CDC Analysis Finds Most People Receive Second COVID-19 Shot on Time


Health care professionals should address potential barriers to completing the COVID-19 vaccination series, including scheduling second-dose appointments at the first-dose administration and sending reminders for second-dose visits.

A new analysis from the CDC has found that among adults who received the first dose of a COVID-19 vaccine and for whom sufficient time had passed to receive the second dose, 88% had completed the 2-dose series.

Of the 3 approved COVID-19 vaccines, the Pfizer and BioNTech vaccine and the Moderna vaccine both require 2 doses. They have different recommended intervals between doses, with 21 days between Pfizer doses and 28 days between Moderna doses. However, the CDC report said up to 42 days between doses is allowable if there is an unavoidable delay.

Investigators conducted 2 analyses of COVID-19 vaccine administration data among approximately 40.5 million people who received their first dose between December 14, 2020, and February 14, 2021, and whose doses were reported to the CDC through February 20, 2021. The first analysis determined whether people who received a first dose had received the second dose at all, whereas the second analysis determined whether the second dose was received within the recommended time frame.

According to their findings, 88% had completed the series and 8.6% had not received the second dose but were still within the allowable interval. Among everyone who had completed both doses, 95.6% received the second dose within the recommended time frame. Furthermore, 3.4% had missed the second dose and were outside the allowable 42-day interval.

The investigators found jurisdictional and demographic differences among those who had not received the second dose on time or at all. In the first analysis, the percentage of those who missed the second dose was highest among non-Hispanic American Indian or Alaska Native peoples (5.1%) and those 16 to 44 years of age (4%). In the second analysis, among approximately 14.2 million people who received a second dose, 95.6% received the dose within the recommended interval, although percentages ranged from 79% to 99.9% among jurisdictions.

When considering age groups, the investigators found that series completion was lowest among older adults. This finding is similar to results from an initial nationwide analysis, according to the CDC report, although they noted that this group was also the least likely to miss the second dose. A large percentage of those included were still in the allowable interval for the second dose.

The study authors noted that the groups eligible for vaccination during their study period were the ones most likely to be vaccinated at their homes or workplaces, including health care workers and long-term care facility residents. This might have facilitated adherence to the recommended schedule, so adherence may decrease as priority groups broaden, according to the study.

Based on these varying rates, the investigators said health care professionals and public health officials should identify and address potential barriers to completing the vaccination series, including scheduling second-dose appointments at the first-dose administration and sending reminders for second-dose visits.


COVID-19 Vaccine Second-Dose Completion and Interval Between First and Second Doses Among Vaccinated Persons—United States, December 14, 2020 – February 14, 2021. CDC; March 15, 2021. Accessed March 16, 2021.

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