In recent months, it has become common to see that some people who begin to show typical symptoms of Covid (cough, nasal congestion, fever…), take a rapid antigen test and the result is negative.
The person continues with the discomfort and, one or 2 days later, a new test is done that confirms the coronavirus infection.
The danger is that, in that period in which there was no correct diagnosis, the necessary measures were not taken, such as isolation and the use of masks, to avoid infecting others with the virus.
This in turn creates new chains of transmission and increases the number of cases of the disease.
But how is this phenomenon of “delayed positivity” explained?
Although there are no clear answers, some experts put forward various hypotheses to the BBC, which help to understand the situation. They also remind you what to do to protect yourself and everyone around you.
In short accounts, although there are still many questions on the subject, the recommendation is simple: if you have the typical symptoms of covid, stay isolated and avoid contact with people so that the rapid antigen test that you have done the first or second day has negative.
And if possible, try to repeat the test between the third and fifth day to be more certain about the diagnosis.
Has the virus accelerated?
The first factor that explains this situation is the arrival of new variants of the coronavirus, especially those that emerged from Ómicron such as BA. 2 and BA.5.
After having spread throughout the world, there was an important change in the incubation period, which is the time between when the virus begins to invade the cells of our c body and the onset of symptoms.
“The virus infiltrates the cells and enters the 100 and 1,000 new copies of itself, which go and infect other cells, and so the process continues until the reaction of the immune system, which causes symptoms such as nasal congestion, sneezing, fever…”, explains virus expert José Eduardo Levi, researcher and development coordinator at DASA (Diagnósticos de América, an expert diagnostics firm in Brazil).
In comparison, according to a report from the United Kingdom’s health security agency, the alpha variant’s incubation period was an average of five to six days.
That period decreased to 4 days during the delta variant wave .
With omicron, the period between viral invasion and the onset of symptoms it was reduced even more and it is only three days.
In other words: if before it took almost a week for a person to start developing typical Covid symptoms after coming into contact with someone infected, today that process is much faster and can happen almost overnight. the morning.
“What we see most in our offices are patients saying they went for a walk on Sunday and that they were already experiencing symptoms on Tuesday or Wednesday,” says infectious disease and virus specialist Nancy Bellei, who is also a professor at the Federal University of Sao Paulo (Unifesp).
But why does it happen?
Different viruses, updated defenses
Within the theories that could explain why omicron was less aggressive than the previous strains, some experts assure that the genetic mutations of the variant were the that shortened the incubation time.
Others underline the role of the immune system in this process.
In a series of posts on Twitter, the immunologist and epidemiologist Michael Mina, who worked at Harvard University and is currently the scientific director of a testing company, assures that vaccination “has fundamentally changed the relationship between Covid and viral load”.
Previously, the symptoms of the covid were beginning to appear just as the amounts of the virus in the body were reaching their peak.
“Why are people reporting symptoms but giving negative results?” he asked.
“The symptoms we experience are usually 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”, he wrote on the social network.
With the advance of the pandemic and vaccination, the symptoms have become more moderate. (Photo: GETTY IMAGES)Later in her explanation, Mina points out that the rapid immune response helps to suppress the virus for a while, until the pathogen is cleared from the body or eventually wins the battle and begins to replicate more ferociously.
“Understanding this, a test created to detect a certain amount of virus will give negative results in the first few days, before the viral load increases”, assured the scientist.
In other words, one of the theories says that with omicron, the short incubation period and earlier onset of symptoms means that the viral load (the amount of coronavirus in action) during the first few days of an infection is not high enough to be detected by antigen tests.
It is worth saying that this is just one of the possible explanations for this phenomenon, and that the idea is far from being proven or being adopted by some kind of consensus among experts.
“The immune system also depends on the activation of a certain number of viral particles so that a response is started. So it seems to me that the immunity that is created through vaccination or previous cases of covid contributes more to ending the infection quickly than interfering with its beginnings, ”says Levi, who also does research at the Institute of Tropical Medicine of the University of São Paulo (USP).
In any case, there is evidence that the amount of viral proteins in those first days of infection could be rather low in omicron waves.
“With this, there is a risk that antigen tests fail to detect these cases, since there are not enough particles to obtain a positive result”, sums up virologist Anderson F. Brito , scientific researcher at the All for Health Institute.
“So we have to be careful when interpreting these initial results, so that we don’t create a false sense of security” , warns.
Human errors and test interpretation
Also, within this discussion it is impossible to ignore the impact of the human factor on the number of errors that is seen in the results.
The antigen test, which can be done by anyone at home, requires performing a series of very specific procedures – separating materials, washing hands, scrubbing the back of the nose with a swab for a minimum period of time, mix with the saline solution, wait a few minutes…
If any of these steps is not done correctly, the result it can give a false negative.
“You cannot trust the conclusion of a poorly done test”, says Bellei, also a member of the Brazilian Society of Infectious Diseases (SBI).
The doctor highlights the variation that exists between the quality of the tests that are available in pharmacies.
“ There is a big difference between the sensitivity and specificity of the different rapid tests. Some can detect the 51% of cases, while in others this rate drops to 70 or even to 50%”, he warns.
Although there is no easy and accessible way to know which products are the best quality, Bellei recommends that people give preference, if possible, to the best-known brands, companies that have been working in diagnostics for decades.
A second important recommendation is to read carefully follow the instructions that come with the test and follow each step exactly to reduce the chance of errors and wrong results.
In this sense, there were also many doubts whether antigen tests would be able to detect omicron variants, because to the mutations so different from those of other variants.
This hypothesis, however, turned out to be false. “The antigen test mainly detects the N protein of the coronavirus,” explains Levi.
“And we know that omicron has more mutations in the S protein, which is not usually the main target of these tests. ”, he added.
In other words, rapid tests continue to be relatively effective in finding new variants.
Perhaps the heart of the matter is trying to understand what is the right time to take the test, and from there, learn how to interpret the results and take the necessary actions.
What changes with the diagnosis?
Given all these changes observed in the behavior of the virus and in our immune system, the main message is relatively simple: yes you have the typical symptoms of covid, such as sore throat, cough, sneezing and fever, avoid contact with other people so as not to transmit the agent to them infectious.
This rule applies even to people who took a test in the first few days and got a negative result. As explained, it is still not possible to be 100% sure and the viral load may not be high enough to be detected by the test.
Quarantine periods depend on a variety of conditions, but a period of between 5 and 7 days is usually sufficient for most people.
If you have been tested the first or second day of symptoms and the result was negative, it is worth repeating it again between the third and fifth day if possible.
A An unpublished study from the London School of Hygiene and Tropical Medicine, in the United Kingdom, revealed that the peak of the viral load usually occurs on the third day of the onset of symptoms.
With this, if you really have covid, it will be more feasible to detect it if you take a test at the point in e l that the load is at its peak.
“And it is important to notify the results to the health unit closest to your home, to eventually confirm the diagnosis through more accurate methods such as the PCR test, and that the case may be taken into account in the official statistics”, assured Brit.
Of course, it is also important to limit contact with other people, especially those most vulnerable to covid such as the elderly or immunosuppressed, during the period of uncertainty between one test and another.
Another key point is to keep the vaccination up to date.
“All those who have their doses up to date transmit less coronavirus than those who do not have their vaccination schedule. n complete”, says the professor and immunologist at the Federal University of Health Sciences of Porto Alegre.
A study carried out at the University of Seoul, in South Korea, showed just it’s. Vaccinated individuals who become infected with covid shed the virus for an average of four days. The period changes dramatically even in the partially vaccinated, who can transmit the virus for up to 8 days.
“This is in addition to fact that it is the vaccine that allows the infection to not get complicated most of the time”, affirms the specialist.
This article appeared published in its original Portuguese in: https://www.bbc.com/portuguese/international -61986889
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