CDC Data Shows That 92% of Partially Vaccinated Americans Are Returning for Their Second Dose

Of those Americans receiving the two-dose regimen of the Pfizer-BioNTech and Moderna Covid-19 vaccines, approximately 92 percent are completing both doses, according to new Centers of Disease Control and Prevention data. The follow-through rate is historic and significantly higher than double-dose vaccinations for other diseases, like the shingles vaccine, which draws a roughly 75 percent vaccination completion rate, reports Rebecca Robbins for the New York Times.

However, more than 5 million people, or eight percent of Pfizer-BioNTech or Moderna vaccine recipients in the United States, have not gone back to receive their required second dose. In mid-February when less of the population was eligible for vaccination, the missed second dose rate was closer to 4 percent, reports Daniel Politi for Slate. While some people may choose to opt-out of a second dose, others are facing accessibilty challenges, including hurdles like providers cancelling second appointments, vaccine sites shuttering between doses, and appointment systems sending Moderna recipients to Pfizer-stocked locations or vice versa.

Two doses of the Pfizer-BioNTech and Moderna Covid-19 vaccines are required for full vaccine effectiveness. Missing a second dose may trigger a weaker immune response, possibly leaving recipients susceptible to other virus variants, reports the New York Times. A second dose is required three-to-four weeks after the first dose, reports Christina Maxouris, Dakin Adone, and Maggie Fox for CNN.

Even if recipients only receive a single dose, it is unknown how long protection against Covid-19 will last without the second vaccine because it was not studied in either company's vaccine trials. After the first dose, both mRNA vaccines protect above the 50 percent minimum threshold, set by emergency use authorization criteria for Covid-19 vaccines. However, the vaccines' effectiveness was tested in two-dose regimens, reported Sanjay Mishra for the Conversation in January.

Some European countries opted for delaying a second dose to get as many people as possible at least vaccinated with the first dose. Some U.S. health experts wanted to follow this trend, reports Slate, but ultimately, Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, disagreed with this approach and it did not become part of the country's vaccination strategy.

Recipients of the first vaccine may not return for their second dose because of frustrations and hassles they face when trying to book their second dose, reports Lucy Yang and Eyewitness News for ABC7. Some vaccine providers have to cancel appointments when they do not have enough vaccines on hand due to late shipments, the New York Times reports. Other people have reported skipping their second dose because their appointment was automatically scheduled with a provider that did not have any doses available of the brand they needed. Some people may miss their second dose to avoid negative side effects.

"Obviously whenever you have a two-dose vaccine, you're going to see people who for one reason or other — convenience, forgetting, a number of other things — just don't show up for the second vaccine," Fauci tells CNN. "I'd like it to be a 0 percent, but I'm not surprised that there are some people who do that."

In total, the counts of missed doses were taken through April 9 by the CDC. The data covers only those who received the first Moderna dose by March 7 or the first Pfizer dose by March 14, the New York Times reports. However, it is not an exact count because some states do not report information right away. The CDC gathers their information from mass vaccination sites, pharmacies, and other miscellaneous vaccine providers, reports CNN. A CDC spokesperson told CNN that if a person were to go to two different locations for their vaccinations, the two vaccinations would not have been reported together, making it seem like someone missed a dose.

In the United States, the only vaccine designed as a single dose is produced by Johnson & Johnson. Vaccine distribution for Johnson & Johnson was paused this month after it was linked to a rare and severe side effect that caused blood clots in 15 individuals out of 6.8 million people who received the vaccine. On Friday, health officials reauthorized use of the Johnson & Johnson vaccine; however, the pause on distribution and production issues may make this vaccine a less viable option for people, the New York Times reports.

Elizabeth Gamillo | | READ MORE

Elizabeth Gamillo is a daily correspondent for Smithsonian and a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.