COVID-19: Incubation period Positive nucleic acid test and Infectiousness
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COVID-19: Incubation period Positive nucleic acid test and Infectiousness.
You may hear that the cunning degree of the new coronavirus is far beyond imagination. Along with such statements, there are often “miracles” far beyond imagination.
For example, the nucleic acid test is positive long time, or the sudden change of positive after being negative for many times. These “miracles” have been fermented and amplified, making everyone bystanders at risk, and all kinds of suspicion and “criminal” for those who test positive. Nucleic acid positive = COVID-19 infection = contagious.
This series of “equal signs” has been deeply rooted in the hearts of the people, but in fact this simple equal sign is not accurate and can easily lead to misleading.
1. What is a positive nucleic acid test?
After a person is infected with the new coronavirus, the virus will replicate in his (her) body and expand its power. An important part of the replication is the replication of the genome. After the virus has replicated to a certain extent, take samples from the person’s respiratory tract and other places. There will be a sufficient number of virus particles in the sample-each virus particle has a virus genome. With more viruses, there are more copies of the total genome. .
The nucleic acid test measures the genome of the new coronavirus-to be more accurate, the nucleic acid test also has a “replication process”, which continuously copies several gene fragments of the new coronavirus through a biochemical reaction (PCR) until these fragments are copied to the number.
The degree that can be detected by professional testing equipment (test offline). The number of gene fragments detected by the instrument is fixed, and in the process of the biochemical reaction of PCR, we can also set or record the number of cycles of replication (that is, the ct value) in advance.
According to the detected downline and how many cycles of the gene fragments in the sample have been replicated to reach the downline, we can infer how many gene copies of the new coronavirus are in the sample itself.
It can be seen from this detection process that nucleic acid detection can be both qualitative and quantitative. From a qualitative point of view, it can determine whether there are new coronavirus gene fragments in the collected samples, thereby confirming whether there are viral gene fragments in the subject’s body. From a quantitative point of view, it can calculate the original number of gene copies in the sample and calculate the amount of poison in the subject.
But whether it is qualitative or quantitative, it must be noted that the results of nucleic acid testing directly determine the number of gene copies of the new coronavirus in the sample. Based on nucleic acid testing, we say that XX is now infected with COVID-19, or what is the amount of the virus in the XX infected with COVID-19. These conclusions are indirect inferences.
This kind of indirect inference is not random speculation, it is based on scientific principles, but these principles are also applicable. It does not mean that you can get a nucleic acid test result at random, and you can copy and copy without thinking.
Judging by the results of nucleic acid testing to determine whether it is in the infectious period, the amount of virus loaded, and whether it is infectious is based on an extremely important premise, that is: the number of gene copies detected comes from a “live” virus.
This premise is indeed applicable in many cases. For example, in patients with COVID-19 who are in the infection stage, the virus is rapidly replicating in their bodies, and all the nucleic acids detected cannot be said to be all, but at least a large part of the gene copy contributes to the living virus.
However, the genes of live viruses can be detected, which does not mean that the detected viruses will always be live viruses. A dead virus that has lost its vitality, can no longer replicate, and can no longer infect people, its genome may still be basically intact. Such a “corpse” can also make a nucleic acid test positive.
For example, crime prospectors can detect the DNA of a criminal suspect or victim based on blood stains, hair, and body fluids collected at the crime scene. This is testing the genome, so are these bloodstains, hairs, and body fluids all living cells? To expand a bit more, scientists still have the genomes of ancient human or animal remains excavated from archaeology. Are the owners of those genomes still alive?
2. Incubation period and infectious period
It is precisely because there is an important premise behind the interpretation of nucleic acid test results that “what is directly measured is the uncertain viral genome”. When determining the specific infection time and whether it is still contagious based on the positive nucleic acid, it is necessary to use other information during the test. Consider it.
Any virus, even any life, has its specific life cycle, and COVID-19 is no exception. Therefore, to analyze the incubation period (when it was infected) and the infectious period of a COVID-19 infection case, not only the nucleic acid test results, but also the life cycle of COVID-19 itself must be taken into consideration.
We discovered that the new coronavirus is indeed less than two years old. But in less than two years, we have invested the world’s most and best scientific research resources in COVID-19 research. In addition, the COVID-19 has infected more than 200 million people worldwide, and scientists have no shortage of research objects to study this virus. Finally, the virus replication cycle is short, you have to study tortoises, for 100 years, you may also study two generations of tortoise dad and tortoise baby. The virus replicates fast, and only n generations have passed in one week. Not to mention the primitive generation of viruses, even evolutionary issues with large time scales can be studied.
Therefore, we must have confidence in the general laws of the life cycle of the new coronavirus itself that are summarized by current observations. Conversely, if a “miracle” that violates universal laws suddenly emerges, especially a very “special” case, we should interpret it with caution. Don’t just come up with the case and say that the previous law was wrong, but consider whether the “case” is problematic, or if there are other explanations that are more reasonable.
Judging from a large number of studies, COVID-19 is not a virus that can be lurking in the human body for a long time. This is also reasonable. For the entire coronavirus family, there is no long-term incubation feature. If COVID-19 is an exception, it will be abnormal. In addition, as a virus that has crossed from the animal kingdom to the human body not long ago, COVID-19 and the human immune system “intersect very shallowly.”
The same is true for the infectious period of an infected person. After being infected, the time that a person infected with COVID-19 can infect others is relatively fixed, and it does not last indefinitely. Relatively stable and limited incubation period and infectious period, the corresponding mechanism is the life cycle characteristics of the new coronavirus itself. After infecting the human body, the virus will begin to replicate and expand.
The incubation period is equivalent to the point where the virus has not expanded to cause symptoms or a positive nucleic acid test. When the replication reaches a certain level, the amount of virus in the human body is enough, and it may be possible to “provide” enough virus to infect others, that is, during the infectious period.
But as long as the infected person’s immune system is normal, it will recognize and attack the virus. This makes it impossible for COVID-19 to replicate and update in the infected person’s body and will be gradually eliminated. The number of viruses that can replicate then drops to a certain level. Even if the virus does not die and is clean, the amount of live virus is too small to spread, and the infectious period is over.
The incubation period and infectious period caused by the life cycle of the virus itself will not be “largely changed” casually. It is known that among hundreds of millions of infections around the world, only a few cases of severe immunosuppression have really clearly broken through this conventional incubation period and infectious period. These people’s immune systems are severely suppressed for other reasons and cannot clear the virus, so there are cases where COVID-19 continues to replicate and the human body is in a state of infection for a long time.
COVID-19 is often cited as two special ones-asymptomatic infection and sequelae, which do not change the incubation period and the infectious period. Asymptomatic infection is also an infection. It’s just that for COVID-19, there are a large proportion of infected people who have no obvious symptoms during the entire infection process.
Even if there are no symptoms, the virus in these people has experienced a cycle of amplification from the beginning to the peak of the virus load, and then gradually decline to the final elimination. The asymptomatic infection of COVID-19 and the carriers of diseases such as hepatitis B are not a concept at all, and should not be confused as long-term carriers of the virus.
The sequelae are the same. The infectious period of the virus corresponds to the presence of a live virus in the body, and the amount of the virus is sufficient to spread to others, which has nothing to do with whether the infected person still has symptoms.
Not to mention the sequelae. When some infected people become severely ill, the amount of virus in their bodies has passed the peak, and they are no longer infectious. Symptoms, especially severe ones, correspond to the damage of human tissues and organs.
The so-called geese passing by, the virus may have finished its life cycle early, but its damage to human tissues can survive, allowing symptoms to exist. Even the human immune system is activated by the virus, and under stress, it sometimes over-exerts. Even if the virus is gone, it is still producing anti-viral weapons such as inflammatory factors, which can cause physical damage.
3. Based on science, do not neglect humanity
Since the incubation period and the infectious period are determined by the life cycle of the new coronavirus itself, when we see some very “strange” nucleic acid test results, we should not blindly judge that the subject has a special incubation period or infectious period.
For example, if you see an infected person who is still positive in the nucleic acid test long after the infection, or suddenly becomes positive after several negative tests. Very long infection period or very long infection period is a more “intuitive” judgment, but it is very likely to be wrong. As explained above, the new coronavirus has a life cycle and is not a virus that can exist for a long time in the human body-it can exist for a long time in the population through continuous transmission, but it cannot survive or hide in individuals for a long time.
Compared with long-term infection or ultra-long infection period, which is contrary to scientific principles, a more reasonable possibility is that this kind of long-term or short-term rejuvenation is the result of nucleic acid detection catching the remaining virus corpse in the body. An imperfect verification method is to take the sample to the laboratory and cultivate it to see if there is a live virus that can infect cells. Of course, this verification is affected by experimental methods and other factors, and the accuracy is not necessarily high.
The other is to observe whether these so-called long-term positve are contagious in reality. As far as I know, so many COVID-19 studies seem to have no clear evidence to point out the spreading incident caused by long-term positve. The side verifies that these nucleic acid tests only caught dead viruses.
The incubation period is the same. The so-called ultra-long incubation period is often based on our preconceived belief that it is impossible for an infected person to encounter the virus in a certain place for a period of time, so the virus can only take root in this person before that, and it has been found to be positive recently. It means that it has been an incubation period before.
But this kind of thinking is based on a certain degree of prejudice. So much scientific evidence points to the fact that COVID-19 is not a virus that can stay “quietly” in the human body-even if it is asymptomatic, there is no virus replication that leads to a positive nucleic acid test? It even stimulates the body’s immune response and causes antibody positivity. Rather than stick to prejudices, consider whether there are loopholes in the hypothetical transmission route.
Finally, we must not forget the purpose of studying the incubation period and infectious period, and using nucleic acid and other testing methods to determine whether the subject is infected. From a micro perspective, we want to confirm whether a person is an infected person and whether it is still at a stage where the infection can continue. From a macro perspective, we want to block the spread of the virus by searching for and isolating people who are at risk of infection and close contacts.
Identifying a person as an infected person or judging who needs to be isolated will cause a lot of inconvenience to these people, and even other effects. The law summarized by a large number of COVID-19 infection cases in the world, from the occurrence of the infection to the diagnosis, the upper limit of the incubation period is about two weeks.
For confirmed cases, the time period for the spread of the virus, that is, the infectious period, is about 10 days. Although it is said that in order to prevent and control the epidemic, it is not unreasonable to increase the quarantine time on the basis of these rules to further reduce the risk. But the minimum scientific laws must be respected.
There is no need to keep increasing the isolation time for people who have already passed the incubation period or the infectious period, otherwise, wouldn’t they fall into a logical vicious circle of isolation for the sake of isolation?
(source:internet, reference only)
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