ABSTRACT
OBJECTIVE: To assess the characteristics and associated disability of headache as an adverse event following vaccination. BACKGROUND: According to clinical trials and post-licensure surveillance, headache is a common symptom of vaccines, yet systematic investigations of post-licensure reports of this adverse event are lacking. METHODS: This was a retrospective database analysis study. We searched the Vaccine Adverse Events Reporting System (VAERS) database completed from July 1990 to June 2020 (a 30-year period prior to the start of COVID-19 pandemic) to identify reports of headache. We evaluated epidemiological features, including event characteristics, patient demographics, and vaccine type. RESULTS: In those aged 3 years or older, headache was the fifth most reported adverse symptom, present in 8.1% (43,218/536,120) of all reports. Of headache reports, 96.3% (41,635/43,218) included the code "headache" not further specified. Migraine was coded in 1973 cases, although almost one-third (12,467/41,808; 29.8%) of headache reports without a migraine code mention nausea or vomiting. The onset of symptoms was within 1 day of vaccination in over two-thirds of cases. The majority of reports were classified as not serious; about one-third involved emergency room or office visits. Of the 43,218 total headache reports, only a minority involved hospitalizations (2624; 6.1%) or permanent disability (1091; 2.5%), females accounted for 68.9% (29,771) and males for 29.5% (12,725), patients aged 6 to 59 years represented 67.3% (29,112), and over one-third of cases were reported after herpes zoster (8665; 20.1%) and influenza (6748; 15.6%) vaccinations. CONCLUSION: In a national surveillance system, headache was a commonly reported post-vaccination adverse event; a small subset of reports was considered serious. The development of standardized vaccine-related case definitions could be useful for better evaluating headache as an adverse event during vaccine development, and may reduce vaccine hesitancy especially in headache-prone individuals.