U.S. COVID-19 vaccine safety surveillance results for the over 65s

In a recent study posted to the medRxiv* preprint server, researchers conducted near real-time safety monitoring in the United States (US) elderly population for 14 safety outcomes weekly following the coronavirus disease 2019 (COVID-19) vaccination. In particular, the researchers focused on safety signals following vaccination with Pfizer BioNTech (Comirnaty), Moderna (mRNA-1273), and Janssen (Ad26.COV2.S) COVID-19 vaccines.

Study: Surveillance of COVID-19 vaccine safety among elderly persons aged 65 years and older. Image Credit: Prostock-studio / ShutterstockStudy: Surveillance of COVID-19 vaccine safety among elderly persons aged 65 years and older. Image Credit: Prostock-studio / Shutterstock


Available information concerning the safety of COVID-19 vaccines in the 65+ elderly population of the US is limited. They were among the first and disproportionately affected by COVID-19, especially those living in long-term care facilities or nursing homes. Thus, the US advisory committee on immunization practices recommended vaccines for the high-risk US elderly population. Subsequently, the Centers for Medicare & Medicaid Services (CMS) data covering 30,712,101 US elderly persons showed that more than 17 million Medicare-insured persons aged 65 years and above received nearly 34 million COVID-19 vaccine doses.

The US Food and Drug Administration (FDA) and CMS have routinely used a safety surveillance method for influenza vaccines in the past decade. For COVID-19 surveillance, this method deployed sequential hypothesis testing prospectively as the vaccine data accumulated to detect potential safety signals during the earlier phase of surveillance. It also proved sensitive enough to quickly screen vaccine safety signals for further evaluation in robust epidemiologic studies. Further epidemiological studies would provide more robust safety signals with adjustments for confounding.

About the study

In the present study, researchers recruited the US Medicare Fee-for-Service (FFS) beneficiaries aged 65 years or older who received a COVID-19 vaccine since December 11, 2020. The team enrolled these elderly on their vaccination date first. They were continuously involved in the study during an outcome-specific pre-vaccination clean window and also monitored through a post-vaccination clean window.

Subsequently, the team described the findings of the weekly sequential testing for 14 safety outcomes to generate rate ratios (RR) of observed outcome rates compared to historical rates. These outcomes were pulmonary embolism (PE), acute myocardial infarction (AMI), disseminated intravascular coagulation (DIC), and immune thrombocytopenia (ITP).

The researchers compared the observed number of each outcome in the COVID-19 vaccinated population to an expected number based on the historical outcome rate in a similar COVID-19 unvaccinated population before the pandemic, adjusted for the delay in claims processing and standardized by nursing home residency status, age, gender, and ethnicity. The researchers calculated annual background rates within the strata of the standardized variables, where feasible, between 2017 and 2019, January 1, 2020, and December 10, 2020. If annual rates varied, the team selected the minimum rate; otherwise, the median annual rate.

Study findings

The early warning safety system could not detect statistical signals for the Moderna and Janssen COVID-19 vaccines for 14 monitored safety outcomes. However, the researchers identified four new signals for modestly elevated risks (RR<2) of AMI, PE, DIC, and ITP following the Comirnaty administration. Although, these four statistically significant signals might be unrelated to COVID-19 vaccination.

Additional analyses indicated that the association was less than double the historical rates and related to factors not accounted for in the study method. The study method could explain only the PE signal by differences in rates between the pre-COVID-19 and post-COVID-19 periods but not the AMI, DIC, and ITP signals. However, intriguingly, some observed outcomes might have arisen due to follow-up care to an existing condition preceding the vaccination in the US elderly population.


According to the authors, this is the first study to identify a time-varying correlation for PE, AMI, DIC, and ITP following Pfizer-BioNTech vaccination in the elderly. However, these findings require cautious interpretation because the early warning system could not prove that vaccines caused these safety outcomes. Fortunately, more robust epidemiologic studies with adjustments for confounding factors, including age and nursing home residency, are in the process of further evaluating these signals. Nevertheless, the US FDA advocates that COVID-19 vaccination outweighs the potential risks of infection.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Neha Mathur

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.


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