If I am positive for Covid after seven days, am I still infectious?

Spanish scientist Nuria Izquierda-Useros explains that the virus replicates quickly in the first days of infection, but how long it remains in a system varies depending on the person

A woman in isolation for Covid-19 in December.
A woman in isolation for Covid-19 in December.Dos Santos (Europa Press)

After isolating for seven days and testing positive for Covid-19 via an antigen, it is possible for a person to continue to infect other people. It has to be taken into account that not everyone is aware that they are infected at the moment of contagion. That’s why there’s no reason why the recommendations made according to the changing epidemiological situation should be followed the same way in all cases. Indeed, with the omicron variant, which is speculated to have a much faster infection process, recent studies in Japan show that approximately 19% of cases continue to test positive for Covid-19 in antigen tests seven days after infection. That’s why antigen diagnostic kits are very useful, they allow us to detect the presence of large amounts of the virus via a quick and trustworthy test that can be taken as many times as needed.

A positive result from an antigen test indicates that we still have large amounts of the virus in the upper tract, be it in nasal passages or in saliva, and this reflects the potential risk of infecting another person by emitting the virus as aerosols.

In the case that the antigen test comes back negative, this cannot be taken as a sign that the infection is gone, and even less so as a sign that we are no longer contagious. That’s because what antigen tests detect is the moment we are most contagious, they do not tell us whether or not we are still infected. Testing negative does not mean that we can no longer infect other people, it simply reflects that the likelihood of doing so is lower.

The risk of infecting other people falls over time after infection. In general, people start developing immunity against the virus – antibodies produced by the immune system that neutralize the virus little by little, meaning that it is increasingly less infectious. This process is faster among those vaccinated against Covid-19. Indeed, in the laboratory, it is quite difficult to isolate the virus in samples from people with no symptoms 10 days after the infection.

The recent study by the National Institute of Infectious Diseases in Japan found that in approximately 10-12% of samples taken from asymptomatic cases, the percentage of the replicative virus isolated during the first days of infection was similar to what was isolated seven to nine days after infection. The fact that we can isolate the virus in asymptomatic people indicates that they can also infect others. However, among symptomatic patients, the virus could be isolated between seven and nine days after infection in nearly 19% of samples. In both cases, the virus was most often isolated – in up to 50% of the samples – between three and six days since the onset of infection.

During the natural course of a SARS-CoV-2 infection, the virus replicates very quickly in the first few days after infection and remains at high levels for a period of time that varies depending on the infected person and their ability to neutralize and contain the virus. That’s why the length of time in which an infected person remains contagious can vary. In the case of people with compromised immune systems or with difficulties building a strong immune response, they will be contagious for a longer time. Similarly, people who are not vaccinated may take longer to control the replication of the virus, and for that reason, will continue to be contagious until the virus is contained. That’s why it is difficult to give totally categorical answers because in each case it could be different.

The rules on quarantines and isolation periods are changing at a dizzying speed. On January 7, the European Center for Disease Control and Prevention (ECDC) published guidelines on the isolation of positive Covid-19 cases in the wake of the arrival of the highly transmissible omicron variant. This document mentions different ways to adapt the isolation rules to the current situation, given the need for essential workers “to uphold critical functions in society.” The guide – which is based on a “pragmatic approach” and not scientific evidence – recommends longer isolation periods for unvaccinated Covid-19 cases. The ECDC suggests either a 10-day quarantine and for the patient to show an improvement of symptoms, including the resolution of fever, or two consecutive negative tests, with a minimum interval of 24 hours, and the same demonstration of clinical improvement.

In the case of vaccinated Covid-19 cases, the ECDC recommends the same criteria, but reduces the isolation period to six days. These guidelines also put forward other quarantine rules for the healthcare sector and other industries that are under great strain due to the high number of infections. In these sectors, the document suggests cutting the isolation period to five days for the unvaccinated and three for the vaccinated. However, as the guide points out, reducing the isolation period increases the risk of contagion, and for this reason, it recommends wearing an FFP2 mask without a valve at all times and reducing non-essential contact with other people, especially vulnerable individuals.

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