The operation principle of the immune system is the starting point for understanding everything
The human immune system is a highly intelligent defense network used to identify and eliminate foreign pathogens. It consists of two major systems: innate immunity and acquired immunity. The former is fast and broad but lacks specificity, while the latter can respond precisely to specific viruses or bacteria and has a "memory" function.
When a person is first infected with a virus or vaccinated, the immune system recognizes the "antigen" structures on the virus's surface and generates specific antibodies and memory T cells. If the same or similar virus is encountered again in the future, the immune system can quickly activate its defense mechanisms to prevent infection.
However, a key prerequisite for this mechanism is that the antigen structure of the virus remains stable. Once the virus alters its shell structure through mutation, it may evade immune recognition, entering the realm of "immune escape."
Immune escape: The virus's invisibility and survival strategy
"Immune escape" refers to the virus's ability to change its surface antigens through continuous mutation, making it difficult for antibodies formed by infection or vaccination to recognize it, thus allowing it to reinfect the host. It is a result of natural selection in the virus's evolutionary process.
Taking the novel coronavirus as an example, the original strain spread rapidly in 2020, and subsequent variants such as Alpha, Delta, and Omicron each exhibited stronger transmissibility and higher immune escape capabilities. The Omicron variant, due to over 30 mutations in its spike protein, has made it difficult for many existing antibodies to fully neutralize it, allowing even those who have received two doses of the vaccine to potentially become reinfected.
The mechanisms of immune escape mainly include:
Antigen drift: Small-scale mutations occur in the virus's surface proteins during replication, gradually altering antigen characteristics.
Antigen shift: Gene recombination between viruses or cross-species infections produce entirely new antigen structures, commonly seen in viruses like influenza.
Glycosylation masking: The virus "masks" key sites by adding sugar structures, preventing antibodies from binding accurately.
Such mutations typically do not make the virus more lethal but enhance its transmission potential and weaken the protective effect of vaccines. For this reason, many vaccines need to be updated regularly; for example, the influenza vaccine is adjusted almost every year.
Immune escape does not equate to vaccines being "ineffective." Although the effectiveness in blocking infection may decrease, vaccines can still effectively prevent severe cases and death. This has been validated by a large amount of real-world data following COVID-19 vaccinations.
Herd immunity: Transforming individual defenses into collective barriers
In contrast to immune escape, there is another key concept: herd immunity.
The basic principle of herd immunity is that when a sufficient proportion of the population gains immunity (through infection or vaccination), the virus has difficulty spreading within the population, and even a small number of unimmunized individuals can receive "indirect protection." The transmission chain is interrupted, and the epidemic gradually declines naturally.
The realization of herd immunity relies on two key parameters:
Basic reproduction number R₀: The average number of people an infected person transmits the virus to under no intervention conditions. The higher the R₀, the higher the proportion of the population needed for herd immunity.
Vaccine efficacy E: The ability of the vaccine to prevent infection; the higher the efficacy, the easier it is to form an immune barrier.
The calculation formula is: Herd immunity threshold = 1 – 1/R₀ ÷ E
For example, if R₀ is 5 and vaccine efficacy is 90%, then at least about 78% of the population needs to be vaccinated to achieve herd immunity.
Historically successful cases of achieving herd immunity include:
Smallpox: Eradicated through global vaccination;
Measles: High vaccine coverage has led to zero transmission in most areas;
Polio: Oral vaccination has significantly reduced incidence rates.
However, in reality, herd immunity is also affected by many variables, such as virus mutations, vaccine hesitancy, and immune decline. Particularly, the novel coronavirus, due to its immune escape ability, has made complete herd immunity difficult to achieve, leading more countries to choose a "controlled coexistence" strategy.
The interaction between the two: A dynamic offensive and defensive confrontation
Immune escape and herd immunity actually constitute an **"evolution-response" mechanism**. The virus enhances its transmission ability through escape, while humans strengthen their collective defenses through vaccination. The dynamic game between the two forms the main theme of contemporary infectious disease prevention and control.
This process includes the following stages:
Initial stage: Virus outbreak, population without immunity, rapid spread;
Intervention stage: Vaccine development and vaccination, establishing partial herd immunity;
Escape stage: Virus mutation, breaking through some immune barriers;
Response stage: Updating vaccines, booster shots, precise prevention and control;
Stabilization stage: Achieving a balance between high immunity levels and coexistence with the virus.
Each round of mutation in the evolution of the novel coronavirus reflects the virus's adaptation to human interventions. For example, from Delta to Omicron, although pathogenicity has weakened, transmissibility and escape ability have significantly increased, prompting a shift in vaccine strategy from "blocking infection" to "reducing severe cases."
This tug-of-war between offense and defense will not end in a short time. Understanding this logic helps the public maintain reasonable expectations regarding policy adjustments, vaccination rhythms, and the ebb and flow of the epidemic.
Real-world cases: Insights from COVID-19, influenza, and other diseases
COVID-19: The most controversial herd immunity experiment globally
In early 2020, some countries (such as the UK and Sweden) attempted to establish herd immunity through "natural infection," but the uncontrolled epidemic led to a large number of deaths. Later, they shifted to large-scale vaccination and public health interventions. Although vaccines did not block all transmission, they significantly reduced hospitalization and mortality rates, proving the value of "partial herd immunity."
Influenza virus: An annual example of dealing with immune escape
The influenza virus undergoes antigen drift every year, leading to immune escape, which is why the vaccine is updated annually. Even so, the vaccine can reduce hospitalization risk by 40-60%. This "catch-up defense" may not eradicate influenza but effectively controls its scale and mortality rate.
Measles and polio: Successful models of herd immunity
The measles vaccine has an efficacy of up to 97%, with a herd immunity threshold of about 95%. In areas with sufficiently high vaccination coverage, measles has nearly disappeared. However, once vaccination rates decline, outbreaks can occur. The resurgence of measles in multiple areas in the U.S. in 2019 due to vaccine hesitancy serves as a warning.
These cases indicate that the transmission characteristics of different diseases and the properties of vaccines determine the difficulty of the game between herd immunity and immune escape. When facing newly emerging pathogens, flexible and scientific strategies are particularly important.
Bridging the gap from scientific consensus to public understanding
Although "immune escape" and "herd immunity" have long been basic concepts in virology and epidemiology, these terms are often too abstract for the public. Bridging the information gap is necessary to transform them into effective social consensus and behavioral guidance.
The following points help build healthy cognition:
Recognize that vaccines are not absolute barriers but "filters" that reduce risk: breakthrough infections may occur, but vaccines significantly reduce severe cases and transmission.
Understand that the continuous change of viruses is a natural law: humans cannot control whether viruses mutate, but they can delay their spread by enhancing collective immunity.
Abandon the absolutist thinking of "complete eradication": In the face of globalization and highly mutable viruses, moderate tolerance of the virus's existence and maintaining social operation is a realistic choice.
Support scientific responses rather than panic avoidance: Understanding "immune escape" does not mean vaccines are useless, but rather requires us to respond more flexibly to viral evolution.
Advocate for a collective defense from society: Herd immunity requires a sufficient proportion of the population to participate in vaccination; it protects not only oneself but also others.
In the future, facing more unknown pathogens, humanity will need not only advancements in vaccine technology but also a collective understanding and cooperation regarding disease prevention logic. "Immune escape" and "herd immunity" are the core keywords of this logic.