Covid-19: To avoid further confinements, contagious people must isolate themselves more quickly

The time elapsed between a person's contact with the SARS-CoV-2 coronavirus and their isolation is a central indicator of the effectiveness of the measures put in place as part of the fight against the Covid-19 epidemic. In our country, this indicator is not currently precisely estimated or monitored, however the surveillance carried out by Public Health France suggests that it is of the order of six to seven days. While this period of time has decreased compared to what it was in the spring, it is still too high and remains higher than that observed in the countries with the best control of the epidemic, such as Hong Kong, Taiwan, New Zealand, or Japan in particular.

SIf we do not manage to move this number of days towards zero, France risks being condemned to the repetition of measures concerning the whole society, such as generalized confinement, as long as antiviral treatments or an effective vaccine are not available. not available.

Here are some ways to meet the challenge of early isolation.

Why isolate rather than let the virus circulate?

First of all, let us remember that in infectious epidemiology, the reproduction number, R, which corresponds to the average number of people infected by each infected person, at a given time, is an essential indicator. Its estimation and monitoring of its evolution over time make it possible to assess the effectiveness of the measures put in place to control the epidemic.

We distinguish the initial reproduction number, denoted R0, corresponding to the number of people infected on average by each infected subject at the time when the pathogen is introduced into a population in which all the individuals are sensitive, without measurement control, the effective reproduction number (Rt), which is calculated after the start of the epidemic. The latter is generally lower than R0, on the one hand because the proportion of unimmunized subjects tends to decrease and on the other hand because of the introduction of preventive interventions. In the spring, when the epidemic began in our country, the R0 of SARS-CoV-2 was estimated at 3, in other words an infected person contaminated 3 others on average.

Lowering the number of reproduction is the goal of any non-pharmaceutical intervention. In the absence of a vaccine or drug treatment to reduce contagiousness, some have suggested decreasing the proportion of susceptible subjects by letting the epidemic spread without strict containment and with minimal interventions, in order to try to achieve "immunity". collective ”. Without going into details, let us specify that this is a very "costly" process for society, not only in financial terms: this would imply, at the level of a country like France, to tolerate the infection of tens of millions of people. With the consequences of a colossal number of cases of illness, hospitalizations and ultimately several hundred thousand deaths. In addition, the saturation of hospitals would lead to many additional deaths, due to the impossibility of treating other pathologies due to the care of Covid-19 patients.

A patient will be evacuated by medical flight to another hospital, at Bron airport near Lyon, on November 16, 2020.Philippe Desmazes / POOL / AFP

This option has been tried in some countries, such as Great Britain (at the very start of the epidemic), some states in the USA, or Sweden. It is now abandoned by most governments, even if Sweden persists in this strategy, in a very specific context of collective responsibility.

An alternative which makes it possible to reduce the number of reproductions of the epidemic is to limit the number of “at risk” contacts.

Reduce contact with contagious people

Barrier gestures such as wearing a mask, washing hands, physical and social distancing, or ventilation of the premises have been implemented to limit the likelihood of contact with the virus. If they are effective, they are not sufficient on their own, as shown by the epidemic resumption observed in the fall of 2020. Confinement allows for its drastically lowering the number of "at risk" contacts that everyone can have, by restricting contact outside the home.

However, only contact with contagious people is dangerous: that a non-contagious person meets zero or a hundred people during the day has no impact on the spread of the virus if all these people are themselves non-contagious ... A less restrictive approach than generalized confinement is therefore conceivable: it consists in reducing the number of contacts of only potentially contagious people, that is to say “contact cases” and confirmed cases for 7 days, by isolating them. Indeed, if an infected person is isolated quickly after their infection, even before becoming contagious, they will not spread the virus, regardless of whether or not they develop symptoms. This is what the triptych “test-tracer-isolate” (TTI) advocated, now unfortunately replaced by “test-alert-protect” (TAP), a more vague and less effective formulation to control the epidemic which does not further emphasizes the most important terms: "trace" and "isolate".

Theoretically, if it were possible to implement a "perfect" isolation of potentially contaminated people, at the scale of the whole of society, that is to say by isolating 100% of contagious people, at the end of the day. two to three weeks (the maximum duration of the illness), there would be no more cases of infection.

In reality, measures to achieve this goal are unlikely to be put in place in a fully effective manner. In addition, there may be imported cases of infection. Under these conditions, the disease will persist, but at low noise. If the few cases that are not immediately isolated are so quickly, limiting the number of risky contacts they may have, the epidemic will remain under control, especially as the rest of society will respect. barrier gestures. This is the current situation in which Australia finds itself, for example.

When to isolate infected people?

The ideal time to isolate a person who has been in contact with the virus, following a dinner with an infected person for example, is immediately after said contact. Indeed, the contagious phase begins about two or three days after meeting with the SARS-CoV-2 coronavirus. By immediately isolating the person who has found themselves in a risk situation, we therefore prevent them from infecting other individuals.

If it turns out that immediate isolation has not been possible, it can also be implemented as early as possible during the contagious phase, before any symptoms develop. But in this case, the person may have infected others before being isolated.

Finally, isolation can be considered after the onset of symptoms (remember that only two-thirds of infected individuals develop them), provided, however, that less than seven days have elapsed since their onset. Beyond this time, it is likely that the person is no longer contagious. Isolation is therefore unnecessary. In a context of a shortage of tests, carrying out a diagnostic test beyond 7 days may therefore be considered unnecessary, or even harmful from the point of view of the health of the community (if it delays the completion of the tests. tests in potentially infected people who are at an earlier stage after contact with the virus).

Minimizing the average time between contact with the virus and isolation is essential to effectively combat the epidemic. It is fundamental to maximize the number of isolated cases in situation n ° 1.
Author provided

It would be central that the distribution of people between situations 1, 2 and 3, and 4, be better measured and monitored, and that every effort be made to shift the highest possible proportion of cases to situation 1 of isolation. before the onset of the contagious period. The evolution of this distribution, which conditions the time between contact and isolation, is a very good indicator of the efficiency of the response of society.

With the equivalent number of newly infected people, there is a huge difference between a community that succeeds in isolating cases before the contagious period and a community where many cases are isolated in the contagious period, or later: this is the existing difference. between a society that is on the path to controlling the epidemic and a society where the disease continues to spread.

The importance of tracing

The contact tracing step is essential. It is not only a question of identifying the individuals who may have been infected by the detected case (called “descending cases” or “contact cases”, which are identified by what the Anglo-Saxons call “forward tracing”. »), But also to identify the case which is likely to be at the origin of the contamination of the individual considered. This case is called "ascending case", and it is identified by "backward tracing".

The identification of descending cases is greatly facilitated by early isolation: if we succeed in identifying and isolating potential cases just after their contamination, before the start of the contagious phase, there will be no, or few, descending cases. to isolate. Conversely, if, as often happens today, we isolate only on the basis of a test carried out three or four days after the onset of symptoms (so around five to six days after the start of the contagious phase), contact cases will be more numerous. In addition, they will be difficult to identify: it is not easy to remember all of our social contacts several days later. Above all, some of these contacts may have already been the source of new contaminations.

Case tracing involves the tracing of top-down cases (which may have been contaminated by the identified case, sometimes called forward tracing) and bottom-up case tracing (whatever may have contaminated the case, or backward tracing, and which corresponds to the classic cluster investigation approach).
Author provided

The approach of investing energy in “parent” cases (“ascending” cases) is particularly profitable at the start of an epidemic wave. It is also important if the cases tend to occur in clusters. However, in the case of coronaviruses such as SARS-CoV-2, it has been shown a strong tendency to aggregate cases, that is to say to development in clusters or epidemic foci in the early stages of the disease. epidemic.

This means that there is, at this time at least, a small fraction of subjects who are responsible for a disproportionately large fraction of contamination cases.. Identifying them is not straightforward and requires a more detailed examination going back further than when looking for top-down cases. This approach would however be particularly relevant, in particular in a phase where the epidemic was well controlled, to reduce the risk that it will restart.

Japan used it successfully at the start of the current phase of the epidemic. The widest possible use by the population of "contact tracing" applications such as the "TousAntiCovid" can greatly facilitate the retrospective identification of aggregations of cases, while respecting the confidentiality of personal data.

Once this identification has been made, isolation must be put in place immediately. Specific recommendations regarding a self-isolation strategy « combining the promotion of the duty of solidarity with compensatory measures » have already been formulated by the Covid-19 Scientific Council.

A hazy situation in France

In France, it is difficult to have a precise vision of the situation in terms of the time to isolation of cases, because the time between contact with the virus and the isolation of the person concerned is not very precisely followed.

The precious weekly epidemiological points However, they provide data making it possible to estimate it indirectly, such as the average value of the time between the date of the first clinical signs and the date of the sample taken to carry out the RT-PCR test. This value is known for 51% of the identified cases (of which it is not known, however, whether they are representative of all cases). It was, for the second week of November 2020, 3,1 days. By the spring she had passed ten days.

Knowing that the time between the start of the contagious phase and the appearance of the first symptoms is of the order of two days on average, and that the time between the sample and the result of the RT-PCR test and then isolation can still be add one or two days, we can estimate that the time between the beginning of the contagious phase and the isolation, was still in mid-November at least on average of the order of 6 to 7 days in our country. It's too much.

Average number of days between the first clinical signs and the date of sampling to perform the RT-PCR test, per week in France between May 13 and November 8, 2020 (end of week 45). For week 45, the estimate is based on 51% of the 247 new cases reported.
Public Health France, weekly epidemiological bulletin of November 12, 2020, Author provided

Since the start of the epidemic, New Zealand succeeded in reducing the time before isolation from 7,2 days to -2,7 days. In other words, the cases were, at the onset of infection, isolated about a week after the onset of their symptoms; they are now on average about 3 days before the onset of symptoms.

In France, we see that with a delay probably of at least three days between the first symptoms and the performance of a test, estimated moreover on a probably non-representative sample of cases, we are very far, around a week or more, of the New Zealand situation. And this, even though an alert on the slowness of the "test-tracer-isolate" had been launched. from last July by the Scientific Council.

Immediate isolation: concretely how to do it?

Being able to isolate as quickly as possible means being able to perform diagnostic tests soon after contact with the virus is suspected (allowing the few days it takes for the viral load to be detectable) or, but it is already late, as soon as the slightest symptom appears in infected subjects who were not identified as a contact case.

It seems difficult to implement in France today. In order to improve the situation, certain conditions seem important:

- Succeed in testing and diagnosing the majority of SARS-CoV-2 cases before the first symptoms, or at the latest when they occur, and not 3 to 5 days after : setting up a single operator, responsible for collecting symptom reports, performing tests (including assistance in making appointments), tracing ascending and descending cases, as well as the accompaniment of isolation could make it possible to achieve this objective. The “entry point” into this test-trace-isolate system would be the Covid toll-free number and an associated website.

- Trigger isolation before the diagnostic test, on the basis of a short standardized questionnaire and a simple algorithm when calling the toll-free number : if we cannot test-trace-isolate effectively, we must at least trace-isolate, rather than risking not to trace and isolate people that we cannot test. Better to quarantine a little too many people than the other way around, if that can decrease the duration of confinement for an entire country. reducing the time to isolation is the priority. This implies having a significant number of respondents to this toll free number; IT assistance could make it possible to somewhat limit the human resources required. Management from the point of view of prevention is often the result of a diagnostic test, which is too late.

- Centralize the management of the appointment queue for carrying out diagnostic tests: at present, this is carried out without precise specifications by private and public operators. In some cities, it is not possible to make an appointment on the Internet or over the phone, you have to go to a laboratory to do so, and said appointment is then sometimes offered only several days later. There is no clear logic for prioritization, despite recommendations that are not clearly explained or applied (even if the situation tends to improve).

- Convince society to play the game: Drastically reducing the time between infection and isolation can only be the result of a collective effort. The success of this approach implies that people possibly infected agree to contact the toll-free number or the website at the slightest doubtful contact or clinical sign relating to the symptomatology of Covid-19. These people must not only be listened to, informed and advised, but also that they adhere to the advice they receive, as well as to the isolation methods proposed. Receiving an SMS is not an adequate response to induce isolation, which constitutes a radical change in lifestyle, even if it lasts only a few days. This situation requires a personalized dialogue.

Contact tracing applications such as “TousAntiCovid”, which replaced the criticized and little-used “StopCovid” application at the end of October, are an important tool in the implementation of early isolation. Damien Meyer / AFP

Note that this strategy should obviously not prevent contact with a doctor if the person is considered to be at risk of developing a severe form of the disease, or if they have worrying symptoms. The individual therapeutic response must take place in parallel with the public health response.

Finally, these proposals should not dispense with an in-depth study of the measures put in place by the countries where isolation works effectively.

The cost of isolation

Let's face it: the price to pay for such an approach, which involves supporting and isolating a large number of people, is very high. People who will reveal themselves ultimately uninfected could end up in isolation while waiting to get an appointment to get tested and receive test results. If the tests drag on, the cost of this approach could turn out to be significant. However, this cost would remain a priori lower than that of generalized confinement. Beyond the financial cost, isolation can prove difficult for some people, especially if it is done outside the home, in order to protect other members of the household.

Moreover, the isolation of cases can be perceived as more unequal than generalized confinement. However, it should be kept in mind that generalized confinement is in no way egalitarian: during the first confinement, from March to May 2020, people belonging to the least advantaged social categories encountered much greater difficulties than better-off people.

Isolating, for a maximum of ten days, the only people who have recently been in prolonged contact with an infected person or those having symptoms is less of a constraint for the community than having to isolate the entire company repeatedly for four to six weeks, which amounts to profoundly modifying its operation several times a year. The example of countries in Asia and Oceania suggests that if the first approach is properly implemented, it may eliminate the need for the second.

It is therefore important to act collectively now to implement these measures, to prevent or delay a new epidemic outbreak after this second confinement. This threat weighs in a very real way on the first half of the year 2021, unless vaccination were to change the epidemic landscape ...

Alfred Spira, Honorary Professor of Public Health and Epidemiology, National Academy of Medicine, Inserm

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