About 92% of Americans who got the COVID-19 vaccine returned for their second shot. That's good, but experts say the rest should do it now.

About 1 in 12 Americans haven't gotten their second scheduled dose of COVID-19 vaccine on time, and while that worries epidemiologists, the follow-through is far better than other adult two-dose vaccines.

According to recent data from the Centers for Disease Control and Prevention, about 92% of people who got the Pfizer-BioNTech or Moderna COVID-19 vaccine made it back for their second shot.

Experts noted such completion numbers for the two-dose regime of the most widely used vaccines against the coronavirus are a cause for celebration. 

"With the biggest mass vaccination program in history, 92% of people coming back for their second shot is a huge victory," said Dr. Monica Gandhi, an infectious disease specialist at the University of California, San Francisco.

The second shot matters because it provides stronger immunity against COVID-19. The first dose, called the priming dose, introduces the body to the spike protein on the surface of the SARS-CoV-2 virus that causes COVID-19. The second boosts that immune response so the body is ready to fight off the virus if exposed. 

"That's the reason why we've got to continue to outreach to people to convince them of why it's so important for themselves, for their family and for their community to complete the full component of the vaccines," the nation's top infectious disease expert Dr. Anthony Fauci said Monday at a Harvard Chan School of Public Health town hall.

"It’s the cherry on top of the ice cream," said Dr. William Schaffner, a professor and infectious disease expert at the Vanderbilt University School of Medicine in Nashville. "It boosts your immune system and helps to ensure you have long-term protection."

The high full-course vaccination rates may be in part because people have a better understanding of the risks of COVID-19 compared relative to other vaccine-preventable diseases, said Katia Bruxvoort, an infectious disease epidemiologist at Kaiser Permanente Southern California Department of Research & Evaluation.

"Most people’s lives have been directly affected by COVID-19,  through illness, losing a loved one, disruptions in work and school, and other changes," said Bruxvoort, who studies multiple-dose vaccine uptake. 

"There is a greater sense of urgency to be fully vaccinated to protect themselves and others, and to get life back to normal."

The more people who are fully vaccinated with these powerfully effective COVID-19 vaccines, the better. Of the first 75 million Americans to be fully vaccinated with both doses only 5,800 had breakthrough infections afterward, according to the CDC.

"People between the first and second shots can definitely get infected, especially in places with high case rates, so we encourage everyone to get their second shot," Gandhi said. 

Pfizer-BioNTech and Moderna's vaccines require two doses given 21 days and 28 days apart, respectively. They make up 224 million of the 232 million doses so far administered in the U.S. 

The Johnson & Johnson vaccine, which was briefly paused while health officials investigated a very rare side effect, requires just one shot. Because it's "one and done," it is especially useful for hard-to-reach populations and for people who have a hard time making it back for a second dose, Schaffner said.

The CDC notes the actual number of people who missed their second dose of the Pfizer-BioNtech or Moderna vaccine is probably lower than 8% because some may have gotten doses from different organizations that might not be connected via computer records.

The rates may get better over time, as it becomes easier to schedule second doses and as records are updated, said Dr. Gregory Poland, director of the Mayo Clinic's Vaccine Research Group.

"It's a bit of a moving target; the government systems are always a little bit behind," Poland said. 

Public health officials always knew a two-dose vaccination campaign for adults was going to be hard. Children get many vaccines that require multiple shots, but because they see their pediatricians fairly frequently, they are easier to administer. 

Getting adults to come back for second shots is tricky. Shingles is a painful rash caused by the chickenpox virus that affects 1 in 3 unvaccinated people by the time they're 80. Even with such high numbers, only 85% of people come back for the second dose of the Shingrix vaccine, according to the CDC.

Hepatitis B can cause liver disease and death. Vaccines to prevent it come in both a two-dose and three-dose series. For the two-dose vaccine, only 45% of adults completed the full sequence. For the three-dose schedule, only 26% did, according to Bruxvoort's study from the Kaiser Health System.

People don't come back for that booster shot for many reasons, experts say. For some, it's too complicated to schedule.

"A lot of people don't even know what their schedule is going to be in four weeks," Dr. Ashish Jha, dean of Brown University’s School of Public Health and a pandemic expert, said during a news briefing Tuesday.

Even he had a hard time fitting in his appointment for his second COVID-19 shot.

"We've just got to be patient and we've got to give people more opportunities to come back," he said.

Others had a strong reaction to the first dose and are hesitant to get the second. 

"They might believe if they got walloped by the first dose, it's better if they wait to get the second," Poland said. "That's fine, as long as they go back and get it."

Waiting longer than the recommended three or four weeks isn't a problem. People should get the booster when they can, Schaffner agreed.

"You won’t be yelled at!" he said. "You'll get a big smile and a thank you."

Nurse practitioner Sophia Thomas goes door-to-door administering COVID-19 vaccines in New Orleans on Saturday, April 17, 2021.