By Christina Maxouris, Dakin Andone and Maggie Fox, CNN

(CNN) — A growing number of Americans have missed their scheduled second dose of a Covid-19 vaccine, according to data from the US Centers for Disease Control and Prevention.

The vaccines by Pfizer/BioNTech and Moderna require two doses — administered three and four weeks apart, respectively — to be considered fully effective. But data shows about 8% of Americans have missed that important second dose — up from about 3.4% in March.

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It’s not an exact count. The CDC is collecting data on vaccinations, but states don’t report information immediately and must gather it from mass vaccination sites, retail pharmacies and various other vaccination efforts.

“If a person received the two doses from different reporting entities, those two doses may not have been linked together,” a CDC spokesperson said.

“For example, if a person received their first dose at a clinic run by the state, and second dose from a tribal health clinic, they might not be linked and it could look like they missed the second dose.”

The news comes as the United States continues its effort to vaccinate as many Americans as quickly as possible. The CDC reported Sunday that 95 million people — about 28.5% of the population — have been fully vaccinated. About 140 million people — 42.2% of the population — have received at least one dose of a Covid-19 vaccine.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN’s Jim Acosta on Sunday he was not surprised some people are missing the second dose, saying it happens frequently with multi-dose vaccines.

“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 said.

“I’d like it to be a 0%,” he said, “but I’m not surprised that there are some people who do that.”

Similarly, the CDC said Americans missing second doses was expected. Groups initially prioritized for vaccination, such as health care workers, were more likely to get vaccinated at their work site, “potentially reducing barriers and increasing adherence to the recommended vaccine schedule,” a spokesperson said.

“The reasons behind the delayed or missed second doses, however, require further analysis,” the spokesperson said, and officials should work to understand whether this is due to access or vaccine hesitancy.

Covid-19 vaccinations declined last week

About 229 million doses of Covid-19 vaccines have been administered in the United States, according to CDC data published Sunday — about 3 million more administered doses reported since Saturday.

That puts the seven-day average of administered doses at about 2.8 million doses per day, a slight drop from earlier in the month, when the average pace of new doses administered peaked at 3.4 million shots per day on April 13.

Saturday, the CDC’s Dr. Amanda Cohn said the recent pause on the Johnson & Johnson Covid-19 vaccine had contributed to the decline.

“Last week was the first week that we saw a decline in vaccination, in terms of the total number of people who got vaccinated over the course of the week, and there is clearly the contributory factor of the pause in the J&J vaccine,” Cohn said.

The CDC and the US Food and Drug Administration paused use of the vaccine following reports of a rare blood clotting syndrome among six women who were recently vaccinated.

Further search turned up a total of 15 cases out of nearly 7 million people vaccinated, and Friday, the agencies gave the OK for use of the vaccine to continue, saying the vaccine label would be updated to warm of blood clot risks.

Senior White House coronavirus adviser Andy Slavitt on Sunday acknowledged vaccine efforts may begin to slow, and educational campaigns will be more important to inoculate people who remain reluctant.

“I think we are going to continue to make progress. They might not be as fast as the first 50%,” Slavitt said. “I think they’re just going to be slower, but I think we’re going to continue to get there.”

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Some place see ‘unsettling gaps’ in vaccine coverage

After several weeks of reporting concerning Covid-19 case increases, the United States could be seeing the beginning of a hopeful trend, a leading health official says.

The country’s seven-day average of new reported infections is going down, CDC Director Dr. Rochelle Walensky said at a White House Covid-19 briefing Friday.

Former US Food and Drug Administration Commissioner Dr. Scott Gottlieb believes that decline could stick this time, telling CBS’s “Face the Nation” Sunday that even hard-hit areas such as Michigan are starting to see cases come down.

“Right now, the declines that we’re seeing we can take to the bank,” he said. “I think we can feel more assured because they’re being driven by vaccinations and greater levels of population-wide immunity — not just from vaccination, but also from prior infection.”

What’s concerning officials now, Walensky said Friday, are the “unsettling gaps” in Covid-19 vaccine coverage in different parts of the country.

“Some areas are doing very well with greater than 65% coverage for those over the age of 65 … but many areas have far less coverage, less than 47%,” she said. “Because this virus is an opportunist, we anticipate that the areas of lightest vaccine coverage now might be where the virus strikes next.”

Experts have stressed for months that the best way Americans can protect themselves — and their communities — is through Covid-19 vaccinations which can, when enough people are vaccinated, suppress the spread of the virus.

“I think it’s really important to understand that vaccines work best at a population level, not at the individual level,” infectious diseases specialist and epidemiologist Dr. Celine Gounder told CNN on Saturday. “If you’re in a community that is swimming with virus, 95% reduction is good, but you’re still at risk.”

“Really the best way to reduce the risk for all of us is for as many people to get vaccinated as possible,” Gounder added.

Reports warn of vaccine ‘tipping point’

But in some parts of the country, local officials are reporting drops in demand for Covid-19 shots.

And in just a few weeks’ time, the US could hit a “tipping point” on vaccine enthusiasm and supply will likely outstrip demand, a Kaiser Family Foundation report said.

“Once this happens, efforts to encourage vaccination will become much harder, presenting a challenge to reaching the levels of herd immunity that are expected to be needed,” the report said.

Some experts, including Fauci, have estimated somewhere between 70% to 85% of Americans need to have immunity to the virus — either through vaccination or previous infection — to control its spread.

Behind the slowing vaccine demand are several factors, experts say, including vaccine hesitancy.

In its latest Covid-19 briefing, the University of Washington’s Institute for Health Metrics and Evaluation wrote that the “slow erosion of vaccine confidence unfolding over the last two or more months is cause for concern.”

“Facebook runs a survey every day, and we look at that data on a daily basis and that’s shown that vaccine confidence in the US has been slowly but steadily going down since February,” IHME Director Dr. Chris Murray told CNN on Friday.

“There’s a lot of people out there, and it’s a growing fraction of people, who are not sure they want to get the vaccine, and that’s really important that we overcome that,” he added.

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