The COVID-19 vaccine booster shot is launched amid controversy. What is the controversy about the third shot? Is the third shot necessary?

With the third wave of the epidemic hitting, confirmed cases of COVID-19 have appeared in many provinces and cities. At the same time, the third booster shot of the COVID-19 vaccine has begun to be administered in many cities.

The launch of the third booster shot has always been controversial. Since the development of the vaccine, scholars from various countries have launched a heated discussion on whether the booster shot is necessary. In the early stages of the research, everyone even reached a consensus that the booster shot was not necessary. Why did the third shot be restarted later? What is the controversy about the third shot? Is the third shot necessary? Today, we will unveil the mystery of the third booster shot of the new crown vaccine.

How does the COVID-19 vaccine work in the body? As the saying goes, knowing yourself and your enemy ensures victory in every battle. To understand how the COVID-19 vaccine works, we need to first understand how the coronavirus causes disease.

When the new coronavirus enters the human body, facing the iron wall of the cell, the cunning virus shows its "key" - the spike protein (S protein). The spike protein can bind to a substance called angiotensin-converting enzyme 2 on the cell membrane. After the binding, the cell's door is wide open, and the virus takes the opportunity to enter the cell and replicate, gradually growing and eventually successfully infecting and causing disease.

Understanding the pathogenicity of the new coronavirus makes it easier to understand the mechanism of action of the new coronavirus vaccine. Since the virus enters the human body through its own "key" - the spike protein, we stick to the spike protein in advance so that it cannot open the door, then the virus naturally cannot replicate and infect, and the new coronavirus vaccine plays an extremely important role in this process.

We can think of the COVID-19 vaccine as a kind of advance rehearsal. Take the inactivated vaccine we received as an example. The vaccine is very similar to the COVID-19 virus, but it is not pathogenic to the human body. When the vaccine enters the human body, it stimulates the immune system to respond. It can not only quickly produce antibodies against the virus, but also remember the appearance of the virus in a cell called "memory cells". When the virus really invades, the human body will quickly mobilize antibodies and "memory cells" to fight the virus. The antibodies in the body first recognize the "key" of the COVID-19 virus - the spike protein and bind to it, so that the virus cannot bind to the cell membrane angiotensin-converting enzyme 2, and it cannot easily enter the cell; on the other hand, memory cells will quickly recognize the virus and produce more antibodies to fight the virus, thus preventing us from being infected.

Why is there controversy over the third booster shot? The controversy over the third booster shot of the COVID-19 vaccine has a long history. Since the first vaccine was released, there has been debate over whether to get a booster shot. Even in the early stages of the epidemic, people generally believed that there was no need for a booster shot.

First of all, the scale of the epidemic at that time was only a local outbreak, far from the current global spread. Everyone generally believed that with the warming weather and vaccination, the new coronavirus would soon be controlled or even eliminated.

Secondly, although the level of neutralizing antibodies in the body will gradually decrease over time, due to the existence of "memory cells", when the virus enters the human body, the memory B cells will quickly recognize the virus and produce a large number of antibodies through humoral immunity, which will also play a role.

For the above reasons, scholars have always believed that booster shots are not necessary, but as the epidemic becomes more and more serious, the number of confirmed cases increases day by day, especially breakthrough infections occur more and more frequently, and the call for booster shots is getting louder and louder. The so-called breakthrough infection means that the new coronavirus is still infected after two shots of the vaccine, which means that the immune barrier constructed by the two shots of the vaccine can no longer effectively prevent the new coronavirus.

There are two main reasons for the frequent occurrence of breakthrough infections. The first is that as the virus mutates, the effectiveness of the vaccine is decreasing. The second is that as the time since the second vaccination increases, the level of neutralizing antibodies in the body gradually decreases. The increase in the level of neutralizing antibodies requires a third booster shot to increase.

Why do we need to get a third booster shot of COVID-19? The third booster shot is to restore the level of neutralizing antibodies in the body to a relatively high level. Some people may say that when the virus strikes, the memory B cells in the body will also produce a large number of antibodies, so why do we need to get a booster shot?

It takes time for memory B cells to be activated and produce antibodies. For some viruses with a long incubation period, memory B cells are activated before the virus replicates, and the body has enough antibodies, so there is no need for booster shots. However, the new coronavirus belongs to the coronavirus family, which has a relatively short incubation period. The Delta strain that is currently prevalent has a shorter incubation period through mutation, so memory B cells have no time to fully activate the virus before it replicates quickly, which has led to a large number of breakthrough infections.

The famous medical journal Nature Med published an article on neutralizing antibody levels and prevention of COVID-19 infection. The study found that when the neutralizing antibody level in the body reaches 20.2% of the neutralizing antibody level of recovered patients, more than 50% of people will avoid symptomatic infection. When the neutralizing antibody level in the body reaches 3% of the neutralizing antibody level of recovered patients, half of the people will still avoid becoming seriously ill, which also explains why many breakthrough infection cases are mild patients. This study shows that the level of neutralizing antibodies in the body plays an important role in preventing infection with the new coronavirus, and also indirectly reflects that the third booster shot is very important.

Summary: With the delay of the second dose of the vaccine, the level of neutralizing antibodies in the body gradually decreases, and the level of neutralizing antibodies directly determines the ability to prevent the new coronavirus. Although a large number of memory B cells will also be produced after exposure to the virus, the new coronavirus represented by the Delta strain has a short incubation period, and the virus often replicates before the memory B cells are fully activated, so a third booster shot is needed to avoid breakthrough infection.