The seventh wave of infections Covid that Spain is experiencing, caused by the uncontrolled expansion of the new variants BA.4 and BA.5 -more contagious and that escape the immunity of the vaccine-, has caused a curious phenomenon with the antigen test as protagonists: the delayed positivity.
Not all the people who have compatible symptoms with this disease —cough, fever, nasal congestion…— they are giving a positive result from the first moment in those diagnostic self-tests. This causes many to initially give up contagion ruled out of coronavirusbut a few days later, after repeating the test, it comes out positive.
This is what is known as delayed positivitya phenomenon that occurs with these new sublineages of omicron having reduced incubation period compared to previous variants.
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Immunologist and epidemiologist Michael Mina addressed this question in a thread posted on Twitter: “Why are people reporting symptoms, but giving negative results?”, he asked himself on the social network.
As he explains, “the symptoms we experience are the result of the immune response. Vaccines can make our bodies detect the virus more quickly, before the number of copies reaches its peak. That is literally the purpose of vaccination.”
“Understanding this, a test created to detect a certain number of viruses will give negative results in the first daysbefore the viral load increases,” Mina added.
IMPORTANT:
RAPID TESTS DO WORK WITH OMICRON
“But why are some people staying negative in the first days they have symptoms??”
This is expected. Symptoms don’t = contagious virus
This is literally a reflection of the fact that vaccines are doing their job!PLEASE READ pic.twitter.com/YBJvNovQXL
— Michael Mina (@michaelmina_lab) December 18, 2021
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In this sense, the antigen tests in pharmacies are sensitive to a given viral loadso if the incubation period is shortened and symptoms appear earlier, that viral load still won’t be high enough to be detected in this test.
The danger of delayed positivity is that in that time lapse between the first and second antigen tests, the person has continued to lead a normal life, without paying attention to preventive measures such as the use of masks or isolation, which helps create new virus transmission chains.
Virologist Anderson F. Brito, a researcher at the Brazilian institute Todos pela Saúde, has warned about this. “We have to be careful when interpreting these initial resultsin such a way that we do not generate a false sense of security”, he warned.
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