Why is it necessary to do nucleic acid testing for dynamic zero-COVID, and do it again and again?

The novel coronavirus has been raging on the world for almost three years. According to the official epidemic data released every day, as of May 5, the number of infections worldwide has reached 514,200,444, and the death toll has reached 6,257,258. This epidemic has not only seriously threatened human health and life, but also dealt a huge blow to the world economy.

At present, the spread of the new coronavirus is still rampant, and the strains are constantly mutating, and have transformed into five generations, namely the Alpha variant, Beta variant, Gamma variant, Delta variant, and Omicron strain.

The new generation of Omicron is more rampant and highly contagious. Therefore, although some countries in the world have adopted a "lie flat" policy, that is, open social interaction, China has always insisted on adopting a strict dynamic zero-clearing policy, the most notable feature of which is lockdown and nucleic acid testing.

Why is it necessary to do nucleic acid testing? Today we will discuss this topic specifically.

First, let's understand what nucleic acid is. Nucleic acid is the general term for DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). It is a biological macromolecule formed by the polymerization of nucleotide monomers and is one of the most basic substances of life.

Nucleic acids are widely present in all animal and plant cells and microorganisms. As long as there is a life with a cell form, such as animals, plants, and microorganisms, they contain both DNA and RNA, and use double-stranded DNA molecules as carriers of genetic material. But viruses are a special existence, and each virus contains only one type of nucleic acid, either DNA or RNA.

According to the different types of nucleic acids, viruses can be divided into two categories: DNA viruses and RNA viruses. Since different viruses contain different numbers and arrangements of ribonucleic acid nucleotides, they have certain specificities, which become markers to distinguish different viruses.

The new coronavirus is an RNA virus. Chinese scientists completed the decoding of the entire genome sequence of the new coronavirus in a very short time. By comparing it with other genome sequences, they found specific nucleic acid sequences in the new coronavirus, which provided conditions for distinguishing whether someone is infected through nucleic acid testing.

The new coronavirus prefers to settle in the human alveoli. Once it invades the human body, it will enter the respiratory tract through various channels, and then enter the human lungs along the respiratory tract, and finally settle in the alveoli. Therefore, the respiratory tract is the first part of the body infected by the new coronavirus.

In this way, by collecting people's respiratory mucus or blood samples and undergoing specific tests, it is possible to detect the presence of viral nucleic acid, thereby determining whether a person is infected with the new coronavirus. A positive result proves infection, while a negative result proves that no infection has been found for the time being.

The method and principle of nucleic acid testing is first of all sampling. Conventional sample types include throat swabs, nasal swabs, sputum, bronchial lavage fluid, alveolar lavage fluid, etc. The type of sample to be taken depends on the actual situation of the person being tested. The current universal screening sampling mainly takes throat swab samples.

After obtaining the patient sample, it is quickly placed in a sample storage tube and escorted to the laboratory under strict packaging. After the staff unpacks the package, they enter the information of each specimen into the system. Then professional technicians unscrew the screw caps of the tubes one by one in the biosafety cabinet, manually draw out the samples, and extract nucleic acids according to the procedures.

After complex preliminary preparation, the samples can be sent to the PCR instrument for amplification detection. This process takes one and a half hours and cannot be interrupted during the process, so there must be no power outages or machine failures. Any problem in any link may lead to rework.

After completing the above steps, the tester needs to analyze and review the amplification results, and give a final report after ensuring that they are correct. Based on the information entered at the beginning, the results will be transmitted to the health code management center. In this way, the complete report will arrive on the user's mobile phone.

The key technology here is PCR technology, namely polymerase chain reaction technology. This technology was invented by American chemist Kelly Mullis in 1983. This invention has a revolutionary impact on the biological community. Since then, biology has been divided into two eras, the pre-PCR era and the post-PCR era. For this, Kelly Mullis deservedly won the 1993 Nobel Prize in Chemistry.

To put it bluntly, without PCR technology, no genetic testing can be carried out today, including gene sequencing, paternity testing, and of course the current COVID-19 nucleic acid test. Now, professionals can magnify this trace evidence to reveal its true face by obtaining a tiny bit of DNA, which is widely used to solve various difficult cases and identify ancient creatures and historical figures.

The reaction template of PCR technology is only for DNA, and the new coronavirus is an RNA virus. Therefore, after obtaining the sample, the nucleic acid must first be extracted and reverse transcribed into cDNA, and then amplified in the PCR reaction system. The current PCR technology has developed to the third generation. The PCR instrument for detecting the new coronavirus is actually a temperature control device. The DNA undergoes a series of different temperature reactions in the instrument to obtain the test increment and complete the test.

The specific method is not described in detail, and is quoted as follows:

The most common method for detecting novel coronavirus-specific sequences is fluorescent quantitative PCR (polymerase chain reaction). Because the PCR reaction template is only DNA, the viral RNA needs to be reverse transcribed into cDNA first, and then amplified for detection. The PCR reaction system contains a pair of specific primers and a Taqman probe, which is a specific oligonucleotide sequence with a reporter fluorescent group and a quencher fluorescent group labeled at both ends.

When the probe is intact, the fluorescent signal emitted by the reporter group is absorbed by the quencher group; if the target sequence exists in the reaction system, the probe binds to the template during the PCR reaction, and the DNA polymerase uses the exonuclease activity of the enzyme to degrade the probe along the template, and the reporter group separates from the quencher group and emits fluorescence. For each amplified DNA chain, a fluorescent molecule is generated.

The fluorescence quantitative PCR instrument can monitor the number of cycles (Ct value) at which the fluorescence reaches the pre-set threshold value, which is related to the concentration of viral nucleic acid. The higher the concentration of viral nucleic acid, the smaller the Ct value. Products produced by different manufacturers will determine the positive judgment value of their products based on the performance of their own products.

PCR amplification and detection should use the fluorescent quantitative PCR instrument specified in the approved product instructions. The size of the sample Ct value obtained by fluorescent quantitative PCR can be used to determine whether the patient sample contains the new coronavirus.

PCR uses the fact that DNA will denature into a single strand at a high temperature of 95°C in vitro. At low temperatures (usually around 60°C), the primers and single strands will combine according to the principle of base complementary pairing. The temperature is then adjusted to the optimal reaction temperature of DNA polymerase (around 72°C). DNA polymerase synthesizes complementary strands along the direction from phosphate to pentose (5'-3'). The PCR instrument manufactured based on polymerase is actually a temperature control device that can well control the denaturation temperature, annealing temperature, and extension temperature.

Nucleic acid testing is currently the world's gold standard for confirming new coronavirus infection. It is the most effective standard, and theoretically the accuracy of the test can reach 100%.

Why are multiple nucleic acid tests needed? According to CCTV News on April 16, as of now, China has completed 11.5 billion nucleic acid tests. Combined with the large-scale tests in recent period, the average person has been tested nearly 10 times.

According to media reports, there are 13,100 medical and health institutions with testing capabilities across the country, and 150,000 technicians are engaged in starlight virus nucleic acid testing. Now 51.65 million tubes of testing can be completed every day, which means that no matter which major city or province in China has an epidemic and needs to test all employees, as long as we concentrate our efforts, it only takes one day to complete the testing.

This is an astonishing statistic that no other country can achieve.

During the nucleic acid testing process, we have enriched our experience, innovated our methods, and developed five-in-one, ten-in-one, and twenty-in-one mixed sampling testing technologies. We have continuously optimized testing strategies and improved efficiency, and now test results can generally be obtained within 6 hours.

However, this repeated testing has caused confusion among many people. Some people have already been tested dozens of times, so why do they need to be tested again and again? If the test is said to be 100% accurate, why are there still false positives and false negatives?

Experts explained this approach:

1. The characteristics of the Omicron variant are rapid transmission and strong concealment. Repeated screening as soon as possible can find all potential infected people more quickly, which is an important strategy to achieve dynamic zeroing; 2. There is a window period from infection to laboratory screening of viral nucleic acid. The incubation period of Omicron is about 3 days. Continuous screening is to find temporarily negative infections.

There are many reasons for false negatives. One of them is that the infected person has not formed a sufficient amount of nucleic acid sample during the window period mentioned above; another is errors in operation, such as inadequate collection of throat swabs, or errors in the storage and transportation process; another is that the detection instruments and test kits are unqualified and the sensitivity is not high, etc.

As for false positives, they are even rarer, and the reasons are basically errors, such as sampling contamination, instrument failure, operating errors, etc. For example, a netizen posted a post about a "positive" false positive during the nucleic acid test. After repeated investigations, the cause was finally found. It turned out that tubes and caps from different manufacturers were used during the operation, resulting in individual wells and caps not being sealed. The high temperature during the amplification process caused the liquid to evaporate, resulting in problems with the test results. (Picture above)

Therefore, this does not negate the accuracy of nucleic acid testing as the gold standard. From my personal understanding, repeated nucleic acid testing should also have a rule that if there is no infection today, it does not mean there will be no infection tomorrow. Only by doing nucleic acid testing every day can we ensure timely detection of infection and ensure safety.

But when will this practice end? Is there a better way to get society back to normal as soon as possible? What do you think? Welcome to discuss, thank you for reading.

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