Important factors that determine the child clinical COVID-19 attack ratio

Scientists have reported that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is the virus responsible for the coronavirus disease 2019 (COVID-19), can infect all age groups, including infants and young adults. However, it has been especially difficult to obtain accurate epidemiological estimates of COVID-19 among children.

Study: COVID-19 attack ratio among children critically depends on the time to removal and activity levels. Image Credit: Halfpoint /

Can children transmit COVID-19?

A systematic review that included surveillance studies conducted during the first wave of COVID-19 found that children were mostly asymptomatically infected or exhibited mild symptoms. This review revealed that during the study period, 21.1% of children were asymptomatically infected and 3.8% of children were severely infected by SARS-CoV-2.

The authors pointed out the lack of evidence related to COVID-19 transmissibility in children. Also, the susceptibility of children to contract COVID-19 infection was highly heterogeneous across studies.

Very recently, a study on Norwegian families revealed that young children can transmit SARS-CoV-2, with the rate of transmission being similar to that of adults. The meta-analysis of household surveys indicated that children are less frequently infected and, in the case of contracting the infection, they mostly experience flu-like symptoms.

Researchers have shown that lower secondary attack rates in children are because they are less frequently tested and exposed as compared to adults. This observation has been supported in a recent study that followed a dense sampling method in 55 Dutch households comprising 187 household contacts.

This study calculated a secondary attack rate of 35% among children and 51% among adults. Specifically, in the context of children, this study revealed a very high rate of transmission as compared to the reports of previous studies.

Some studies have found evidence supporting a shorter SARS-CoV-2 shedding period in children as compared to that in adults. Scientists have observed differential antibody kinetics in response to COVID-19 infection between children and adults.

There is a scarcity of reports related to the epidemiological trends associated with SARS-CoV-2 variants and how it affects children. As schools are reopening, it is extremely important to understand the role of children in the transmission of the virus.

Along with contact tracing studies and seroepidemiological surveys, mathematical models would play an important part in the formulation of effective strategies to contain the pandemic.

Some previously published mathematical models have simulated the effects of decreasing children-to-children contacts. Another group of researchers designed an age-structured agent-based simulation model to understand the effects of closing schools in Ontario, Canada on the COVID-19 epidemic curve. This model revealed the importance of isolating asymptomatically infected children.

Scientists found that self-isolation reduced the further spread of the disease. All of the available mathematical models recommended continual implementation of non-pharmaceutical interventions, such as self-isolation and frequent COVID-19 tests.

A mathematical model to estimate attack ratio among children

A new modeling study published on the medRxiv* preprint server aimed to determine the attack ratio among children between the ages of 0-11 years in Ontario, Canada. The authors developed a new methodology based on the analysis of a disease transmission dynamics model to evaluate the age-stratified attack ratio in a population.

In this study, the researchers designed a new algorithm to calculate the attack ratio of COVID-19 among children as a function of different adjustable model parameters such as levels of activity and time to remove from the transmission chain.

In the current study, the researchers made some assumptions, which included that all individuals below the age of 12 years should be half as susceptible to infection by SARS-CoV-2 than those above 12 years. They also assumed that the SARS-CoV-2 Delta strain has completely replaced all other circulating strains. The latter assumption might cause a slight overestimation of the transmission probability per contact, as the Alpha strain is still present in Ontario.

The potential differences in activity levels among children associated with the susceptibility to infection by SARS-CoV-2 and vaccination coverage rates have been considered in this study.  

The researchers stated that the developed algorithm is generic; hence, the attack ratio analysis could be performed in any geographic region. As the Delta strain is known to be highly infectious, a decrease in contact per day from 12.73 to 10, for example, within the vaccine-eligible population in Ontario is necessary. This study indicates that the attack ratio among children depends greatly on how fast infected individuals could be removed from the transmission chain.  


As some COVID-19 outbreaks, such as those that occurred in Israel, have been linked with school communities, the study of the epidemiology of COVID-19 among children is extremely important to gather more knowledge and make an informed decision about closing or reopening schools.

The authors of this study stated that the rate of infection could be reduced with higher vaccine coverage in the vaccine-eligible population, as well as minimal social mixing among children, with frequent COVID-19 testing and isolation.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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