Why are the COVID-19 epidemiological data on children difficult to interpret?
Text updated on 2020-10-30
Available data on the role of children in the COVID-19 epidemic are difficult to interpret because children are often asymptomatic and probably have little involvement in the early stages of the epidemic.
Recent studies suggest that children are as contaminating as adults, while older studies indicate that they are 2-3 times less contaminating. See the question Children's role in the COVID-19 epidemic: what do we know? Why such differences? Because the interpretation of epidemiological data on children is difficult because of two phenomena.
First, the importance of children in spreading the virus in populations changes with time and context. Most epidemiological studies of children are not representative of situations where the virus is actively circulating in the general population and schools are open. This is because they were conducted when schools were closed (confinement, holidays). Moreover, they correspond to the first phase of the epidemic, when the virus was primarily carried by adults, who travel more than children and on average interact more with other adults than with children. See the question Children's role in the COVID-19 epidemic: what do we know?
Second, several factors lead to an underestimation of the number of infected children.
- One of the main reasons for getting tested is if you have symptoms of COVID-19. However, children are often asymptomatic (see the question What are the symptoms of COVID-19 in children?). Infected children are therefore tested less often, resulting in a lower rate of positivity in children than in adults.
- In some countries, tests are for adults only, or there is little testing of young children (less than 10 years old), partly due to the unpleasant nature of the nasopharyngeal swab. This also contributes to low detection of potentially positive children.
- When an adult tests positive, a child who is around him or her will be tested as a contact case. However, it is possible that in some cases the child may have caught the coronavirus first, remained asymptomatic and passed it on to the adult. If the PCR test is performed late on a child who has cleared the virus in the meantime, the child will appear negative.
Thirdly, the rate of contamination of children in family homes does not reflect the rate of contamination of children when they go to school. Five studies conducted in China and Israel, during periods of confinement and holidays, compared the risk of being infected with the new coronavirus for children and adults in the same household. These studies concluded that among the contacts of the contagious person, children are two to three times less often infected (tested positive at SARS-CoV-2) than adults. However, it must be taken into consideration that within a household, adults in a couple are more likely to infect each other than other household members, including children, for obvious reasons. Only one of these studies took this factor into account and excluded spouses. Again, the risk of infection for children was lower than for adults. Therefore, outside the school setting, children may be less likely than adults to get COVID. When children go to school, their social interactions are more numerous, which may increase their risk of infection.
Fourth, rates of contamination by children may be underestimated. As explained above, cases of coronavirus transmission within households may be incorrectly attributed to an adult when they are actually transmitted by a child. This then leads to an underestimation of the rate of contamination by children.
In conclusion, some studies show that children transmit as much as adults while others indicate that they transmit 2-3 times less. However, as discussed above, the number of contaminated and contaminating children may be underestimated for many reasons. There is no consensus yet on this issue. On the other hand, there is no doubt that children can catch the coronavirus SARS-CoV-2 and transmit it to others, whether they are children or adults.
Recommendations from the Norwegian Institute of Public Health indicating that adults are given priority for testing over children.Norvegian Institute of Public Health. Test criteria for coronavirus. Updated 23.09.2020.
Study of blood samples from ambulatory care in Sweden, where most schools remained open. Seroprevalence (the presence of antibodies indicating a past COVID-19 infection) of youth aged 0 to 19 increased from April to early June 2020 to reach values similar to those of adults aged 20 to 64 in mid-June 2020 (6.8% versus 6.4%). Seroprevalence remained lower for people aged ≥ 65 years (1.5%), who were more isolated than others.Public Health Agency of Sweden. Påvisning av anti-kroppar efter genomgången covid-19 i blodprov från öppenvården (delrapport 1).
Extremely detailed Israeli study of children between January and September 2020, before and after the reopening of schools. 677,982 RT-qPCR tests were conducted on children in Israel between January 27 and September 24, 2020. Of these, 8% (55,288) were positive. In comparison, 157,229 out of 2,548,273 (6%) were positive in adults. The National Serological Survey conducted by the Ministry of Health detected a positive rate in children of 7% compared to 2-5% in adult age groups. It is estimated that approximately 50-70% of children infected with SARS-CoV-2 are asymptomatic. The opening of schools is associated with an increase in the rate of positive children and their closure with a decrease. Excluding transmissions caused by adults, children tend to be infected by other children in their own age group. 7 children each infected 10 people, 3 each infected 12 people, and one child infected 24 persons.Israel, Ministry of Health, COVID-19 Report, 18 October 2020.
In a seroprevalence study of more than 61,000 people in Spain between April and May 2020, 3.4% of children and adolescents had antibodies to SARS-CoV-2 according to a "point-of-care" test, compared to 4.4% to 6.0% of adults. In a subset of almost 52,000 people who had undergone an immunological test, the gap narrowed to 3.8% compared to 4.5% to 5.0%. This study was conducted while schools were closed in Spain since March 2020. The children therefore had less social interaction than when they went to school.Pollán, M., Pérez-Gómez, B., Pastor-Barriuso, R., Oteo, J., Hernán, M. A., Pérez-Olmeda, M., ... & Molina, M. (2020). Prevalence of SARS-CoV-2 in Spain (ENE-COVID): a nationwide, population-based seroepidemiological study. The Lancet, 396(10250), 535-544.
Analysis of 637 households in Bnei Brak, Israel, of which at least one member tested positive for the coronavirus SARS-CoV-2 by PCR, and all other members were subsequently tested. Children are half as infected (tested positive at SARS-CoV-2) as adults. The joint analysis of PCR and serological data shows that the PCR test does not detect all infections (because not at the right time) and that the underestimation is greater in children. However, the differences in detection rates are not sufficient to explain the differences in PCR positivity rates in the two age groups.Dattner, I., Goldberg, Y., Katriel, G., Yaari, R., Gal, N., Miron, Y., ... & Huppert, A. (2020). The role of children in the spread of COVID-19Using household data from Bnei Brak, Israel, to estimate the relative susceptibility and infectivity of children. medRxiv.
Nice review article dated from July 28, 2020 on the transmission of coronavirus by children. This article summarizes in a table all the studies comparing the rates of contamination of children and adults in family homes. These studies show that among the contact cases of the contagious person, children are two to three times less infected (tested positive at SARS-CoV-2) than adults. The authors consider that comparisons of attack rates between age groups within children are not biased. But there is the same risk of underestimation: younger children have fewer symptoms than older children, leading to their lower identification as index cases than older children.Goldstein, E., Lipsitch, M., & Cevik, M. (2020). On the effect of age on the transmission of SARS-CoV-2 in households, schools and the community. medRxiv.
Analysis of 105 index patients and their 392 contact cases within the household. The secondary attack rate in children is 4% compared to 17% in adults. The secondary attack rate of household contacts with index patients quarantined by themselves since the onset of symptoms is 0%. The secondary attack rate for contacts who are spouses of index cases is 28% versus 17% for other adult household members.Li, W., Zhang, B., Lu, J., Liu, S., Chang, Z., Cao, P., ... & Chen, J. (2020). The characteristics of household transmission of COVID-19. Clinical Infectious Diseases.
Analysis of 2,812 index cases and their 6,690 contact cases in the province of Trento in Italy in March-April 2020, at a time when schools were closed. The contacts of the 14 children under 15 years of age included in this study were found to be more likely to be infected than those of adults (attack rate of 22.4% versus 10.6-17.1%).Fateh-Moghadam P, Battisti L, Molinaro S, et al. Contact tracing during phase I of the COVID-19 pandemic in the province of Trento, Italy: key findings and recommendations [preprint]. medRxiv 2020; https://doi.org/10.1101/2020.07.16.20127357