Asymptomatic COVID-19 infection appears, do you need to worry?

# How to find asymptomatic infected people #

By the end of March 2020, the epidemic situation in China has been effectively controlled, with dozens of new confirmed cases every day, most of which are imported cases. However, the global development of COVID-19 epidemic shows no signs of slowing down, and the total number of confirmed cases has exceeded 654.38+0 million.

At the same time, another concept came into public view last month, which triggered a new discussion-asymptomatic infected people. How many asymptomatic infected people are there around us? Will you be infected inadvertently?

Wang Liming, a professor at the Institute of Life Sciences of Zhejiang University and the author of Get, will answer the concerns of asymptomatic infected people. Welcome to check the following article.

What is an "asymptomatic infected person"

"Asymptomatic infected person" refers to a person who is infected by COVID-19 virus but shows no symptoms of the disease.

Specifically, the gold standard for judging whether a person is infected with COVID-19 is nucleic acid detection-detection of upper respiratory tract samples. If the COVID-19 gene fragment is confirmed, the infection can be confirmed.

However, I want to remind you that there are obvious "sensitivity" problems in nucleic acid detection technology. A considerable number of infected people cannot be detected. If the test is not up to standard, the proportion will even be as high as 50%.

If we find a person who has no symptoms but has a positive nucleic acid test, he may be in three different situations before we follow him up in depth-

The first one is asymptomatic before onset.

I was infected, but during the incubation period, I didn't show any symptoms. This period usually lasts for three to five days, and in rare cases, there is an incubation period of several weeks.

Second, the whole process is asymptomatic.

From the time he was infected to the time the virus was cleared from his body, he didn't show any symptoms at all, or the symptoms were so mild that he didn't even notice.

Third, there are no symptoms after the onset.

After being infected by the virus, the typical symptoms of COVID-19 also appeared, but after rest or treatment, the symptoms disappeared. However, although the symptoms disappeared, the nucleic acid test was still positive, or it turned negative first and then turned positive.

These three kinds of people can be defined as "asymptomatic infected people" for a period of time in the process of being infected by COVID-19.

In fact, there is a difference between the second kind of people who have no symptoms at all and those who have very mild symptoms. However, a person's grasp of his physical state varies from person to person and is also influenced by his mood and state of mind, so it is difficult to distinguish between black and white.

Different asymptomatic infection types will determine different control measures.

How many asymptomatic infected people are there

This is a difficult question to answer. Because patients with mild and asymptomatic symptoms are difficult to find.

Even in China and South Korea, where nucleic acid testing is very intensive, only people who have symptoms and have had close contact with COVID-19 patients will receive nucleic acid testing. According to the data released by Qian Wei Health and Health Commission, at present, there are 1500 asymptomatic infected people in China under medical observation, almost all of whom are found by screening close contacts of COVID-19 patients.

At present, the measures taken by European and American countries to COVID-19 are that if a person does not have COVID-19's symptoms, he will not be tested for nucleic acid, and asymptomatic infected people will not be found at all.

This brings a big problem-there is no systematic method to comprehensively screen those who are completely asymptomatic, and they may become potential sources of infection and continue to spread the virus.

You might say, can we just give everyone a round of nucleic acid testing? Really can't.

On the one hand, the social cost will be unbearable.

At the peak of domestic diseases, the ability of nucleic acid detection is about several hundred thousand copies a day. Even if this number is expanded tenfold, it will take several years to thoroughly screen out the population of China. Moreover, the spread of diseases caused by crowd gathering is also a great hidden danger.

On the other hand, the sensitivity of nucleic acid detection will also become a limiting factor.

As a detection method with low sensitivity, even as low as 50% in some cases, even if the whole population is screened, a considerable proportion of "asymptomatic carriers" will be missed.

Of course, some small-scale studies can help us estimate the approximate proportion of these asymptomatic carriers.

For example, Japanese scientists systematically screened hundreds of Japanese citizens who evacuated overseas Chinese from Wuhan, and found 4 asymptomatic infected people and 9 symptomatic patients, with the proportion of asymptomatic infected people exceeding 30%.

For the passengers on the "Diamond Princess" cruise ship, Japanese scientists did nucleic acid tests on most of them and found that 634 people were nucleic acid positive. Among these people, 328 people, that is, more than 50%, showed no symptoms when they were tested.

On March 25th, the British Medical Journal mentioned that after carpet nucleic acid examination in an Italian village of about 3,000 people, it was found that about 50-75% of the infected people were asymptomatic.

Some mathematical models also support similar conclusions. On March 6th, scientists in China proposed in a paper published by MedRxiv, an open access platform, that at least 59% of people infected with COVID-19 will not be found because they have no symptoms or mild symptoms.

So, although there are some differences in the data, we can make a rough guess-if a COVID-19 patient is found at a certain time and place, it may mean that there are one or more asymptomatic infected people here and now.

How to deal with "asymptomatic infection"

Either way, asymptomatic infected people seem to have the same ability to spread the virus. This has been proved by many studies. Many cases of family transmission also prove that asymptomatic carriers can spread the virus to close family members.

Then of course we will ask: asymptomatic infected people are difficult to identify and it is difficult to spread the disease. Is it possible to completely control the epidemic?

In fact, the treatment measures for the three asymptomatic infected people are quite different.

The easiest thing to deal with is that the third type is asymptomatic after the onset, and the first type is asymptomatic before the onset. There are also more systematic treatment methods. The really tricky thing is that the second one is asymptomatic all the time.

Let's talk about the third one first. After the symptoms disappeared, the nucleic acid test was "re-positive". On the contrary, this situation is easier to identify and control-since it will happen, as long as the patient is found in time and the discharge standard is tightened, the flow of people can be avoided.

According to the current diagnosis and treatment plan, the patient can be discharged after the symptoms disappear and the nucleic acid test is negative twice. If the nucleic acid test is "re-positive", it is most likely that the nucleic acid test is "false negative".

Many studies have shown that COVID-19's patients also carry the virus in their feces. The discharge standard implemented in Shanghai comes from the "Shanghai 20 19 Expert Consensus on Comprehensive Treatment of Coronavirus", which particularly emphasizes that in addition to respiratory tract samples, stool samples of patients before discharge should also be tested negative for nucleic acid. Accordingly, there are very few "relapse" patients in Shanghai.

Even if this standard is not implemented, according to the current diagnosis and treatment plan of the Health and Health Commission, discharged patients still need to be isolated for 14 days, and regular review can also avoid the spread of patients after "recovery".

Let's talk about the first type, the infected person who has no symptoms before the onset and will get sick after the incubation period.

As long as the public health system can quickly and accurately identify it, isolate it for treatment, and track its close contacts for a period of time before the onset, it can be effectively managed.

In the first few months of 2020, we achieved rapid control of the COVID-19 epidemic in China in this way.

Of course, being able to spread in the incubation period means that many public management policies are needed to deal with the epidemic, such as reducing crowd gathering, closing cinemas and wearing masks in crowded places. It may last for a long time.

These measures can ensure that even if new patients appear occasionally, the number of people infected by him can be kept at a very low level, and the second outbreak of the disease can be prevented.

In fact, many recent model studies show that the effective public management measures taken in China in the past two months have rapidly reduced the actual number of virus infections R, that is, the average number of healthy people that a patient can infect, from 3-4 to below 1.

What we really need to worry about is the second asymptomatic infected person-a person with mild or even no symptoms during the whole process from being infected to clearing the virus in the body.

First of all, the proportion of these people is definitely not low. Like the study of the Diamond Princess, Japanese scientists speculate that about 20% of those infected with COVID-19 will have no symptoms from beginning to end.

Secondly, the virus content in this part of the human body may not be low, even if the transmission ability is lower than that of patients with fever and cough symptoms, it is still a source of disease transmission that cannot be ignored.

Moreover, due to the concealment of their diseases, it is impossible to prevent the spread of diseases through isolation.

Will the existence of these people cause public panic for a long time? I don't think so.

We might as well consider the existence of this part of asymptomatic infected people, what will be the result. For these asymptomatic infected people, according to our classification, because they have no symptoms at all, or their symptoms are extremely mild, they themselves do not need special attention and treatment.

What about the people they infected?

First, some infected people may still have no symptoms, or they may be the same asymptomatic infected person. They certainly don't need special attention and treatment.

At the same time, there should be another infected person who will have symptoms of COVID-19's disease, so that they can be recognized by the public health system and receive isolation treatment. Their close contacts will also be isolated and tested for nucleic acid.

In other words, the existence of asymptomatic infected people in the whole process will indeed lead to the long-term existence and spread of COVID-19 in society.

However, as long as we can identify those COVID-19 patients with symptoms in time, and find, isolate and detect their close contacts in time, the disease will not start a large-scale epidemic again.

In fact, there is another possibility-asymptomatic infected people will lose their communication ability because of mild symptoms, and the infected object is more likely to be the same asymptomatic infected person.

A small-scale study recently published in China Journal of Epidemiology also supports this view.

Among the close contacts of COVID-19 patients, 6.3% were sick, and most of them were diagnosed. Among the close contacts of asymptomatic infected persons, 4. 1% were sick, and half of them were asymptomatic infected persons.

If this phenomenon is supported more, it may mean that the pathogenicity of COVID-19 will gradually weaken in the process of transmission, and finally form a stable and less harmful parasitic relationship with human body.

What impact will it have on epidemic control?

1. Asymptomatic infection can be divided into three completely different situations: asymptomatic infection before onset, asymptomatic infection during the whole process and asymptomatic infection after onset.

Secondly, according to the latest research results, it can be roughly estimated that at any time, about 50% of COVID-19 infected people are in the asymptomatic infection stage.

Thirdly, effective control can be achieved by improving discharge standards, continuous diagnosis and treatment, isolation measures and maintaining some epidemic prevention measures.

Fourth, asymptomatic infected people do exist in the whole process, and the proportion is not low. This makes it possible for COVID-19 to coexist with us for a long time. But as long as appropriate measures are taken, the existence of such people will not lead to a second outbreak of the epidemic. Don't panic too much.

Based on the above discussion, I think we can reach a consensus:

At least until the vaccine is successfully developed and applied on a large scale, it will be a long-term and arduous task for China and the whole human world to fight against COVID-19.

For many places outside China, the urgent task is to take the lead in learning from the successful experience of China and South Korea, and curb the violent outbreak of the virus through large-scale detection, compulsory isolation and social grounding.

In China, we have made great achievements in fighting the epidemic in the past two months. Although the whole society has begun to reopen and resume activities, some epidemic prevention measures may still need to be adhered to for a long time to prevent the second outbreak of the disease.

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