The vast majority of Americans who have received their first Covid-19 vaccine shot came back to receive their second shot, according to a recent study published in CDC's Morbidity and Mortality Weekly Report.

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Study details

For the study, researchers conducted analyses of Americans vaccinated with either Pfizer/BioNTech or Moderna's vaccine between Dec. 14, 2020 and Feb. 14, 2021. One analysis considered about 12.4 million people who had received at least one dose and for whom enough time had elapsed for them to have received a second dose. The other analysis examined roughly 14.2 million people who had received two doses.

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According to the first analysis, the vast majority of people who had received the first dose of either vaccines—88%—had since received their second shot. An additional 8.6% had not yet received their second dose but were still within the allowable timeframe of 42 days to do so. Overall, just 3.4% of people who'd received one shot had missed the window for the second, according to the study.

The researchers noted that the populations most likely to have not received their second dose varied by locality, but—among vaccine recipients for whom race and ethnicity were known—populations with the lowest rates of completion including American Indians and Alaska Native individuals. However, the study noted that racial and ethnic data was not always reported for people who received a vaccine.

Meanwhile, according to the second analysis, 95.6% of people who had received both doses received their second dose with the recommended timeframe—between 17 to 25 days for Pfizer/BioNTech, and between 24 and 32 days for Moderna. Just under 3% had received their second dose after the recommended timeframe, yet still within CDC's allowable delay of up to 42 days. Only 1.5% of people who had received both doses received their second dose early.

The states with the highest vaccine completion rates

According to the study, among states that submitted individual vaccination data, West Virginia had highest percentage of people who returned for their second shot at 95.8%, while Utah had the lowest rate at 75.3%.

Good news—with caveats

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According to The Verge, the findings helped relieve some experts' concerns about the pitfalls of administering multi-dose vaccines. Many experts had cited the multi-dose HPV and shingles vaccines as examples of how difficult it is to get people to return for subsequent doses, with about 25% of people who get their first shingles shot never receiving the second, and only about 50% of people who get the first HPV vaccine dose managing to complete the three-dose regime.

But while the study's authors called their findings "reassuring" overall, they cited several caveats, including that many of those who had received both doses did so at their place of work or at a nursing home, which may have boosted the odds of adherence. The researchers warned that "as priority groups broaden, adherence to the recommended dosing interval might decrease."

CDC Director Rochelle Walensky said the agency is working on ways to reduce the number of people missing their second dose. "CDC is working across the government and with state and local partners to identify and address barriers to getting both doses," she said (Scott, Vox, 3/15; Rabin, New York Times, 3/15; Wetsman, The Verge, 3/15).