Many people are concerned about the safety of the COVID-19 vaccines that have been developed so quickly worldwide. The adverse reactions to the latest COVID-19 vaccine are still unknown because it has not been tested or monitored in a large population. With so many situations or dangers unknown, people who are ready to receive the COVID-19 vaccine are full of doubts.

Despite the important role vaccines play in global health, concerns about vaccine safety have increased significantly over the years, for example, mandatory vaccination is recommended and mandated in many European countries to increase vaccination rates, however, in some cases, rare adverse reactions caused by vaccines have been widely reported, some of which have even led to hospitalization and death. Pfizer’s COVID-19 vaccine has recently been reported adverse reactions, including cases of facial paralysis, by major media. So, what is the safety of the vaccine really like? The adverse reactions following vaccination are reviewed and analyzed as follows.

Various adverse reactions with causes difficult to determine, but similar adverse reactions after vaccination

Vaccines are essentially either DNA, RNA, or primitive proteins or protein hydrolysates, therefore, they have some commonalities in adverse reactions. A journal under the prestigious journal Lancet has summarized the causal relationship between vaccines and adverse reactions and the characteristics thereof and assessed the causal relationship between adverse events following immunization (AEFI). The research results have certain authority, and according to the final findings, 12 AEFI have been established a causal relationship with vaccination, including anaphylaxis, arthralgia or arthritis (mild, acute and transient, not chronic), deltoid bursitis, disseminated varicella infection (in immune-deficient individuals), encephalitis, febrile seizures, Guillain-Barré syndrome, hepatitis (in immune-deficient individuals), herpes zoster, immune thrombocytopenic purpura, meningitis, and syncope.

The said adverse reaction of the COVID-19 vaccine possibly based on the pathogenesis of Guillain-Barré syndrome

All the above 12 adverse reactions constitute a possible causal relationship with vaccination, however, the rates of these adverse reactions are in fact extremely low or even very rare, except for mild, acute and transient arthralgia or arthritis. It is worth noting that Guillain-Barré syndrome is an autoimmune peripheral neuropathy characterized pathologically by the demyelinating lesions of peripheral nerves and nerve roots and small-vessel inflammatory cell infiltration, and the classic form of Guillain-Barré syndrome is called acute inflammatory demyelinating polyneuropathy (AIDP), with a clinical manifestation of acute symmetric flaccid limb paralysis. The recently reported facial paralysis caused by the COVID-19 vaccine may be a varied clinical manifestation of Guillain-Barré syndrome.

Overall, the first adverse reaction after vaccination being anaphylaxis

Hypersensitivity reactions after vaccination are rare and can be roughly divided into two categories based on the chronological order and extending time: those occurring immediately 4h after vaccination or those occurring chronically 4h thereafter. Reactions that occur immediately are generally mediated by IgE and require allergy tests. Normally, if patients to be vaccinated have a previously known allergy to the specific components of certain items (such as gelatin, yeast, latex, antibiotics, or other vaccines), they need to receive allergy tests before vaccination.

Normally, the second inoculation of the same vaccine being the most dangerous

Local reactions are the most common adverse events following vaccination. For example, the Arthus reaction, i.e., experimental local anaphylaxis, is relatively rare, where a larger and more severe local reaction usually occurs after vaccination, being a type III hypersensitivity reaction; such a type of reaction is characterized by pain, swelling, induration (hardening of tissues) and edema, and even severe necrosis or ulceration at the injection site. However, most mild cases are self-healing without treatment, and only severe cases require treatment with anti-allergic agents. Therefore, such a type of adverse reaction is often overlooked. According to a study of adverse reactions to vaccination, the rate of Arthus reactions after the first, second and third inoculation was 13.3%, 50.0% and 23.3%, respectively. Therefore, the second inoculation is generally the most likely to cause an Arthus reaction, probably because of the allergens produced in the body during the first inoculation.

Summary

The rates of adverse reactions to vaccines are in fact not high, less than 1‰ in general. Polio-related paralysis cases are even rarer. Although the probability is extremely low, the public needs to establish awareness of the safety of routine vaccines, reduce the fears associated with vaccine use, and maintain the necessary caution. Patients who are allergic or have a history of allergy to similar drugs may need to pay more attention to their reactions following vaccination. However, vaccination has excellent safety as a whole, can provide protection against infectious diseases for individuals and the general population, and can be called an angel for the majority and a devil for the very few.