severe", but the "paediatric role in community transmission has remained largely unknown".