With breakthrough infections occurring frequently, should we get booster shots?

A new round of "tour group COVID-19 infection" has spread to more than ten provinces, cities and districts, with 169 people infected. Almost all of the infected people in this wave of the epidemic have received more than two injections of the COVID-19 vaccine, but there have still been large-scale infections. Everyone can't help but ask, why can people still be infected after completing the COVID-19 vaccine injection? Can the vaccine really effectively protect us from the new coronavirus? Is there still any point in getting a booster shot?

Breakthrough infection

Here, we must first understand a term - breakthrough infection. Breakthrough infection refers to a positive result for the new coronavirus in a vaccine recipient more than 14 days after completing the vaccination (that is, the immune barrier has been established).

Breakthrough infections are not uncommon. According to a report from the Israeli Ministry of Health on July 24, as the country with the highest COVID-19 vaccination rate in the world, Israel has found a total of 5,770 breakthrough infections, including 123 deaths. And among the vaccinated population in the United States, there have also been tens of thousands of breakthrough infections.

(Former US Secretary of State Powell died of COVID-19 on October 18. He had already completed two injections of the vaccine. Powell's death once again triggered attacks on the COVID-19 vaccine by anti-vaccine activists)

Why do breakthrough infections occur?

Prime Minister, let us understand how the vaccine protects against Covid.

After the new coronavirus invades our body, the spike protein on the surface of the virus will bind to receptors such as ACE2 on normal human cells, thereby invading the human body. When we are vaccinated, the immune cells in our body will produce neutralizing antibodies. These neutralizing antibodies are precision-guided missiles that resist foreign invaders (viruses, bacteria and other pathogenic microorganisms). They can accurately identify the spike proteins on the surface of the new coronavirus and bind to them, preventing the spike proteins from binding to the ACE2 receptors, thereby playing a role in resisting the virus from invading human cells.

(Neutralizing antibodies bind to the coronavirus spike protein, preventing it from binding to the ACE2 receptor on normal human cells)

14 days after we complete the COVID-19 vaccine injection, the neutralizing antibodies in the body will increase significantly, thus forming an immune barrier. However, as time goes by, the level of neutralizing antibodies in the body will decrease significantly, and the protective effect of the vaccine will gradually decrease.

(14 days after the second injection of the inactivated COVID-19 vaccine, the level of neutralizing antibodies in the body reached a peak and then began to gradually decline)

All COVID-19 vaccines face this problem. A study by the University of California, San Diego, showed that after more than 10,000 medical staff received two doses of mRNA vaccine, their neutralizing antibody levels decreased over time, the number of breakthrough infections continued to increase, and the vaccine's effective protection rate dropped from 93.9% in March to 65.5% in July.

(The research results of the University of California, San Diego were published in the authoritative medical journal "New England Journal of Medicine". The study showed that the protective effect of mRNA vaccines will decrease over time)

On the other hand, in our immune system, there are two special immune cells, called memory B cells and memory T cells. When a pathogen invades the human body for the first time, the immune system will produce exclusive (specific) memory B cells and memory T cells for the pathogen, which will remember the characteristics of the pathogen. When the pathogen invades the human body again, the memory B cells and memory T cells will quickly awaken the immune system and quickly eliminate the invader. Memory B cells and memory T cells can exist for a long time, and some can even exist for a lifetime (this is why some diseases will never be contracted a second time after being contracted once)

In addition to producing neutralizing antibodies, the COVID-19 vaccine is designed to simulate the invasion of the COVID-19 into the human body and stimulate our immune system to produce memory B cells and memory T cells. When the COVID-19 invades our body, the memory B cells and memory T cells will quickly activate the immune system and eliminate the invading COVID-19 in a timely manner, thereby protecting our health.

However, it takes a certain amount of time for memory B cells and memory T cells to recognize the virus and activate the immune system. With the emergence of the Delta variant, the new coronavirus is more contagious and replicates faster. The virus may have invaded human cells before memory B cells and memory T cells have time to activate the immune system, resulting in infection.

The decline in neutralizing antibodies + viral mutation lead to breakthrough infections.

So is there still meaning in getting a booster shot of the new coronavirus vaccine?

Although the vaccine's effective protection rate will decrease over time, the vaccine has been very stable in reducing the severity and mortality rates, with no significant decline.

(Chile has a high COVID-19 vaccination rate. In April 2021, 75.84 people out of every 100 have completed COVID-19 vaccination)

According to data from the Chilean Health Bureau in August, the COVID-19 vaccine was 58.49% effective in preventing symptomatic infection, 86.02% effective in preventing hospitalization, 89.68% effective in preventing admission to intensive care units, and 86.38% effective in preventing death. The data released in April showed that the vaccine was 67% effective in preventing symptomatic infection, 85% effective in preventing hospitalization, 89% effective in preventing admission to intensive care units, and 80% effective in preventing death. It can be seen that although the vaccine has shown a significant decline in its effectiveness in preventing symptomatic infection, there has been no significant change in preventing hospitalization and severe illness, and there has been a significant increase in its effectiveness in preventing death.

This is because even though the virus has mutated and is more infectious, the memory B cells and memory T cells produced by the vaccination will eventually start at a faster rate, thereby activating the body's immune system to fight the virus, reducing the possibility of the disease developing into a severe illness and protecting the patient's life.

Therefore, even if the virus has mutated to a certain extent, vaccination is still an effective means to deal with the new coronavirus.

Recently, research data on booster shots of Sinovac and CanSino vaccines have been released one after another. After the booster shot, the level of neutralizing antibodies in the human body will be greatly improved. The level of neutralizing antibodies 14 days after the third dose of Sinovac vaccine increased by 3-5 times compared to the level of neutralizing antibodies 14 days after the second dose. CanSino's research also showed that booster vaccines can increase the level of neutralizing antibodies in the body by 7 to 10 times, thereby better protecting the vaccine recipients.

(The third booster shot effectively increased the level of neutralizing antibodies in the body)

Therefore, in order to defeat the epidemic, better prevent the new coronavirus, and build a herd immunity barrier more quickly, vaccine booster shots are an important and effective means, especially among key groups such as front-line medical staff, border port staff, isolation facility staff, and students studying abroad. It is very necessary to promote booster shots.

Author: Zhang Yuanchao, attending physician

Reviewer: Li Bin, Director of the Science Popularization Committee of the Chinese Society of Cell Biology, Deputy Director and Researcher of the Shanghai Institute of Immunology, School of Medicine, Shanghai Jiao Tong University

This article was published by Tencent's "Everyone Loves Science" team

Please indicate that the source of the reprint is from Science Popularization China