Immune Response and Side Effects Are Not the Same Thing
Discomfort after vaccination can generally be divided into two categories: immune responses and adverse events. In a medical context, there is a fundamental difference between the two.
An immune response is the process by which the body recognizes the components of the vaccine and activates its defense system, manifested as local redness, swelling, low fever, fatigue, etc., and is considered a normal physiological response. For example, soreness in the arm after receiving a tetanus vaccine or a slight fever after receiving a flu vaccine is a sign that the immune system is "learning to recognize the enemy."
In contrast, **adverse events** refer to situations that exceed the normal range of reactions and may cause significant discomfort or health risks, such as anaphylactic shock or neurological abnormalities. Fortunately, these serious adverse reactions are extremely rare, with an incidence usually below one in a million.
For example, data from the U.S. Centers for Disease Control and Prevention shows that about 60% of vaccine recipients experience soreness at the injection site after the first dose of the COVID-19 mRNA vaccine, and 30% experience fatigue or low fever, but the incidence of severe allergic reactions (such as anaphylactic shock) is only about 5 cases per million doses.
Therefore, "discomfort after vaccination" should not be uniformly regarded as a "side effect" of the vaccine. Many times, those seemingly uncomfortable reactions are precisely manifestations of the vaccine working.
The Scientific Mechanism Behind "Fever After Vaccination"
Fever is one of the most commonly mentioned discomfort reactions after vaccination. Many parents notice that their children develop a mild fever after vaccination, which often causes anxiety and concern.
In fact, this is a systemic response caused by immune activation. After the vaccine enters the body, the immune system recognizes it as a "foreign antigen" and quickly releases cytokines (such as interferons, interleukins, etc.), which act on the brain's temperature regulation center, causing an increase in body temperature.
This "programmed fever" has positive significance for the formation of immune memory. Studies have shown that moderate fever can enhance antibody production and T cell activity, making the body's recognition of pathogens more lasting and accurate.
For example, in a study in the UK involving infants, mild fever after receiving the DTP vaccine was positively correlated with higher protective antibody titers. This indicates that fever is not a sign of the vaccine being out of control, but rather the body is seriously "rehearsing."
Of course, excessively high temperatures or persistent high fever lasting more than 48 hours should be taken seriously, and medical attention should be sought to rule out other infections or individual abnormal reactions. However, the vast majority of cases of "fever after vaccination" usually resolve naturally within 1-2 days without special treatment.
What Are Common Discomfort Reactions After Vaccination?
Different types of vaccines may have slightly different reactions due to variations in components, dosages, and adjuvants, but they can generally be divided into local reactions and systemic reactions:
Local Reactions (higher incidence)
Redness, swelling, pain, and hardening at the injection site
Mild heat or itching of the skin
Mild pressure discomfort, usually alleviated within 1-3 days
Systemic Reactions (rare, but also common in children)
Fever (low fever is common; attention is needed if it exceeds 38.5°C)
Fatigue, muscle soreness, headache
Nausea, slight decrease in appetite
Individual Adverse Reactions (extremely rare)
Allergic reactions (such as urticaria, asthma attacks)
Anaphylactic shock (such as difficulty breathing, pale complexion, extremely rare)
Neurological reactions (such as facial paralysis, seizures, requiring further medical evaluation)
For example, among those receiving the COVID-19 mRNA vaccine (such as Moderna, Pfizer), about 15% report fatigue or muscle soreness, but most resolve within two days. Inactivated vaccines widely used in China (such as Sinovac, Sinopharm) have a lower proportion of reactions due to their mild components, making them suitable for the elderly.
It should be clarified that discomfort reactions after vaccination are usually short-lived and manageable, far less severe than the symptoms caused by the virus itself. For instance, the risks of fever, cough, chest tightness, and even respiratory failure from COVID-19 infection are significantly higher than the discomfort caused by the vaccine.
Real Cases: Reflection of Vaccination Reactions and Individual Differences
Public concerns about vaccine side effects often stem from individual experiences around them. The following two real cases help clarify the nature of the reactions:
Case 1: A young teacher experiences low fever for two days after receiving the nine-valent HPV vaccine
Ms. Chen, 29, experienced mild low fever (37.8°C) and slight fatigue for about 48 hours after receiving the first dose of the nine-valent HPV vaccine, which resolved on its own. No other discomfort was reported. Subsequent second and third doses had no reactions.
→ The doctor judged it to be a normal immune activation process, requiring no intervention, and advised increased fluid intake and rest. Such reactions indicate that the vaccine is working.
Case 2: A 5-year-old boy experiences high fever and convulsions after receiving the DTP vaccine
A 5-year-old boy in Guangzhou developed a high fever of 39.5°C and experienced febrile seizures on the night of vaccination with the DTP vaccine, requiring hospitalization for observation. After ruling out other infections, the hospital assessed it as febrile seizures, not directly caused by the vaccine components.
→ Pediatric experts explained that high fever and convulsions are common physiological responses in children, related to the timing of the vaccine but not causally linked, and that subsequent vaccinations could continue with enhanced monitoring.
These two cases illustrate that even when receiving the same vaccine, reactions can vary from person to person, with most experiences being safe and mild. Even if there are occasional stronger reactions, it does not mean the vaccine has "problems," but may simply be due to individual differences.
Why Do Vaccines with High Safety Still Need Close Monitoring?
Although the overall safety of vaccines has been rigorously tested in clinical trials and approved for market use, adverse reactions after vaccination still require close monitoring. This does not indicate that the vaccine "has problems," but is a reflection of a high level of responsibility for public health.
Countries around the world have established vaccine adverse event monitoring systems (such as China's CNAEFI system and the U.S. VAERS system) to receive reports of any abnormal reactions after vaccination from doctors and the public, which are then analyzed for causal relationships by professional institutions.
The significance of this mechanism lies in:
Identifying extremely rare side effects (such as allergies to vaccine components in certain populations)
Dynamically assessing the performance of different vaccines in different regions and populations
Optimizing vaccination strategies to improve vaccination safety and public confidence
For example, in 2021, the European Medicines Agency noted that the AstraZeneca COVID-19 vaccine was associated with thrombosis in a small number of recipients, and after data verification, adjusted the recommended population to avoid risk spread.
The existence of such monitoring mechanisms precisely indicates that modern vaccine management not only focuses on "effectiveness" but also on "safety," and is willing to take responsibility for individual uncertainties.
How to Establish Rational Public Awareness of Vaccine Reactions?
In an era of rapid information dissemination, vaccine reactions are often amplified online. Reports of someone developing a fever, fainting, or even being hospitalized after vaccination can easily cause public panic. However, from a scientific perspective, most of these events are coincidental and not causally related to the vaccine.
Establishing rational awareness requires the following points:
Understanding the normal range of vaccine reactions: fever, soreness, and fatigue are common and do not require excessive worry.
Recognizing the criteria for identifying real adverse reactions: persistent high fever, allergic symptoms, and abnormal mental states require timely medical attention.
Viewing individual differences scientifically: one cannot infer the safety of the entire vaccine population based on the reactions of a few individuals.
Consulting professional doctors rather than online comments: those with vaccination concerns should seek individual assessments from doctors rather than relying on online opinions.
Paying attention to safety reports from authoritative institutions: information released by the Chinese CDC and the World Health Organization is more credible.
Vaccines are a public health intervention that plays a significant role in reducing disease incidence, controlling sources of infection, and protecting vulnerable populations. Compared to the slight discomfort caused by vaccines, the risks, complications, and medical burdens caused by diseases themselves are far more concerning.