No abstract

I n 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