Factors Associated With Delayed or Missed Second-Dose mRNA COVID-19 Vaccination Among Persons ≥12 Years of Age, United States

Lu Meng; Neil Chandra Murthy; Bhavini Patel Murthy; Elizabeth Zell; Ryan Saelee; Megan Irving; Hannah E. Fast; Patricia Castro Roman; Adam Schiller; Lauren Shaw; Carla L. Black; Lynn Gibbs-Scharf; LaTreace Harris; Terence Chorba


Emerging Infectious Diseases. 2022;28(8):1633-1641. 

In This Article

Abstract and Introduction


To identify demographic factors associated with delaying or not receiving a second dose of the 2-dose primary mRNA COVID-19 vaccine series, we matched 323 million single Pfizer-BioNTech (https://www.pfizer.com) and Moderna (https://www.modernatx.com) COVID-19 vaccine administration records from 2021 and determined whether second doses were delayed or missed. We used 2 sets of logistic regression models to examine associated factors. Overall, 87.3% of recipients received a timely second dose (≤42 days between first and second dose), 3.4% received a delayed second dose (>42 days between first and second dose), and 9.4% missed the second dose. Persons more likely to have delayed or missed the second dose belonged to several racial/ethnic minority groups, were 18–39 years of age, lived in more socially vulnerable areas, and lived in regions other than the northeastern United States. Logistic regression models identified specific subgroups for providing outreach and encouragement to receive subsequent doses on time.


In December 2020, the US Food and Drug Administration (FDA) issued Emergency Use Authorizations (EUAs) for the Pfizer BioNTech (https://www.pfizer.com) and Moderna (https://www.modernatx.com) 2-dose primary mRNA COVID-19 vaccine series.[1,2] The Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices, part of the National Center for Immunization and Respiratory Diseases, prioritized certain populations to be offered the COVID-19 vaccination first, including healthcare personnel, long-term care facility residents, persons ≥65 years of age, persons 16–64 years of age with high-risk medical conditions, and essential workers.[3] Starting in March 2021, Pfizer-BioNTech and Moderna COVID-19 vaccines have been available at pharmacies and from other medical practice providers for anyone ≥16 years of age. In the 1-year period of this analysis, the recommended intervals between the 2 primary doses were 21 days for the Pfizer-BioNTech vaccine and 28 days for the Moderna vaccine.[4] On May 10, 2021, FDA expanded the EUA for the Pfizer COVID-19 vaccine to include persons 12–15 years of age.[5] During August–November 2021, FDA approved a series of EUAs: 1 for an additional primary dose for immunocompromised persons and 1 for a booster dose for persons ≥18 years of age.[6]

In the summer of 2021, one of every 10 US persons received the first dose of an mRNA COVID-19 vaccine, ≈15 million still had not received the second dose, and many more had received the second dose outside the recommended intervals between doses.[7] Persons who start the primary series are presumably amenable to initial vaccination but may then either delay completing or may fail to complete the series. Delayed or missed recommended COVID-19 vaccine doses can hamper national efforts to reduce COVID-19–associated illness, hospitalization, and death.[8–10] More information about this population is valuable for addressing second-dose vaccination barriers and devising interventions to increase primary series completion.

To support nationwide COVID-19 immunization efforts, we performed an analysis to identify demographic factors associated with receiving 1 dose of the 2-dose primary mRNA vaccine series but delaying the second dose or not completing the series. The study was reviewed by CDC and conducted consistent with applicable federal law and CDC policy.