The authors have no funding or conflicts of interest to disclose. The emergence of the novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in late December 2019, causing an illness later named COVID-19, marked the beginning of an unprecedented pandemic. By early 2022, the number of confirmed cases exceeded 265 million worldwide, with >5 million deaths.
The emergence of more transmissible SARS-CoV-2 variants of concern (VOCs), such as alpha, delta and omicron, pose additional risks to global public health.
The vast majority of severe cases and deaths occur in older adults and high-risk groups with underlying health conditions, while children and young people (NJ) predominantly have a mild course of the disease, with approximately one third of cases who are asymptomatic.3,4
Data from 10 European Union countries from August 2020 to October 2021, a time when the alpha variant was dominant in most European countries, revealed that hospitalization was required in the 1-18 age group. in <1.2% of all reported cases. Less than 0.04% of NJs developed severe disease and the overall mortality rate was equal to or less than 0.01%. On the contrary, NJs suffered greatly from mitigation measures during the pandemic such as distancing and the closure of schools or daycares.
Children and adolescents are at increased risk of developing a rare but serious condition called multisystem inflammatory syndrome in children (MIS-N), typically within 6 weeks after infection with SARS-CoV-2.5
Similar to adults, although apparently at lower rates, children can develop long-term sequelae, often summarized under the term "long COVID", an ill-defined condition, which does not appear to correlate with the initial severity of the disease. , and can lead to significant morbidity.
Even 2 years into the pandemic, the extent to which NJs become infected and can transmit SARS-CoV-2 remains uncertain. However, it appears that the likelihood of transmitting SARS-CoV-2 may at least in part be related to the reduced susceptibility of this specific age group.
| Susceptibility of children to SARS-COV-2 infection |
Before the circulation of VOCs, a meta-analysis that included 77,758 people with reported household transmission found that only 17% of exposed NJs under the age of 18 became infected compared to 28% of exposed adults.6
A more recent study from Israel looking at 15 household groups during the dominance of the more virulent alpha strain found that secondary household transmission in exposed children and adolescents was around 70% compared to 90% in adults.7
A meta-analysis comparing a total of 40,000 children with 250,000 adults suggests that those under 14 years of age have approximately 50-60% lower risk of infection than older age groups. Interestingly, the same analysis found that over 14 years of age the risk of infection was similar to older age groups.8
Still, much of that data was collected at a time when many countries implemented mitigation measures such as distancing and closing schools and daycares. The underreporting of mild or asymptomatic pediatric cases and the circulation of less transmissible virus variants at the time some of these studies were conducted may partly explain these findings.
Unlike previous studies conducted at the beginning of the pandemic, the most recent data suggest that the probability of children becoming infected is similar to that of adults, especially since the emergence of the most virulent VOCs.9
However, children appear to be less likely to develop symptomatic COVID-19. To explain this finding, several possible factors have been hypothesized in the literature. Among others, age-specific differences in immune response, divergent expression of angiotensin-converting enzyme 2 (ACE-2 receptor), lower prevalence of comorbidities that predispose to COVID-19, and differences in social behavior have been proposed. .5
| Infectivity of children |
Compared to the lower risk of developing severe illness or symptoms, the question of whether NJs are naturally less infectious than older age groups, and their potential to drive transmission of SARS-CoV-2, is much more controversially discussed. since the beginning of this pandemic.
Although PCR-based studies comparing nasopharyngeal viral loads of NJ and adults reveal conflicting results, some authors have emphasized that even with comparable viral loads, asymptomatic individuals may be less likely to transmit SARS-CoV-2 as a result. early elimination of the virus and lower production and emission of infectious aerosols.10
A recent meta-analysis including results from 3,385 individuals showed that compared to symptomatic COVID-19 patients, asymptomatic people experience a 55% shorter viral RNA clearance time. Furthermore, subgroup analysis from the same review revealed a significantly shorter clearance in individuals under 18 years of age compared to adults.11 Additionally, there is evidence that asymptomatic people can neutralize the virus more quickly and have 42% less likelihood of spreading SARS-CoV-2.6,11
Since the start of the pandemic, findings from studies focused on transmission have varied widely, especially when focusing on NJ. Given the generally less severe course of COVID-19 in children and a large proportion of asymptomatic cases, SARS-CoV-2 is likely to be substantially under-reported in this age group.2
However, it is difficult to generate high-quality evidence since PCR-based studies cannot make firm assumptions about whether RNA fragments detected in a sample reflect replication-competent particles.
Furthermore, viral loads in nasopharyngeal samples appear to fluctuate during the course of infection and do not necessarily reflect infectivity. Poor understanding of the antibody response to SARS-CoV-2 infection, especially in oligo or asymptomatic patients, complicates the interpretation of seroprevalence studies.11
Children and young people represent a very heterogeneous group in terms of basic physiology and functioning of the immune system.2
To date, published data on the role of children in the transmission of SARS-CoV-2 show highly variable transmission rates, possibly due to variations in the prevalence of SARS-CoV-2 in the community and the implementation of mitigation measures. at the time of the study, differing strategies for the detection of secondary cases and variable compliance with infection control measures in homes and daycares.2 In addition, the prevalence of different VOCs and their impact on transmission must also be taken into account. .12
| Home transmission |
A Canadian study investigating 6,280 pediatric index cases in homes between June and December 2020 found that children under 3 years of age were least likely to be an index case, while siblings and caregivers for the same group showed the highest risk. of secondary infection. On the contrary, the risk of being the index case of SARS-CoV-2 in the household increased with age.13
Compared to alpha dominance time, the domestic secondary attack rate with the delta VOC increases across all age groups by 70%. Even then, individuals under 30 years of age infected fewer household members than all other age groups.12
From the study of other respiratory tract viruses such as respiratory syncytial virus (RSV), it is widely accepted that proximity and exposure time have a positive relationship with the probability of infection.14 Although young children have a lower volume of expired air , parents and other household members will likely remain close to young children to provide care and comfort.
Successfully implementing mitigation measures such as mask wearing or hand hygiene can be challenging for parents when caring for a sick child, especially toward the younger age spectrum.13,14
In older children and adolescents, it is easier to implement mitigation measures. However, beginning in adolescence, individuals develop lung capacities similar to those of adults and, more importantly, have a greater range of interaction outside the home in which they live, potentially resulting in a more prominent role in the spread. of the transmission.14
| Transmission at school or daycare |
Existing data on transmission in educational settings are very inconsistent. Such data are likely influenced by differences in public health measures implemented at the time of data collection, community transmission rates, vaccine availability and vaccination rates, and circulating virus strains.2
Although transmission of SARS-CoV-2 in schools and daycares has been widely documented, the risk of transmission to and from children in these settings appears to be low, especially before adolescence.
In particular, at times of low transmission rates in the community and, more importantly, when public health measures are implemented in schools, such as improved ventilation, mask wearing and physical distancing.15 Based on first principles, it is likely that once viral activity increases in the community, transmission in schools also increases. However, a large UK study found that most school outbreaks were caused by staff members, rather than school pupils.16
Extracurricular activities such as overnight camps appear to pose a higher risk of transmission, especially when compliance with physical distancing and mask wearing is low.15
With the emergence of the highly transmissible delta variable, schools once again became the center of public and expert debates about their role in driving the pandemic. In particular, an analysis of data from 783 schools in the United States done at a time when the delta strain was dominant that included data from 59,561 students and 11,854 teaching staff in kindergarten through 12th grade found that the risk of transmission within school was lower than the risk of transmission in the community.
The US counties included in this study had an average rate of 47% of citizens fully vaccinated during the study period. In schools, distancing of at least 3 feet was recommended, mask-wearing was mandatory, and quarantine was imposed for close contacts of cases.
Importantly, institutions where suggested public health measures were strictly implemented experienced very low transmission rates in school, even with cases increasing exponentially in the community. However, it should be noted that the secondary attack rate during the period of delta dominance was estimated to be 2.6% compared to 1% when the α strain was dominant.17
| Conclusions |
Children and young people have received much less attention than adults since the start of the pandemic. While NJ shows the lowest disease burden with respect to severity and long-term sequelae, the physical, mental and economic damage to this age group, caused primarily by public health mitigation measures, should not be underestimated.
Although available data on NJ’s role in SARS-CoV-2 transmission is inconsistent, there is no compelling evidence to date, 2 years into the pandemic, that children are the primary drivers of the pandemic.
Existing data support that even with the occurrence of highly transmissible VOCs, educational and child care settings are relatively safe spaces if prevention strategies are followed. This should be taken into account by public health experts and health authorities, especially in light of a global wave of omicron VOCs, when making decisions about mitigation strategies.
