Friday, March 29

If I am positive for covid after the seven-day quarantine, can I spread it? | The scientists respond | Science


A person wearing a mask during a quarantine at home last December.
A person wearing a mask during a quarantine at home last December.Dos Santos (Europe Press)

After a seven-day quarantine for covid and with a positive antigen test, you can continue to infect other people. Keep in mind that not everyone is aware that they have been infected at the same time. Therefore, the recommendations that are made based on the changing epidemiological situation do not have to be complied with in all cases in a particular way. Even with the omicron variant on which it has been speculated that the entire infection process is faster, recent studies from japan show that there are approximately 19% of cases that continue to be positive by antigen tests after seven days of the course of infection. That is why antigen tests are very useful tools, they allow us to detect the presence of large amounts of virus through a very fast and reliable test that can be repeated over time.

The positivity of this test tells us that we still have a large amount of virus in the upper respiratory tract, either in the nasal passages or in saliva, and this reflects the potential risk we have of infecting other people by emitting these viruses as aerosols.

In the event that after seven days of quarantine the result of the antigen test is negative, it cannot be taken as an absence of infection and, much less, as an absence of contagious capacity because, in the end, what the antigen tests detect is when we are most contagious, but they do not give us information about whether or not we are infected. Having a negative test does not mean that we cannot infect other people, it simply reflects that the probability of doing so decreases.

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The risk of infecting other people decreases as the days after infection go by. In general, people develop immunity against the virus, antibodies produced by the immune system that gradually neutralize the virus, making it less and less infectious. This is a process that is accelerated in people vaccinated against covid. In fact, in the laboratory it is quite difficult to isolate virus from samples from people who do not have symptoms after ten days of infection.

In the study of the National Institute of Infectious Diseases of Japan published at the beginning of January, it has been seen that between days 7 and 9 after the start of the infection process, a percentage of replicative viruses can be isolated in the laboratory, very similar to those isolated during the first two days after infection, in approximately 12%-10% of samples from asymptomatic people. The fact that we can isolate viruses from people who do not have symptoms indicates that they can also infect others. However, in cases of patients with symptoms, this percentage of virus isolation increases to almost 19% of the samples between days 7 and 9. And in both situations where more virus is isolated, up to 50% of the samples, is between days 3 and 6 after the start of the infection.

During the natural course of SARS-CoV-2 infection, the amount of virus increases very rapidly after the first few days after infection and remains high for a period of time that will vary depending on the person infected and their ability to neutralize and contain the virus. Therefore, the periods in which infected people are contagious may be different. In the case of people with a compromised immune system or with difficulties in mounting a good immune response, the time during which they will be contagious may be longer. Likewise, people who are not vaccinated may also take longer to control the replication of the virus and, therefore, continue to infect until then. That is why it is difficult to give totally categorical answers because each case can be different.

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Regulations regarding quarantine and isolation are changing rapidly. On January 7, the ECDC (European Center for Disease Control) published a guide on the isolation of positive cases of covid 19 in the current epidemiological situation of expansion of the omicron variant. In this publication, different isolation options are mentioned to adapt to the current situation, indicating that they are not based on scientific evidence and recommending them for those workers who perform essential jobs. The guidance recommends longer isolation periods for unvaccinated people. For these people, it suggests isolation for 10 days with no fever and improvement of symptoms in the 24 hours prior to the end of isolation or the same improvement in terms of symptoms and two successive negative antigen or PCR tests with an interval of 24 hours between them. hours. In the case of vaccinated people, the same criteria are recommended, but the isolation is shortened to six days. This same guide also includes other possibilities for health systems or societies highly stressed by the number of infected, shortening the isolation period to five days in the case of unvaccinated people and three days in the case of vaccinated people. However, as the guide highlights, reducing the time of isolation increases the risk of contagion, which is why the use of FFP2 masks without a valve is recommended continuously and minimizing non-essential interaction with other people, especially with those more vulnerable.

Nuria Izquierdo-UserosShe is a doctor in biology, head of the emerging pathogens group at IrsiCaixa.

Question sent via email byTamara Castro

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