Conclusion
Vaccines currently routinely recommended to the general population in the U.S.* can rarely cause syncope up to an hour after vaccination, most frequently among adolescents, and especially among females 11-18 years of age.
Potential injury from syncope after vaccination can be prevented by careful monitoring of vaccine recipients and having them sit or lay down if symptoms develop 1. The ACIP recommends that recipients always receive the vaccine while sitting and that providers observe adolescent and adult patients for 15 minutes after vaccination 2,3. To avoid a hysterical reaction among peers to a post-vaccination syncope case, it is also recommended that adolescents are vaccinated out of sight of others awaiting vaccination 4.
Epidemiological evidence
The 2012 report by the Institute of Medicine (IOM), now called the National Academy of Medicine (NAM), found no relevant studies of quality in the literature assessing an association between vaccination and syncope, since the only applicable studies available either had limited power or serious methodological limitations, or used passive surveillance systems and therefore lacked an unvaccinated comparison group 5. However, numerous case studies have provided strong mechanistic evidence, as described in the proposed biological mechanism section below.
A 2013 study by the U.S. Armed Forces estimated annual rates of syncope associated with immunization to be between 4.4 and 14.1 events per 100,000 immunizations 6.
Proposed biological mechanism
Syncope is usually caused by a vasovagal reaction in which sympathetic nervous system stimulation brings a sudden onset of hypotension. Potential stimuli for a vasovagal reaction include invasive medical procedures such as venipuncture, as well as simply the sight of blood in some persons 1.
The 2012 IOM report described a number of cases of syncope after vaccination 1,7-16. Due to the consistency of the prodromal symptoms, such as dizziness and pallor, and that most cases had a latency of 15 minutes or less between vaccine injection and the development of vasovagal syncope, the IOM concluded that this mechanistic evidence was strong and presented definitive clinical evidence 5. Syncope following vaccination has also occasionally been reported via passive surveillance systems 17.
* These conclusions do not necessarily consider vaccines recommended only for special populations in the United States such as Yellow Fever vaccine (international travelers) or Smallpox vaccine (military personnel), or vaccines no longer recommended to the public such as the Janssen (J&J) COVID-19 vaccine.
References
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