Higher viral loads were discovered in the nasopharynx of asymptomatic children. ST PHOTO: MARK CHEONG
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 KKH's Infectious Disease Service, said that this trend could be due to the lower expression of the angiotensin-converting enzyme 2 (ACE2) receptors - receptors that the Sars-CoV-2 virus which causes Covid-19 uses for host entry - in the respiratory airways of younger children, making them more resistant to infection.
ACE2 receptors are found in various parts of the body, including respiratory airways, which are used as entry points by Sars-CoV-2.
Other possible reasons could include differences in children's immune responses against the virus compared with adults.
Around 4 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 53 per cent in children from ages five to nine and 64 per cent in the 10 to 16 age group.
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.
The studies looked at how they got the virus, their symptoms, viral load and how they were tested. Here are the five main findings.
1 Children under five at lowest risk of being infected by adults
Of the 137 households surveyed with adults who contracted Covid-19, the adult-to-child transmission rate was at 6.1 per cent.
For children below the age of five, the adult-to-child transmission rate was the lowest at 1.3 per cent, meaning this group was least likely to be infected by adult Covid-19 patients.
2 No evidence of young children infecting others with Covid-19 in pre-schools
According to a review conducted on children in three schools (two pre-schools and one secondary school), there was no evidence of Covid-19 transmission among children.
3 Nearly four in 10 of the children who tested positive for Covid-19 were asymptomatic
About 61.5 per cent of children with Covid-19 had symptoms, according to a study conducted on 39 children admitted to KKH between January and May.
The highest proportion of symptomatic cases (75 per cent) were found in children from birth to four years old.
4 Higher viral loads were more likely in symptomatic children
In a study conducted among 17 children, 10 were symptomatic, displaying mild illness with upper respiratory tract infection.
They did not have any symptoms by day five or have any complications.
The remaining seven did not develop any symptoms.
5 Buccal swabs and saliva testing, though non-invasive, were not useful in testing for Covid-19 in children
Buccal swabs, where DNA is collected from the inside of a person's cheek, contained lower viral loads compared with nasopharyngeal swabs.
Similarly, saliva testing also found poorer sensitivity and lower viral loads when used for testing on children.
Higher viral loads were discovered in symptomatic children, indicating the possibility of higher transmissibility.
However, both asymptomatic and symptomatic children 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. Their disease was mild throughout and they 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 - tested negative for Covid-19.
In contrast, when an adult staff member tested positive for Covid-19 at the second pre-school, 16 staff and 11 cases from their households tested positive.
None of the 77 children identified tested positive.
Dr Yung said that the impact of Covid-19 on children ranged from asymptomatic to moderate rather than severe, but that the community transmission among children has remained largely unknown.