As of January 10, 2021, a reported 4,041,396 first doses of Moderna’s COVID-19 vaccine had been administered in the US. Reports of 1,266 (0.03%) adverse effects (AEs) after receipt of the vaccine were submitted to the Vaccine Adverse Event Reporting System (VAERS), according to researchers from the Centers for Disease Control and Prevention (CDC) COVID-19 Response Team and Food and Drug Administration in a Morbidity and Mortality Weekly Report early release .
The researchers screened VAERS reports that described suspected severe allergic reactions and anaphylaxis and collected information from medical records and outreach to healthcare facilities, providers, and recipients. They identified 108 reports for further review as possible severe allergic reaction, including anaphylaxis, a rare vaccination reaction. Ten cases were determined to be anaphylaxis—or a rate of 2.5 cases per million vaccine doses administered. Nine of the cases were people with a documented history of allergies or allergic reactions; 5 had a history of anaphylaxis.
The median interval from vaccine receipt to symptom onset was 7.5 minutes. Eight people had follow-up information available; all had recovered or were discharged. Of the case reports that were determined not to be anaphylaxis, 47 were assessed as nonanaphylactic allergic reactions and 47 were considered nonallergic adverse events. Four cases lacked enough information to be determined.
Based on those preliminary findings, it appears anaphylaxis is rare after the Moderna vaccination, but the researchers note that comparisons with other non–COVID-19 vaccines are constrained due to the limited information available this early in the vaccination program. They did cite an analysis of the Pfizer-BioNTech COVID-19 vaccine, also an mRNA vaccine, which estimated an initial rate of 11.1 cases per million doses after the first shot.
The researchers found a “strong female predominance” of anaphylaxis for both vaccines. All 10 anaphylaxis cases reported with the Moderna vaccine were in women. However, during the analytic period, 61% of first doses were given to women, vs 36% to men. Similarly, two thirds of first doses of the Pfizer-BioNTech vaccine were administered to women, and women were more affected.
Postvaccine COVID-19 Infections
But patients shouldn’t be too hasty to assume that symptoms after the vaccination are vaccine related, researchers at Israel’s Sheba Medical Center warn. The mere availability of a vaccine may lead to a certain laxity of precautions and a consequent rise in COVID-19 cases. “Thus, almost every physical complaint after vaccination poses a true diagnostic dilemma,” they point out, “as to whether an adverse reaction or a new COVID-19 infection is the cause.”
They studied 4,081 healthcare workers given the Pfizer-BioNTech vaccine. Of the vaccinated healthcare workers, 22 (0.54%) later had laboratory-confirmed COVID-19. Thirteen were tested because they had symptoms, usually an influenza-like illness that included fever, chills, cough, headache, myalgia, and sore throat. The median time between the first dose of vaccine and first symptoms was 3.5 days (one HCW had symptoms before immunization).
The vaccine, BNT162b2, is not likely to protect against clinical disease during the first days after receipt of the first dose, the researchers say. Efficacy was 52% a week after the first dose and positive COVID-19 cases were described among vaccinees even early after the second dose.