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A second dose of a COVID-19 vaccine can be administered up to 6 weeks after the initial shot. Paul Bersebach/MediaNews Group/Orange County Register/Getty Images
  • Experts say that it’s important for people to follow through with second doses of Moderna and Pfizer COVID-19 vaccines.
  • They say that the odds of being hospitalized or becoming seriously ill drop dramatically after a second shot.
  • A second dose can be administered up to 6 weeks after a first shot.

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It’s the new rallying cry of infectious disease experts across the United States.

“Get your second dose.”

With nearly a third of the U.S. population now fully vaccinated, a worrisome trend has popped up, according to those experts.

Some people are choosing to take the first of the two-shot series required with both Pfizer and Moderna vaccinations but are opting out of the second shot.

The Centers for Disease Control and Prevention (CDC) reports that about 8 percent of people who have received a first shot of the two-shot vaccines have missed their second shot.

Officials are digging into the situation and speaking out about why getting both shots in the two-shot series is critical.

They say that the second dose not only builds herd immunity, but also strengthens the protection from serious COVID-19 illness and complications.

“Many have the illusion they are completely protected (with one of the two shots), but they are not,” Dr. William Schaffner, an infectious disease expert at Vanderbilt University School of Medicine in Nashville, Tennessee, told Healthline. “Some may lose their prevention ability sooner, and they won’t know it.”

The first shot is “priming the pump,” Schaffner said, and “the second dose brings up the water.”

Dr. John Zaia, the director of the City of Hope’s Center for Gene Therapy in the Los Angeles area and a specialist in vaccine research, told Healthline the trend of skipping second doses concerns him.

The virus and its variants, he explained, seek out “hosts.” That means that with more people vaccinated, the virus may hone in on those who aren’t fully vaccinated.

With strong variants immerging, Zaia added, he hopes to see everyone take both doses.

Dying from COVID-19, he noted, looks to be almost fully avoidable with two shots.

Zaia points to a study by a team at Houston Methodist Hospital that drilled down on the chances of both developing COVID-19 or dying from it for the fully and partially vaccinated.

In the study, which hasn’t been peer reviewed yet, less than 1 percent of those who had taken both shots were hospitalized. That number jumped to more than 3 percent for those who opted for just one of the two shots.

In addition, the study found that the two-shot total dose is 98 percent effective at preventing death from COVID-19, while choosing to stop at one shot drops that down to 64 percent.

Why are people skipping a second dose?

Schaffner sees it as “not one big reason” but many small ones.

He points to things such as believing one shot protects them enough, fearing sickness from the second dose, preoccupation and difficulty scheduling, and “COVID fatigue.”

Experts say that if you received your first shot and — for whatever reason — didn’t schedule a second, now is the time to do just that.

“It’s not too late,” Zaia said.

According to the CDC, both the Pfizer and Moderna vaccines can be administered up to 6 weeks after the first dose.

No data is available yet on whether getting a second shot after that 6-week period is effective enough.

Zaia said a person’s best plan is to get both within the time frame or as close to it as they can.

If you do decide to get the second dose, be sure to know which shot you had the first time. Most sites will ask to see your vaccination card to confirm that on site.

Schaffner hopes the public listens to the plea of infectious disease experts and rethink the second shot if they’ve decided to skip it.