Like other Americans who have received a coronavirus vaccine, 76-year-old Elizabeth Larson of Arizona struggled to book her first dose.

When it came time to schedule her booster shot, Larson recounted initially facing technical glitches that prevented her from selecting an appointment. She felt hopeless.

“I’ve been so frustrated,” Larson said. “It’s almost literally like tearing my hair out.”

The vast majority of recipients of the two-dose Pfizer and Moderna vaccines are able to get their second shot within the recommended period, according to vaccine data reviewed by The Washington Post. Yet, some people are still encountering problems navigating local programs and unplanned delays like bad weather or supply shortages. More than a dozen people wrote to The Post, saying they faced barriers to getting a second appointment or were worried that they would. These anxiety-inducing hurdles come as an international debate is unfolding over how to best roll out the limited vaccine supply: prioritizing fully inoculating people who have had their first dose or offering partial protection with single doses to more people.

The United Kingdom chose the former.

New data from the U.K. show fewer infections after the country concentrated only on distributing first doses and holding off on giving second doses, while other recently released research found that delaying the second dose of the AstraZeneca vaccine to 90 days may be more effective and confirmed reports that a single dose may be enough to immunize people who were previously infected. But health experts in the United States are divided about halting the distribution of second doses, with some expressing concern that a population with limited protection could deal a blow to vaccines’ effectiveness as immune-resistant variants evolve.

When Larson, a Phoenix resident, saw no available slots on Arizona’s scheduling website for her second dose, she wondered if she was going to be able to get the vaccine at all — especially within the recommended 21 days for the Pfizer-BioNTech vaccine. The suggested interval between the Moderna vaccine doses is 28 days, but the Centers for Disease Control and Prevention says there’s wiggle room for both vaccines, advising that a six-week period is acceptable.

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After days of refreshing her browser and waiting on hold, she got an appointment.

“I was afraid,” Larson said, “that they were going to say ‘well, let’s vaccinate everyone and you’re not going to get a second dose.'”

The Arizona Department of Health Services said second-dose appointments are “guaranteed,” and that the issues with people not receiving the follow-up email or finding available slots were resolved. The State Farm Stadium vaccination site in Phoenix, the state’s first 24/7 facility, now offers second-dose appointments to people during their initial visit, spokesperson Steve Elliott wrote in an email.

Health departments across the country have contended with supply snags as they attempt to ensure everyone receives their second dose: Maryland officials were accused of “hoarding” supply to guarantee people could get their booster shots, while a county health department in Iowa announced Tuesday that thousands of residents would experience delays getting their second shot because of an error calculating how many doses they were allocated. Delaware warned its residents their second appointments would be delayed as they distributed first doses.

“This is an ethical dilemma we have to deal with every day,” Delaware medical director Rick Hong told WHYY. “With more supply coming in, we’re hoping we can meet both missions: get as many people vaccinated who have never been vaccinated before, as well as completing the series for those who need the second dose.”

The recent storms have also complicated distribution. All 50 states reported weather-related delays in vaccine shipments as two deadly winter storms tore through the center of the country, making it impossible for many people to keep their second appointments.

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People waiting for their second dose told The Post they worry about their protection.

There is little data available about the efficacy of vaccines when the second dose isn’t administered on schedule, but experts say there is likely no reason to believe a delay would hinder an immune response.

The limited information has sparked a discourse among public health experts who do not agree whether officials should prioritize giving more people one dose or commit to distributing second doses.

Researchers in favor of focusing efforts on offering first doses say the initial shot effectively shields people from severe infection. Canada-based researchers Danuta Skowronski and Gaston De Serres, in a letter published last week in the New England Journal of Medicine, pointed to data that shows the Pfizer vaccine efficacy is 92.6% after the initial dose and Moderna’s is 92.1%. Offering people some protection can keep them from becoming severely ill or dying.

Protection will also be needed if variants that are more virulent become predominant in the United States, experts believe.

As a surge in cases of the deadly variant first identified in the U.K. is likely, second doses should be delayed while seniors older than 65 are inoculated with their first dose, according to epidemiologist Michael Osterholm, a member of Biden’s coronavirus advisory board during the transition. In a paper published Tuesday by the University of Minnesota’s Center for Infectious Disease Research and Policy, Osterholm and other researchers estimate that there would be 559,000 fewer infections, 112,000 fewer hospitalizations and 39,000 fewer deaths if second doses are deferred in favor of offering first shots to seniors.

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One dose of the approved vaccines appears to be effective, possibly against known variants, which so far have occurred as a result of natural infection and not through building vaccine resistance, Osterholm told The Post.

But other experts disagree, arguing the two-dose regimen has shown to be very effective and that it’s not known how the virus could mutate if transmitted among people who are not fully protected. Reacting to the U.K. decision to delay second doses, Paul Bieniasz, a virologist at Rockefeller University, wrote in Oxford’s Clinical Infectious Diseases journal that a population that is not fully inoculated could be a breeding ground for a vaccine-resistant variant.

While the data emerging from the U.K. offers researchers new insight into how second-dose delays influence infection numbers, more information likely won’t be gathered by the time many Americans receive their vaccines, said Anthony S. Fauci, the nation’s leading infectious-disease expert.

“If you want to really study it — the amount of time that it will take, the amount of people you would have to put in the study — by that time, we will already be in the arena of having enough vaccines to go around anyway,” Fauci told NBC’s “Meet the Press” host Chuck Todd earlier this month.

That uncertainty was nerve-racking for Steve Fleury, 65, of Scottsdale, who encountered similar problems as Larson when scheduling his second dose of the Pfizer vaccine at the same Arizona site. After he finished cancer treatment in December, his doctors advised him to get immunized as soon as he could, given his weakened immune system. When he wasn’t able to find an available second appointment, he wasn’t sure what to do. On a whim, he and his wife drove to the site without an appointment 22 days after his first shot. It worked. He said other people with jobs might not be able to do the same.

“The only reason we didn’t have to wait was we decided we’re retired, let’s just drive down there every night until they let us in and we get the second shot,” he said. “Luckily, they let us in the first night.”

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New York resident Jane Barowitz, 79, was not as fortunate. She was scheduled to wait 40 days between her doses of the Moderna vaccine, and she said she wasn’t told why there was a delay. But Barowitz said that just having the first dose was a relief.

“Still having half protection feels wonderful or something short of wonderful, like breathing more easily,” she said. “The idea that we’ll have the full protection that Moderna offers is very encouraging.

“Very,” she repeated. “I’m tearing up. We’ll be able to see our grandchildren again.”