CDC Reports Adverse Events for Pfizer-BioNTech, Moderna COVID-19 Vaccines


With millions of mRNA vaccine doses administered, the US Centers for Disease Control and Prevention (CDC) provide information on these events.

CDC  covid-19 vaccines

This article, Adverse Event Rate Low in First Month of COVID-19 Vaccine, was originally published on HCPLive.

A new report from the CDC shows relatively low rates of adverse events from the coronavirus disease 2019 (COVID-19) vaccine.

Overall, between Dec. 14 and Jan. 13 there was more than 13.7 million vaccine doses administered and 6994 reports of adverse events following vaccination. Of the adverse events, 6354 (90.8%) were deemed non-serious and 640 (9.2%) were classified as serious.

To monitor vaccine safety and report adverse events, the CDC relies on the Vaccine Adverse Event Reporting System (VAERS), a spontaneous reporting system and v-safe, an active surveillance system.

Adverse Events

The most frequent adverse reactions were headache (22.4%), fatigue (16.5%), and dizziness (16.5%). There were also a total of 113 deaths reported to VAERS, including 78 (65%) among individuals in long-term care facilities. The median age of vaccine recipients reported in VAERS was 42, while 5505 (78.7%) of the adverse events reported occurred in women.

However, the CDC said the available information from death certificates, autopsy reports, medical records, and clinical descriptions from VAERS reports and health care providers did not suggest any causal relationship between COVID-19 vaccination and death.

The report also highlighted rare cases of anaphylaxis following the receipt of both vaccines, with 4.5 reported cases of anaphylaxis per million vaccine doses administered.

In addition, for the individuals who received both doses of the Pfizer-BioNTech vaccine, adverse reactions were more frequently reported to the v-safe system following the second dose.

The authors also identified 6844 (97.9%) adverse events from individuals not residing in long-term care facilities, 5533 (80.8%) from the the Pfizer-BioNTech vaccine and 1311 (19.2%) in individuals that received the Moderna vaccine.

“The initial post-authorization safety profiles of the two COVID-19 vaccines in current use did not indicate evidence of unexpected serious adverse events,” the CDC states. “These data provide reassurance and helpful information regarding what health care providers and vaccine recipients might expect after vaccination.”

VAERS is jointly operated by the CDC and US Food and Drug Administration (FDA). Reports of an adverse event to VAERS are followed up with direct outreach, including telephoning contacts listed in the VAERS report to gather additional clinical details and collecting medical records.

In December, the FDA issued Emergency Use Authorizations (EUA) for both the Pfizer-BioNTech BNT162b2 vaccine and Moderna’s mRNA-1273 vaccine.


In the FDA’s benefit-risk assessment of BNT162b2, they observed an overall benefit of reduced risk of COVID-19 at least 7 days following a two-dose vaccination regimen in individuals without prior history of SARS-CoV-2 infection.

They additionally assessed benefit consistent across subgroups stratified by age, race, gender, and comorbidity.

In observed risks, they noted the reported high rate of local and systemic adverse reactions, severe adverse events possibly associated with the vaccine including lymphadenopathy, limited data among observed adolescents 16-17 years old, and limited follow-up duration, plus risk assessment of pregnant women.


The mRNA-1273 is encoded for a prefusion stabilized form of the SARS-CoV-2 Spike S protein, a protein complex that is critical for membrane fusion and host cell infection. The vaccine was created in collaboration between Moderna and investigators from the Vaccine Research Center at the National Institute of Allergy and Infectious Diseases (NIAID).

Moderna’s mRNA-1273 is a new approach to vaccines to protect against infectious diseases. Although the technology of nucleoside-modified messenger RNA platform-hosted vaccines have been studied for decades, none have ever been employed for use in the US prior to COVID-19.

At the interim analysis provided to FDA in the EUA application, mRNA-1273 was found to have an efficacy of 94.5% (95% CI, 86.5%-97.8%) against confirmed COVID-19 14 days after the second dose in participants without evidence of a prior SARS-CoV-2 infection. In total, 11 cases of severe COVID-19 were observed, but all of them occurred in the placebo group.

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