Incidence and Characteristics of Delayed Injection Site Reaction to the mRNA-1273 SARS-CoV2 Vaccine (Moderna) in a Cohort of Hospital Employees

Jacobson, Mark A; Zakaria, Adam; Maung, Zaw; Hart, Colin; McCalmont, Tim; Fassett, Marlys; Amerson, Erin

BACKGROUND: COVID-19 vaccines, primarily mRNA types, are administered to 2,000,000 individuals per day in the US under FDA emergency use authorization. METHODS: Observational cohort study of hospital workers who received their first SARS-CoV2 mRNA vaccination between December 14, 2020 and January 8, 2021, including cases reporting onset of an injection site reaction > 48 hours after administration of their first or second dose to an employee hotline. RESULTS: Thirteen female employees, who received mRNA-1273 SARS-CoV2 vaccine (Moderna) during the first three weeks of SARS-CoV2 vaccine rollout at San Francisco General Hospital, reported a pruritic rash at the injection site appearing 3-9 days after receiving their initial dose. Five had milder or similar reactions with earlier onset after the second dose. One additional female employee reported this delayed reaction only after the second dose. None of these 14 employees reported serious adverse events, such as anaphylaxis, or had symptoms severe enough to seek medical attention. These cases represented 1.1% of the 1,275 female employees who received their first mRNA-1273 dose, and 2.0% of the 557 who were 31-45 years old, during this initial vaccine rollout. None of 675 males initiating mRNA-1273 or 3,612 employees of either sex initiating BNT162b (Pfizer) vaccination during this period reported this delayed onset reaction. CONCLUSIONS: These results suggest that delayed-onset, injection-site pruritic rashes after mRNA-1273 SARS-CoV2 vaccine administration, lasting up to a week, occur commonly in females, do not lead to serious sequela, and should not deter receipt of the second vaccine dose.