Covid-19: what is hiding the increase in the incidence rate among the youngest?

FIG DATA – More than ever put forward, the statistics relating to cases of Covid-19 in children under 10 years old should be interpreted with caution.

Since the start of the 2020 school year, the youngest had been relatively spared the material consequences of the Covid-19 health crisis. They are now singled out. In particular, the responsibility of the school in the transmission of the virus.

Faced with pressure from the teaching staff and part of the public, Jean-Michel Blanquer announced on Friday March 26 the systematic closure of classes as soon as a certified case appears in the confined departments.

Valérie Pécresse, the president of the Île-de-France region, had previously suggested bringing the school holidays forward by two weeks in the region.

The evolution of the incidence rate among the youngest shows a significant increase. The 10-19 age group is overtaking the 20-29 age group, becoming the most affected age group in terms of proven cases.

Equally worrying, children under 10 saw their incidence rate double in just 20 days. It has passed the milestone of 200 new weekly cases per 100,000 people and is reuniting at the rates observed in other age groups.

A balance sheet to measure in this specific case. This is primarily due to the nature of the incidence rate, which is difficult to interpret by itself under certain conditions.

To have a fair view, it is necessary to consult the related statistics. Starting with the positivity rate.
If the incidence rate counts the number of new cases reported to a total population. The positivity rate tells us about the ratio of positive people among those tested.

This is part of the problem in the direct interpretation of the incidence rate. The change in the positivity rate among those under 10 does not follow the dynamics common to other age groups, i.e. an upward trend since the beginning of March. Over the same period, the positivity rate of those under 10 fell by 25%, from 8 to 6% of positive cases.

This is the signal for an anomaly. To fully understand what is going on, we also need to look at the weekly screening rate, which is the number of people screened in a week and compared to the total population.

The picture is completed: the youngest are tested 4 times more than they were even a month ago. The increase among 10-19 year olds is also noteworthy. They become the most tested age group.

Thus, the recent increase in the incidence rate among children under 10 is here largely mechanical and caused by the more systematic implementation of tests. Among 10-19 year olds, the situation appears to be significantly different. The weekly screening rate doubled in less than a month, from 3% to 6%. At the same time, the positivity rate has remained stable around 8%. This is in line with the hypothesis of a strong circulation of the virus in this age group.

This does not mean that there is not a health problem in leaving nursery schools and primary schools open, or even that the incidence rate of the youngest is artificially low. Simply, it is dangerous to compare incidence rates over time as test conditions change.

Likewise, comparing these figures by age group when their screening rates are beyond compare can also be misleading.

SEE ALSO – Covid-19: the variants cause an “increase in the severity of the disease in young adults”, worries Olivier Véran


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