The CT value is used to evaluate whether a person is positive or not. This is a viral load, in other words, the number of cycles required to find the virus. The level of CT is currently reported by all laboratories involved in PCR testing in Slovakia. However, not everyone will know this value. What does the CT value, ie the viral load, mean?
Viral charge vs. infectivity and symptoms
In addition to positivity, this value is associated with two factors – the extent to which a person is infectious and how they can go. In general, countries have the same level of viral load. In Slovakia, for individual CT values, if the value is below 25, it is a very high viral load, from 25 to 35 it is medium and a viral load with a value higher than 35 is considered low.
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In neighboring Austria, one considers oneself to be infectious at the value of 30. The organizer of the Winter Olympics China, in turn, considers CT values above 35 to be either a false positive result or a positive one. In Slovakia, everyone with a CT value of up to 40 is considered positive.
The symptoms corresponded to a viral load
However, despite the fact that it is determined by all laboratories in Slovakia, not everyone will know this value. As a result, it is reported only by private laboratories. State PCR tests do not provide this information. However, some people who had this value stated that their symptoms and course corresponded to the CT value. “I was surprised by the result, I was fine. If I had to say, the only symptom was fatigue. “ revealed to Lenka for Topky.sk, which had the stated value of 35.52.
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Julia had a viral load of 20.94, very high. “It caught me again, but it sounded fast. I had COVID for the second time, in both cases the course was moderate, but last year it took 2 weeks, now basically 3 days, but I was surprised that it was quite difficult to have three doses, ” said for Topky.sk. Therefore, some experts point out that relatively non-infectious persons sometimes have to comply with quarantine. In both cases, however, it was probably a combination of several factors and the impact of vaccination. Why?
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It is not positive as positive
However, the virologist of the Slovak Academy of Sciences Tatiana Betáková and the head of the Department of Anaesthesiology and Intensive Care Jakub Hložník agree that this is a relative value on the basis of which it is not possible to determine exactly whether and when a person is infectious and what his symptoms will be. The Public Health Office is also considering, which points out that on the basis of a single CT value, it is not possible to determine with sufficient reliability whether such a person poses a risk to their surroundings. “This number only reflects the current state of the virus load at the time of sampling. On the basis of one result, it is not possible to assess whether the viral load will increase or decrease, so it is not clear whether this is the onset of infection or a lingering infection. ” Marek Eliáš, a spokesman for ÚVZ SR, told Topky.
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Therefore, the length of the quarantine or isolation cannot be shortened based on CT. He is not a positive person as a positive person. “The length of isolation based on the CT value could only be reliably determined if the dynamics of the CT value were monitored (ie how it changes over time for different samples),” he explained. Thus, if it is a fading infection, a person with a CT value above 35 can be considered practically non-infectious, although in some foreign literature, the CT number 37-40 is mentioned. “CT over 35, however, can also mean the beginning of the disease, in 1-2 days it can be infectious for the environment. It should also be emphasized that the CT value may also depend on the quality of the consumption, “ he added.
This is also the reason why they do not report the charge on all results. “The number concerning the viral load is only current at the time of sampling, therefore its presentation in the results of the PCR test is considered irrelevant by experts at ÚVZ and MZ SR, it can often even be confusing. Based on one result, it is not possible to assess whether the viral load will increase or decrease (ie it is not clear whether this is the onset of infection or a lingering infection). “ the Ministry of Health responded.
The human body is not an engine, it cannot be said exactly
According to Hložník, we can roughly say that the lower the CT value, the more virus there is and the higher the infectivity. “In our experience, it’s only rough, because people often have higher values in the beginning and it happens that the patient’s number is higher after more than seven or ten days and then decreases.” described Hložník, stating that I take patients on the ward that are still PCR positive, but have a CT value higher than 32 than non-infectious ones if they have been lying with them for a long time. “It also needs to be correlated with the clinical condition and duration of symptoms,” he said for Topka.
As for the symptoms, those with the charge height may not be related to the value of the viral charge. “Someone can have a CT value of 16 and it’s okay, someone can end up in ventilation. It cannot be accurately predicted. The human body is not an engine, you can’t say by number how it will react, plus there are other factors, such as the sampling technique. “ he added.
The CT number says nothing about the amount of infectious virus
According to Tatiana Bioáková, a virologist at the Biomedical Center of the Slovak Academy of Sciences, who also works at the Faculty of Science, Charles University, talking about a viral load using a CT number is misleading and does not say anything about the patient’s infectivity. “People who have overcome covid-19 may have residual RNA on their mucosa and the PCR test may be positive half a year after healing.” explains. “The amount of infectious virus is determined on the cells, where the sample is applied to the cells and the multiplication of the virus is observed.” stated. You can find out more about the CT value in the article below.
“The CT value says nothing about the infectivity of an individual and the rate at which the virus multiplies, etc. However, it can serve as a guideline for the doctor, especially if the patient belongs to a risk group. “ clarified virologist. According to her, an individual may have a very high CT and may develop lethal COVID-19. “Conversely, CT may be low and an individual will not develop COVID-19. We know that with some viruses, such as noroviruses, it is enough for a person to become infected with a single viral particle and be able to die of a viral infection. For influenza and coronavirus viruses, 10-100 viruses have been found to be sufficient for infection. The rest then depends on the immune system, “ she said that the immune system works better in young and healthy people than in the elderly and sick.
A few virus particles are enough for infection
Study co-authors Patrice Delafontaine and Xiao-Ming Yin of the Tulane University School of Medicine in New York, who studied students’ CT, also found that those with low viral loads could infect someone. “In our study, we found that the CT number did not correspond to the real transmission of the infection. There is a group of patients with a low viral load who have infected others. For this reason, all those infected should remain in quarantine. ” stated. On the contrary, the association of symptoms with the CT value was demonstrated, as those who had symptoms had a lower CT and thus a higher viral load.
So what is the connection between CT value and infectivity? Is it possible to judge in general when a person is no longer infectious or, conversely, when he is most infectious? “We also do not have a reliable answer to this. We know that people are infectious and can infect others without having the symptoms of covid-19. Conversely, people who have symptoms may not be infectious to others. A few virus particles are enough for infection, “ explains Betáková.
The virus remains infectious in the elevator, bus or apartment for several hours
If they work in a laboratory and infect animals, they know how many infectious particles they have in the sample that infects the animals. “There, the more virus, the sooner the symptoms, the worse the course of the disease and the longer the treatment. In addition, when infected with a high infectious dose, the virus spreads to other organs. “ clarified virologist.
We can influence the spread of the virus among people by wearing respirators. “The exhaled virus is caught in a respirator and does not spread into the air. The virus remains infectious in the air (elevator, office, apartment, bus, tram, etc.) for several hours. All you have to do is get on the elevator, the bus, the office, take a breath and get infected. The more virus there is, the more virus particles you inhale. The longer you stay in a contaminated environment, the more virus you inhale and the higher the risk of infection. ” stated.
Does a particular coronavirus variant affect infectivity or symptoms?
Another measure is hand washing. “Contaminated hands will catch your mouth, nose, eyes – you can get infected. The problem with pathogens and viruses is that the thing is not black or white. There are always a myriad of factors that lay people have no idea about and that experts are considering. That information will not fit into one article. “ he says. Does coronavirus variants have any effect on this? Is there a difference between omicron and delta in any of the parameters such as infectivity or symptoms?
“Every virus is different and each variant has slightly different symptoms of the disease. In the case of the omicron variant, in addition to the surface “spike” protein, other parts of the virus that help the virus penetrate the cell and multiply have mutated. The virus can attach better to the cell, penetrate the cell faster, and multiply faster and more efficiently in the cell. That’s why the virus is more infectious and spreads faster. “ added Betáková.
What is a CT value or a viral load? Explains Tatiana Betáková, virologist at the Biomedical Center of the Slovak Academy of Sciences, who also works at the Faculty of Science, Charles University.
The CT value is the number obtained during the RT-PCR assay. Indicates the number of cycles required to amplify the sample in order for the sample to be considered positive. During RT-PCR, nucleic acid (RNA) is isolated from the sample and transcribed into DNA (RT reaction) and then copies of genetic material (DNA) are produced (amplified) in thermocyclers using enzymes so that DNA can be detected. If we have one DNA molecule in the sample, we have two after one cycle, we have 4 after the second cycle, we have 8 after the third cycle, and so on. Usually 35-40 cycles are done. Thus, at CT below 35-40, the sample is considered positive. The smaller the number, the more genetic material is in the sample, because the genetic material can be detected after only a few cycles. However, it should be emphasized that the CT number does not indicate the infectivity of the virus. Each laboratory uses different purification techniques, different enzymes, and amplification instruments, and therefore this number may not be the same if RT-PCR is performed in different diagnostic laboratories. The positivity of the sample is also affected by the quality and quantity of the material collected, storage and transport of the material to the laboratory, the time for which the sample was in the refrigerator, freezer and the like.
Talking about viral load using CT numbers is misleading. The CT number says nothing about the amount of infectious virus in the body. People who have overcome covid-19 may have residual RNA on their mucosa, and the PCR test may be positive half a year after healing. The virus multiplies in the upper and lower airways and swabs from the nasal cavity cannot give an objective picture of the amount of virus in the body. It was found that people who had positive RT-PCR from saliva usually had a worse course of covid-19. Of course, it can be assumed that the more RNA we find in the sample, the more virus there will be in the body. But it doesn’t have to be that way. It depends on when and how the sample is taken. The amount of infectious virus is determined on the cells, where the sample is applied to the cells and the multiplication of the virus is observed.
Not positive as positive: Experts on viral load! It says nothing about infectivity, you can also get infected in the elevator