In Arkansas and Illinois, health officials have directed teams to call, text or send letters to people to remind them to get their second shots. In Pennsylvania, officials are trying to ensure that college students can get their second shots after they leave campus for the summer. South Carolina has allocated several thousand doses specifically for people who are overdue for their second shot.

recommended restarting use of the vaccine, but the combination of the safety scare and ongoing production problems is likely to make that vaccine a viable option for fewer people.

The C.D.C.’s count of missed second doses is through April 9. It covers only people who got a first Moderna dose by March 7 or a first Pfizer dose by March 14.

Roughly three-quarters of adults come back for their second dose of the vaccine that protects against shingles.

In some cases, problems with shipments or scheduling may be playing a role in people missing their second doses. Some vaccine providers have had to cancel appointments because they did not receive expected vaccine deliveries. People have also reported having their second-dose appointments canceled or showing up only to find out that there were no doses available of the brand they needed.

Some people can be flexible about being rebooked. But that’s harder for people who lack access to reliable transportation or who have jobs with strictly scheduled hours, said Elena Cyrus, an infectious disease epidemiologist at the University of Central Florida.

Walgreens booked some customers for their second appointments at places that didn’t have the same vaccine that they had received for their initial doses. The company said it fixed the problem in late March.

come under fire for, until recently, scheduling second doses of the Pfizer vaccine four weeks after the first shot, rather than the three-week gap recommended by the C.D.C. Pharmacists have been besieged by customers complaining, including about their inability to book vaccine appointments online.

In other cases, though, access to vaccines is not the sole barrier; people’s attitudes contribute, too.

Basith Syed, a 24-year-old consultant in Chicago, nabbed a leftover Moderna vaccine at a Walgreens in mid-February. But when the time came for his second shot, he was busy at work and preparing for his wedding. After the first shot, he had spent two days feeling drained. He didn’t want to risk a repeat, and he felt confident that a single dose would protect him.

C.D.C. says there is limited data on the vaccine’s effectiveness when shots are separated by more than six weeks, although some countries, including Britain and Canada, are giving shots with a gap of up to three or four months.

Mr. Syed’s experience is part of a broader shift in Illinois. When vaccines were mostly being given to health care workers, residents of long-term care facilities and people over 65, almost everyone was getting their second shots. In recent weeks, though, the number dipped below 90 percent, though it has since rebounded slightly, according to the Illinois Department of Public Health.

In Arkansas, about 84,000 people have missed their second shots, representing 11 percent of those eligible for those shots, said Dr. Jennifer Dillaha, the state epidemiologist. Workers recently began calling people who are due or overdue for their second shots.

College students pose a particular challenge. Many recently became eligible to be vaccinated and are getting their first shots, but they will have left campus by the time they are due for their second doses.

In Pennsylvania, health officials have instructed vaccine providers to give second doses to college students even if they did not receive their first doses from that location.

Some vaccine providers have put on special clinics for people who need a second dose. In South Carolina, the health system Tidelands Health started a program specifically for people who received their first Pfizer doses more than 23 days earlier but hadn’t been able to find a second shot. The state health department sent the health system 2,340 doses for the effort.

Demand has been strong, and Tidelands only has a few hundred doses left. The majority of takers have been people who “were having difficulty navigating all the various scheduling systems and providers,” said Gayle Resetar, the health system’s chief operating officer.

In many cases, vaccine providers had canceled second-dose appointments because of bad winter weather. “It was up to the individual to reschedule themselves on a web portal or web platform, and that just became difficult for people,” Ms. Resetar said.

There are rare cases in which people are supposed to forgo the second shot, such as if they had an allergic reaction after their first shot.

Zvi Ish-Shalom, a religious studies professor from Boulder, Colo., had planned to get fully vaccinated. Then, an hour after his first shot of the Moderna vaccine, he developed a headache that hasn’t gone away more than a month later.

There is no way to know for sure whether the vaccine triggered the headache. But after weighing what he saw as the risks and benefits of a second dose, Dr. Ish-Shalom reached a decision about how to proceed.

“At this point in time, I feel very clear and very comfortable, given all the various elements of this equation, to forgo the second shot,” he said.

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