I am in quarantine for covid, I have had several tests done and they have been negative, can I still test positive?
A health worker takes samples from a woman for a PCR test at the military hospital in Seville on May 14.PACO PUENTES

The registry of infections of people who have passed the covid or who are vaccinated is scarce. But they are given and they are a wake-up call: it is still too early for the open bar, for the return to absolute normality. Different studies have detected infections between 10% and less than 1% of people already immunized, depending on the moment, the group, the place of the investigation and the vaccination schedule. These people, asymptomatic in most cases thanks to natural or vaccine-induced immunity, still retain the ability to harbor and spread the coronavirus for a time, even if they do not develop the disease. According to Guillermo López Lluch, professor of Cell Biology at the Pablo de Olavide University (UPO), the key to avoiding the danger of contagion being promoted, is “to achieve the broadest possible group immunity in the shortest period of time.” “It is a race against the virus,” he warns.

The biologist has been able to verify an infection of an immunized person in a close case: a person close to him, 92 years old and vaccinated with the double dose of one of the brands administered in Spain, had to enter a trauma hospital in Cordova. He was routinely tested for covid and tested positive, despite lacking symptoms consistent with the disease.

More information

Jocelyn Keehner and Lucy E. Horton, along with other members of the University of California (UCLA) and San Diego (UCSD) Faculties of Medicine, published last March in The New England Journal of Medicine (NEJM) the results of an internal investigation: “The absolute risk of testing positive for SARS-CoV-2 [coronavirus causante de la covid] after vaccination it was 1.19% among UCSD healthcare workers and 0.97% among UCLA healthcare workers; these rates are higher than the risks reported in the coronavirus mRNA-12731 vaccine trials [Moderna] and the BNT162b2.2 vaccine [Pfizer-BioNTech]”

The study shows the same conclusions, although with different figures, as another published last month by the Centers for Disease Control and Prevention of the United States and other priors reflected in Nature Y British Medical Journal.

It is very likely that the cases of reinfection are more numerous than those detected, since, in most cases, they do not produce symptoms and, therefore, these people, who are considered immunized, do not undergo PCR

Guillermo López Lluch, Professor of Cell Biology at the Pablo de Olavide University

All these studies confirm the existence of infections in immunized patients, although their record is scarce. López Lluch explains one reason: “It is very likely that the cases are more numerous than those detected, since, in most cases, they do not produce symptoms and, therefore, these people, who are considered immunized, are not makes them PCR ”. Mark Pandori, director of the Nevada Public Health Laboratory, agrees. “We are underestimating the cases of reinfection. They are very difficult to determine, specialized teams are needed to do this work or a central laboratory ”, he explains in Scientific American.

López Lluch refers to a recent study in The Lancet to argue that, although the immune response generated by vaccination or by the body itself prevents contagion and suffering from the most serious sequelae of covid, there are immunized people who become infected and can be a source of spread of the virus, even if they show a mild or no symptoms.

The specialist in cell biology explains that the cause is the viral load, the amount of virus that circulates in the blood of the infected person. Immunized people have a defense line made up of B and T lymphocytes. “The former produce antibodies, they hinder the virus from entering the cell. The latter attack it directly and generate intermediaries, such as interferon, the proteins that signal the presence of a virus and prevent its proliferation ”, he simplifies to explain it.

While the immune system works to eliminate the infection, a person, even if they do not develop the disease, can maintain a certain viral load

But while this system works to eliminate the infection, a person, even if he does not develop the disease, can maintain a certain viral load. In this sense, López Lluch explains: “Immunity is a matter of days. While we are not immune, the virus or pathogen has a long time to cause damage that is reflected in the symptoms, which are more serious. Our body’s defenses are inefficient and take time, between 10 and 14 days, to fully activate. When we are immune, the response of B and T lymphocytes is faster and the pathogen has less time to cause damage. However, if someone immunized, but with a viral load, emits aerosols in front of someone who does not have the same defenses, they can infect it. If the other person is immunized, the ability for the disease to spread is minimal. “

For this reason, the UPO biologist points out the importance of maintaining common preventive measures until vaccination is general. That reality will change, as López Lluch warns, the faster it is vaccinated and the greater its scope. “As long as herd immunity is not achieved,” concludes the specialist, “mutations that confer greater infectivity to the virus will have a greater ability to spread to more people.”

The fast-spreading variants of the coronavirus carry mutations that allow it to escape part of the immune response created naturally or through vaccination, according to a new study published in Science by scientists from the USA, Germany and the Netherlands. The researchers mainly focused on three mutations: K417N, E484K, and N501Y. Alone or in combination, they are found in most major SARS-CoV-2 variants and affect the spicule, the pathogen’s gateway to the human cell. The scientists determined that some antibodies lose the ability to effectively neutralize the virus when mutations are present.

Reinfections

The importance of mass vaccination that includes all age groups against reinfections is also supported by neurologist at Mount Sinai School of Medicine (USA) Seth M. Glickenhaus, lead author of a study on reinfection in young people published in The Lancet: “Our findings indicate that SARS-CoV-2 reinfection occurs in young adults. Despite a previous infection, young people can contract the virus again and can still transmit it to others. This is an important point to know and remember as vaccine rollouts continue. Young people should receive the vaccine whenever possible, as vaccination is necessary to increase immune responses, prevent reinfection, and reduce transmission. “

Despite a previous infection, young people can contract the virus again and can still transmit it to others. This is an important point to know and remember as vaccine rollouts continue.

Seth M. Glickenhaus, Mount Sinai Researcher

The study shows, according to the authors, that antibodies induced by SARS-CoV-2 infection are largely protective, but do not completely protect against reinfection in young people. The work tracked more than 3,249 members of the corps of marines (Marines) of the United States between May and November 2020. About 10% (19 of 189) of the participants who had previously had the disease were reinfected. The research found that those without immunity had five times the risk of infection than participants with antibodies, but the latter still had some risk of reinfection.

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