The seventh wave of the coronavirus pandemic has caused an increase in infections due to two subvariants of omicron that are responsible for this uptick in positive cases: BA.4 and BA.5. These new strains are more contagious than the previous ones, although their effects seem to be milder, also due to the high percentage of immunity in the population acquired by previous vaccinations and infections.
The symptom picture is similar to that of other variants, although one of the novelties of these strains is that the incubation period is shorter and symptoms last more days. Sore throat, cough, diarrhea, headache, fever, stuffy or runny nose are the most common symptoms of these sublineages.
The expansion of this silent wave has generated an increase in the demand for antigen tests in pharmacies, but how does it behave? How long does it take to give a negative in a diagnostic test?
Why is it more contagious?
These subvariants can be transmitted with a lower viral load and are more contagious because they better evade the immunity of the immune system, details the Ministry of Health in its latest update. Therefore, they can be transmitted before they can be detected in an antigen test.
In this sense, depending on the patient’s viral load and the evolution of the disease itself, it will take more or less days for a test result to be negative. In the case of antigen tests, it is recommended its use when the person presents symptoms compatible with the disease and is in the most infectious phase.
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This would explain why there are cases that offer a false negative in people who do have an infection. “It can occur when the viral load is very low or when it has been more than seven days since the start of the infection,” indicates the AEMPS. This type of rapid test has a lower sensitivity than PCR, so a false negative result is possible if the number of viruses in the sample is low, “which occurs during the initial or final days of infection.”
Although home isolation or quarantine is no longer necessary, it is crucial to know during how long the viral load is high to avoid infecting other people.
A study by the UK Health Security Agency revealed that after five days from the onset of symptoms, an infected person has a 31% chance of continuing to infect. After a week, this possibility was reduced to 15% and after 10 days, the risk of contagion was between 1% and 5%.
“Now until the clinical picture is advanced and the symptoms are more important, the positive is not seen and there are more false negatives. Until the viral load is not intense and higher than it was in other waves, the test does not detect contagion,” explains to ‘Heraldo de Aragón’ the epidemiologist of the University of Zaragoza, Nacho de Blas.
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