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How to evaluate the effects of re-containment? How to judge the health situation in France? Between the number of contaminations, the incidence rate, the positivity rate, the hospitalizations: every day we are faced with figures that are not always easy to interpret. Decryption to try to see more clearly.
>> Covid-19 in France: follow the indicators on a daily basis, where is your department located?
1. Number of positive cases per day
“This does not mean that these people were taken within 24 hours”
A traffic jam at the end of October-early November in the reporting of the daily data on the number of contaminations reminded us of the caution with which the data must be read. An IT “incident” identified on October 28 at the level of AP-HP (Assistance Publique – Hôpitaux de Paris), whose servers centralize the data of all laboratories in France, led for a few days to “an underestimation of the indicators”, making impossible to accurately interpret the number of positive cases.
How does it work ? Every day, the French public health agency publishes a series of new confirmed cases of Covid-19 in the past 24 hours but “this does not mean that these people have been sampled in the last 24 hours,” he explains. The results can be recorded a few days late and correspond to laboratory tests several days in advance, therefore to even earlier contaminations. “The time to report the tests in some cases can exceed 9 days”, warns the health agency on the website data.gouv.fr.
This is why the weekly tally of positive tests in the last two weeks published every Thursday by Public Health France leads to different results and allows us to have more perspectives.
Note that for the moment this indicator does not take into account the results of rapid antigen tests, in particular authorized for targeted screening (nursing homes, universities, etc.) as well as for the diagnosis of symptomatic patients without risk factors. According to Public Health France, they should be integrated into the database from 15 November.
In any case, these data reflect only the status of positive tests, a necessarily very incomplete picture of actual contamination, as a large percentage of people with Covid-19 do not develop symptoms and do not necessarily have tests.
2. Positivity and incidence rate
The positive rate measures the percentage of positive cases out of the number of tests. Main advantage: don’t settle for a gross number of new cases, which can increase or decrease because there are more or less tests.
But even here, beware of prejudices. This figure “does not represent the frequency of virus transport in the French population, but transport in the population tested”, which has more “symptomatic people and people in contact with symptoms”, warns epidemiologist Catherine Hill.
The incidence rate provides the number of new cases detected over seven days per 100,000 population. Again, we only measure what we test.
3. Admission to hospital and intensive care unit
To analyze this second wave, data on hospitalizations and resuscitation services allow for a reliable comparison with spring, when there were no tests.
The indicators on admissions and ICU admissions are “a reflection of the epidemic situation” of two weeks ago, Olivier Véran, Minister of Health explained in September. One to two weeks: This is the time between the time you get infected and the symptoms worsen.
“We have a week of incubation, a week of symptoms – at this point we will recover or not – and when we do not recover and go to intensive care, it takes another week,” summarized at the beginning of the week. the infectologist Karine Lacombe.
According to all experts, a drop in hospital admissions for several days can therefore be interpreted as a drop in the circulation of the virus from one to two weeks earlier.
4. The rate of reproduction
The “R effective” is the estimated reproductive rate of the virus, which is the estimated average number of people a patient infects. It is calculated from the number of positive tests or emergency visits or hospitalizations for Covid-19.
If it is greater than 1, it means that the trend is upward in the number of cases. If the R is less than 1, “the epidemic is on the decline”. But as stated by Santé Publique France, it is “an indicator of the dynamics of transmission of the virus approximately 1 to 2 weeks earlier (including the time between contamination and testing and the fact that the calculation is done over a period of 7 days). “.
In its bulletins, the agency invites us not to interpret these data “in isolation”, but to put them in perspective “with the other epidemiological data available”.
5. The number of deaths
Public Health France reports the number of deaths in hospital of patients with Covid-19 every day, adding, on Tuesdays and Fridays, deaths in nursing homes.
These figures do not include home deaths, estimated at nearly 1,900 between March 1 and May 31, according to Inserm’s still provisional figures at the end of August.
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