In recent months, a scene has become increasingly common in Brazil and in the world: the person begins to show typical symptoms of covid (cough, runny nose, fever…), takes the rapid antigen test and the result is negative.
She continues to have the discomfort and, a day or two later, repeats the test that, then, confirms the infection by the coronavirus.
The great danger is that, in the meantime, without the proper diagnosis, the necessary measures are not taken to reduce the risk of passing the virus on – such as isolation and the use of masks.
This, in turn, creates new chains of transmission and increases the number of cases of the disease.
But what explains this phenomenon of “delayed positivity”? Although there are no clear answers, experts heard by BBC News Brazil cite the hypotheses that help to understand this scenario. They also point out what to do to protect yourself and everyone around you.
In summary, although the subject is shrouded in many mysteries, the recommendation is simple: if you have typical symptoms of covid, stay in isolation and avoid contact with some people, even if the rapid antigen test done on the first or second day gives a negative result. And, if possible, try to repeat the exam between the third and fifth day to be more certain about the diagnosis.
The first factor that explains this scenario is the arrival of new variants of the coronavirus, especially those that emerged from the omicron, such as BA.2 and BA.5.
Their spread around the world was followed by an important change in incubation, which is the time between the virus starting to invade our body’s cells and the onset of symptoms.
“The virus enters the cells and makes between 100 and 1,000 new copies of itself inside, which will go out to infect other cells and continue this process, until the immune system reacts and causes symptoms, such as a runny nose, sneezing, fevers. …”, explains virologist José Eduardo Levi, research and development coordinator at Dasa.
By way of comparison, according to a report by the UK Health Safety Agency, the incubation of the alpha variant lasted an average of five to six days.
During the delta wave, that window dropped to four days.
Already in the omicron, the period between viral invasion and the onset of symptoms has been reduced further and is only three days.
In other words: if before the person had contact with someone infected and it took almost a week to show the typical signs of covid, currently this process is much faster and can happen almost overnight.
“What we see most in our offices are patients who say they went for a walk on Sunday and already have symptoms of the disease on Tuesday or Wednesday”, says infectious disease specialist and virologist Nancy Bellei, a professor at the Federal University of São Paulo (Unifesp) .
But why is this happening?
Different virus, updated defenses
Among the possible theories that help to understand this lighter action of the omicron, some experts point out that the very genetic mutations that this variant carries shortened the incubation time.
Others also draw attention to the role of the immune system in this process.
In a series of posts on Twitterimmunologist and epidemiologist Michael Mina, who worked at Harvard University in the United States and is currently the scientific director of a testing company, argues that vaccination “It fundamentally changed the relationship between Covid symptoms and viral load.”
Previously, the symptoms of covid used to appear just when the amount of virus in the body reached its peak.
“Why are people now showing symptoms, but the rapid test result is negative?” he asked.
“The symptoms we experience are usually the result of the immune response. Vaccines make our defense system detect the virus faster, before the number of copies of it peaks. That is literally the purpose of vaccination,” he wrote in social network.
Omicron strains shortened incubation time and onset of symptoms — Photo: Getty Images via BBC
Continuing in the explanation, Mina points out that the rapid immune response helps to suppress the virus for a while, until the pathogen is eliminated from the body or eventually manages to win this battle and begins to replicate more forcefully.
“With this, a test created to detect a certain amount of virus will be negative in the first few days, before the viral load increases”, argued the scientist.
In other words, one of the theories put forward points out that, with the omicron, the short incubation time and the earlier appearance of symptoms means that the viral load (the amount of coronavirus in action) in the first days of infection is not high enough to be detected. detected by rapid antigen tests.
But it is worth mentioning that this is just one of the possible explanations for this phenomenon and the idea is far from being proven or being a consensus among experts.
“The immune system also depends on a certain amount of viral particles to be activated and start a response. So, it seems to me that the immunity created from vaccination or previous cases of covid will contribute more to quickly ending the infection than to the beginning of the picture”, evaluates Levi, who is also doing research at the Institute of Tropical Medicine at the University of São Paulo (USP).
In any case, there is evidence that the amount of viral proteins in these first days of infection may actually be lower in the 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”, summarizes virologist Anderson F. Brito, scientific researcher at the Todos pela Saúde Institute.
“Therefore, we have to be careful in interpreting these initial results, even so we don’t create a false sense of security”, he warns.
Moving average of Covid deaths rose 57% in the last two weeks
Human error and test interpretation
Still within this discussion, it is not possible to ignore the human factor behind errors in the results.
The antigen test, which can be done at home by the next person, has a series of very specific procedures—separating materials, washing hands, swabbing the back of the nose and throat for a minimum of time, mixing with saline, wait a few minutes, ping the exact amount on the device…
If one of these steps is not done properly, the result could be a false negative.
“You cannot trust the conclusion of a poorly performed test”, says Bellei, who is also a member of the Brazilian Society of Infectious Diseases (SBI).
The doctor highlights the variation in the quality of tests available in pharmacies.
“There is a lot of difference in the sensitivity and specificity of rapid tests. Some can detect 80% of cases, while in others this rate drops to 70% or 50%”, he compares.
Although there is no easy and accessible way to know which products are the most reliable, Bellei advises that people give preference, if possible, to the best-known brands, from companies that have been working with diagnostics for decades.
A second important tip is to carefully read the instructions that come with the kit and follow each step to the letter to reduce the likelihood of errors and skewed results.
In this sense, many doubts were also raised whether the antigen tests would be able to detect the lineages of the omicron, which carry many mutations compared to the previous variants.
This hypothesis, however, proved to be false. “The antigen test mainly detects the N protein of the coronavirus,” explains Levi.
“And we know that the omicron presents more mutations in the S protein, which is not usually the main target of these tests”, he adds.
Ie: rapid tests continue to work relatively well for catching new variants.
Perhaps the heart of the matter lies in understanding the right time to take this test — and, from there, how to interpret the results and take necessary action.
What changes in the diagnosis?
Given all these changes observed in the behavior of the virus and in our immune system, the main message is relatively simple: if you have typical symptoms of covid, such as sore throat, cough, sneezing and fever, avoid contact with other people so as not to transmit the infectious agent to them.
You can check the complete list of signs of the disease in this BBC News Brazil report:
- The main symptoms of Covid in those who took two or more doses of vaccine
This guideline applies even to people who took a test in the first few days and the result was negative — as explained above, it is still not possible to be 100% sure and it may be that the viral load is not yet high enough to be detected. by the exam.
Quarantine time varies depending on a number of conditions, but a period of five to seven days is usually enough for most people.
See how many days of isolation are needed for each situation in the following link:
- Covid: how many days of isolation are indicated
If you took the test on the first or second day of symptoms and the result was negative, it is worth repeating the test on the third, fourth or fifth day, if possible.
An unpublished research from the London School of Hygiene and Tropical Medicine, in the United Kingdom, showed that the peak of viral load usually occurs up to three days after the onset of symptoms.
With that, if you really have covid, you will be more likely to get that peak of viral load, when the amount of virus will be high enough for the test to be positive.
“And it is important to notify the result to the health unit closest to your home, to eventually confirm the diagnosis through other more accurate methods, such as RT-PCR, and for the case to be accounted for in official statistics”, he points out. Brit.
It is, of course, worth maintaining isolation and avoiding contact with other people, especially those most vulnerable to covid, such as the elderly and immunosuppressed, during this period of uncertainty between one test and another.
Another key point is keep vaccination up to date.
“The variants even ‘appear’ earlier, but those who have all their doses up to date transmit less coronavirus compared to those who do not have the complete vaccination schedule”, informs immunologist Cristina Bonorino, a professor at the Federal University of Health Sciences in Porto. Alegre (UFCSPA).
A study carried out at the University of Seoul, in South Korea, shows just that. Vaccinated individuals who catch covid can transmit the coronavirus for four days on average. Already those who are partially immunized “pass” the pathogen for up to eight days.
“Apart from the vaccine is what allows the infection not to progress to more serious and worrying cases most of the time”, completes the specialist, who is also a member of the SBI.
For now, the Ministry of Health recommends a fourth dose of vaccine for the entire population over 40 years of age. Between 12 and 39, three doses are indicated. For children aged 5 to 11 years, two doses.