Table I.

Risk of varicella zoster virus reactivation after messenger RNA COVID-19 vaccination

Cohort	Persons in cohort	Persons with VZV reactivation	Risk (per 1000 person-years)∗	Risk ratio, 95% CI
mRNA COVID-19 vaccination vs historical cohort†	555,256	673	16	0.91 (0.82-1.01)
	555,256	740	17
mRNA COVID-19 vaccination vs contemporary cohort‡	359,789	492	18	0.98 (0.87-1.11)
	359,789	501	18

The relative risk compares the risk of VZV reactivation within 28 days after mRNA COVID-19 vaccination against persons in control cohorts after matching for age, sex, race, ethnicity, HIV, malignancy, use of antineoplastics, use of immunosuppressants, and receipt of shingles vaccine. The diagnoses in the control cohorts were determined upon by authors to be conditions that do not have a known relationship to VZV reactivation. Multiple diagnoses were included to help increase cohort size for robust propensity-matching.

HIV, Human immune deficiency virus; mRNA, messenger RNA; VZV, varicella zoster virus.

^∗Risk per 1000 person-years were calculated as followed: (persons with VZV reactivation)/[(Persons in cohort) × (28/365)] × 1000).

^†The first (historical) control cohort comprised individuals who received a diagnosis of acne, viral wart, melanocytic nevi, dry skin, lipoma, skin cysts, or seborrheic keratosis between January 1, 2020 and December 1, 2020 and who had no history of COVID-19 vaccination to establish a cohort wherein COVID-19 vaccination was not readily available.

^‡A second (contemporary) control cohort comprised of individuals diagnosed between December 15, 2020 and July 15, 2021 to account for possible seasonal variation in VZV incidence.