Why Do Vaccinated People Still Get Sick? Are Vaccines Ineffective?

Why Do Vaccinated People Still Get Sick? Are Vaccines Ineffective?

One of humanity’s most remarkable discoveries, vaccines have always faced some degree of hesitation. Many people mistakenly believe that vaccines are ineffective when vaccinated individuals still get sick. As a result, some choose to avoid vaccination, assuming they can fight off an illness naturally once exposed. This misconception fuels vaccine hesitancy, not only among those who hold this belief but also among those influenced by them. Vaccine hesitancy can arise for various reasons, but it primarily stems from a lack of awareness or a misunderstanding of how vaccines work. While vaccines do not offer complete immunity, they play a vital role in reducing the severity of illness, preventing complications and limiting the spread of infectious diseases.  

This article explores the reasons why vaccinated individuals might still get sick and highlights why vaccines remain one of the most effective tools in disease prevention.

What is vaccine and how do they work?

A vaccine is a biological preparation containing an inactivated or weakened pathogen, or a component of the pathogen such as nucleic acid, genetic material or a protein designed to stimulate the immune system. The process of administering the vaccine into the body is called vaccination. 1 When administered, the vaccine mimics a natural infection without causing illness, prompting the immune system to recognize a specific antigen from the pathogen. This triggers the production of antibodies and the development of memory cells, allowing the body to mount a quicker and stronger immune response if exposed to the actual pathogen in the future. However, vaccine effectiveness (measure of how well vaccines work in the real world) and vaccine efficacy (percentage reduction in the risk of a disease among vaccinated individuals compared to those who are unvaccinated, measured under controlled conditions) which is determined by reduction of infection, severity, hospitalization and mortality depends on several factors: (2,3)

 

  1. Immune response of an individual:

The immune response of an individual to a vaccine can be influenced by:  

  • Individual’s Age: Age is a key factor that affects vaccine response, particularly in infants and older adults. For example, Infants have an underdeveloped immune system and weaker cell-mediated immune responses, resulting in lower antibody production. Additionally, maternal antibodies may interfere with the effectiveness of vaccinations in newborns. Older adults often exhibit weaker responses to vaccines including seasonal flu vaccines, compared to middle-aged individuals. As people age, their immune system declines, leading to a reduced immune response. 4
  • Sex: Sex differences in vaccine responses are evident across various age groups, from infants to older adults. Generally, females tend to produce stronger antibody responses and are more likely to experience adverse reactions after vaccination compared to males.5
  • Nutritional status: Nutrient deficiencies can negatively impact the adaptive immune system, resulting in a weakened response to vaccines. Improving the nutritional status is found to improve the response to vaccine.6
  • Obesity: Obesity is another factor that influence the vaccine response. For example, obesity in people is significantly associated with the decreased response to hepatitis B vaccines.7
  • Gut Microbiota: The intestinal microbiome aids nutrient digestion and absorption as well as produce metabolites and antimicrobials that help to prevent pathogen adhesion to the intestinal membrane. The microbiome also plays a crucial role in regulating both innate and adaptive immune responses, influencing overall immunity.3
  • Host Genetic factors: One key contributor to variations in vaccine effectiveness across different populations is genetic variation, particularly in immune system-related genes such as the major histocompatibility complex (MHC). For example, the individuals with the homozygous CCR5Δ32 genotype exhibited a reduced immune response to the hepatitis B virus (HBV) vaccine, demonstrating how genetic factors can influence vaccine outcomes.8

 

  1. The types of vaccine:

The types of vaccine we are using also have a vital role in determining the vaccine efficacy, as different vaccine platforms stimulate the immune system in distinct ways. Several factors like immune response strength, duration of protection, and adaptability to mutations or variants influence efficacy and effectiveness of vaccines. Various types of vaccines are Live attenuated Vaccines, Killed or Inactivated Vaccines, Subunit, Recombinant, and Conjugate Vaccines, DNA and mRNA Vaccines, Viral Vector Vaccines, Toxoid Vaccines, Recombinant Vector Vaccines, Whole-Cell Vaccines, Therapeutic Vaccines, Plant-Based Vaccines and Nanoparticle Vaccines. Each vaccine type has its own strengths and limitations, its selection depends on factors such as pathogen characteristics, target population, safety profile, and logistical feasibility, including storage and distribution needs.9

 

  1. Mutation rate of the virus:

Mutations can impact virus-host cell interactions, alter binding affinity, affect Furin cleavage efficiency and reduce neutralization effectiveness by antibodies and vaccines.10

Why do vaccinated people still get sick?

In some cases, individuals may still contract an infection even after being vaccinated. This can happen due to various factors:

  1. Already Infected Before Vaccination:

Vaccines require time to stimulate an immune response, often taking several weeks to become fully effective. If an individual is already infected at the time of vaccination especially during the incubation period when symptoms have not yet appeared, the vaccine will not be able to prevent the illness. In such cases, the infection progresses before the immune system can build adequate protection.11

 

  1. Continuous Evolution and Mutation of Virus:

The rapid evolution of viruses, particularly through mutations, can impact vaccine effectiveness. A notable example is SARS-CoV-2, which has demonstrated how viral mutations can influence disease transmission and immune response. All viruses, including SARS-CoV-2, naturally mutate over time. Factors such as herd immunity, prior infections, medications, vaccination, and antibody responses create immune pressure, leading to the emergence of new variants. During replication, genetic changes can give rise to Variants of Concern (VOCs), such as Alpha, Beta, Delta, Gamma and Omicron, which have been reported worldwide.10 For instance, the Delta variant exhibited high transmissibility and contained mutations that allowed partial immune escape, reducing vaccine effectiveness and increasing the risk of reinfection.12 Mutations can enhance a virus’s ability to spread more efficiently and evade immune responses, potentially diminishing the effectiveness of monoclonal antibodies, convalescent plasma, and vaccines. Additionally, certain mutations may impact the accuracy of molecular diagnostic tests, leading to delayed detection, increased community transmission, and challenges in providing timely treatment.10

 

  1. Weak Immune Response to the Vaccine:

In some cases, vaccines may not generate a strong immune response in the body. Several factors can contribute to this, including underlying health conditions, age, nutritional deficiencies and individual variations in immune function. Additionally, if a vaccine is administered while a person is already ill, the immune system may prioritize fighting the existing infection rather than responding effectively to the vaccine. As a result, the body may not develop sufficient immunity, leaving the individual vulnerable to infection.11

 

  1. Vaccine Side Effects and immune response mistaken for Illness:

Vaccines work by mimicking an infection, stimulating the immune system to develop memory cells that enable a faster and stronger response if the actual pathogen is encountered in the future. This process can cause temporary symptoms such as fever, fatigue or mild body aches, which are signs that the immune system is responding to the vaccine. Additionally, some individuals may experience mild side effects after vaccination, which are often mistaken for illness. These side effects and immune response symptoms are typically harmless and resolve within a few days. However, misinterpreting these symptoms as vaccine failure can contribute to the misconception that vaccines are ineffective. 11

 

  1. Breakthrough infections:

A breakthrough infection occurs when a vaccinated individual becomes infected with the same pathogen they were vaccinated against because the vaccine has failed to provide complete immunity against the pathogen.13 In most cases of breakthrough infections, the symptoms are mild or asymptomatic, indicating that the vaccine effectively reduces disease severity and helps to prevent severe outcomes.14 Breakthrough infections are often caused by viral strains with mutations that enhance transmissibility, increase disease severity, reduce neutralization by antibodies from prior infection or vaccination, decrease effectiveness of treatments or vaccines or compromise diagnostic detection. Highly transmissible new variants have rapidly spread worldwide, replacing pre-existing strains.15

 

  1. Waning Immunity:

Over time, the immune response generated by a vaccine may gradually decline, leading to reduced protection against infection. The rate of waning immunity varies depending on the type of vaccine, individual immune response and the evolving nature of the pathogen. To enhance and prolong immunity, booster doses are recommended, helping to restore antibody levels and strengthen immune defense, particularly against emerging variants.16

Conclusion

Vaccination is a critical tool in controlling the infectious diseases. Before vaccines were introduced, smallpox and measles caused widespread fatalities, with smallpox epidemics wiping out nearly half of affected populations, while measles remained highly lethal among susceptible groups.17 Smallpox was the first human infectious disease eradicated through vaccination, a historic milestone achieved in 1979. 17 Over time, numerous vaccines have been developed, but it is important to remember that no vaccine can guarantee complete protection against infections due to the influence of various factors including host-related aspects such as age, sex, obesity, genetic variations and nutritional status as well as virus-related factors like mutations and the emergence of new variants. However, vaccines significantly reduce disease severity and mortality rates. Therefore, vaccinations should not be neglected, particularly in children. Receiving vaccines on time helps build immunity and ensures better health.   If you have concerns, consulting a healthcare professional can provide accurate information and guidance.  

References
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