Nearly 90% of people who received the first dose of a COVID-19 vaccine completed the series within the recommended time interval, according to research published in the Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report.

In December 2020, the Food and Drug Administration granted Emergency Use Authorization to two COVID-19 vaccines, both of which require 2 doses to complete the series. The recommended time interval between receipt of the 2 doses is 21 days for the Pfizer-BioNTech vaccine and 28 days for the Moderna vaccine; however, if a delay is unavoidable, 42 days between doses is considered permissible.

To assess patient adherence to the full vaccine series, 2 analyses using COVID-19 vaccine administration data between December 14, 2020, and February 14, 2021 were conducted. “The first analysis was conducted to determine whether persons who received a first dose and had sufficient time to receive the second dose (ie, as of February 14, 2021, over 25 days from receipt of Pfizer-BioNTech vaccine or over 32 days from receipt of Moderna vaccine had elapsed) had received the second dose,” the study authors explained. “A second analysis was conducted among persons who received a second COVID-19 dose by February 14, 2021, to determine whether the dose was received during the recommended dosing interval, which in this study was defined as 17 to 25 days (Pfizer-BioNTech) and 24 to 32 days (Moderna) after the first dose.” The analyses were additionally stratified by jurisdiction as well as by patient demographics.

Findings revealed that of the total patients who received the first dose of a COVID-19 vaccine and for whom a sufficient amount of time had lapsed to receive their second dose (n=12,496,258), 88.0% had completed the series, 8.6% had yet to receive the second dose but were still within the allowable administration interval, and 3.4% missed the second dose. Results also showed that of the total patients who received a second COVID-19 dose (n=14,205,768), 95.6% received it within the recommended interval, 2.8% received it within the permissible interval, 1.5% received it early, and 0.1% received it late.

Vaccine series completion rate was lowest for older adults (aged 65 years and over); however, this group was reported to be the least likely to miss the second dose of the vaccine. Data analysis also revealed non-Hispanic American Indian/Alaska Native persons to be the least likely of all racial and ethnic groups to complete the vaccine series and were also found to have the highest prevalence of missed second doses.

“Identifying and addressing possible barriers to completing the COVID-19 vaccination series can help ensure equitable coverage across communities and optimal health benefits for recipients,” the study authors concluded. Scheduling second dose appointments at the time of first dose administration, as well as sending reminders for the second dose visit, may help ensure series completion.


Kriss JL, Reynold LE, Wang A, et al. COVID-19 vaccine second-dose completion and interval between first and second doses among vaccinated persons — United States, December 14, 2020−February 14, 2021. Centers for Disease Control and Prevention. Accessed March 17, 2021.