En español | The majority of Americans who are rolling up their sleeves for the coronavirus vaccines are doing so twice. Both the Moderna and Pfizer-BioNTech vaccines — the most commonly administered COVID-19 vaccines in the U.S. — require two shots, several weeks apart. But not everybody is going back for their second dose.
A small but significant share of people (about 11 percent) have missed their second shot of a two-dose series, according to data from the Centers for Disease Control and Prevention (CDC) that looked at vaccine completion status through June 16. This number has been creeping up for months. In mid-March, a CDC report found 3.4 percent of vaccine recipients didn't go back for a second dose within six weeks of receiving the first; by late April, it had jumped to 8 percent.
Reasons for not returning run the gamut. Earlier in the vaccine rollout, some had difficulty finding a second dose from the same manufacturer as their first; others lack transportation to vaccination sites. Side effects from the shots also remain a concern for many.
Still, “ensuring second dose completion of the vaccine is critical in helping to protect people from COVID-19,” the CDC says. Here's a look at why.
Longer immunity more likely with second shot
It's clear that the Pfizer and Moderna vaccines are powerful when it comes to keeping COVID-19 at bay. In fact, a recent CDC report found that these messenger RNA (or mRNA) vaccines were 80 percent effective at preventing a coronavirus infection after the first dose and 90 percent after the second.
What's still unclear, however, is how long that protection lasts. But what we know from other vaccines is that booster shots help immunity to stick, and the same is expected from the Pfizer and Moderna vaccines. In the two-dose COVID-19 regimen, the first dose primes the immune system; the second dose ensures “that immunity really takes and is longer lasting,” explains Mark Rupp, chief of the Division of Infectious Diseases at the University of Nebraska Medical Center.
Angela Rasmussen, a virologist at VIDO-InterVac and Georgetown University's Center for Global Health Science and Security, likens the mRNA vaccines to a college course that teaches your body to recognize and fight off an infection. The first shot — think of it as “COVID 101” — introduces your immune system to the coronavirus, but the second shot “is like advanced topics in SARS-CoV-2,” she says.
"Your immune system is more likely to retain that knowledge for a longer period of time. And those immune responses, although we don't know for sure yet, are more likely to be durable or longer lasting,” she adds.
It provides stronger protection against variants
Another reason to go back for the second shot: That extra dose makes it more likely that your body will win a fight against the coronavirus variants that are sparking concern, including the highly transmissible delta variant, which was first identified in India and is now the dominant strain of coronavirus circulating in the U.S.
Some mutations seen in the variants make it easier for the virus to infect the cells of the body, explains Emily Landon, an infectious diseases specialist and associate professor of medicine at the University of Chicago Medicine. But the flood of antibodies that come after the second shot gives the body an extra boost of protection that can keep the virus from binding to the cells.
Pointing to a handful of new studies, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in a July 1 news briefing that the two-dose mRNA vaccine from Pfizer-BioNTech is about 80 percent effective at preventing infection from the delta variant and 88 percent effective at preventing symptomatic disease. But at one dose, the mRNA vaccines were just 33 percent effective at preventing symptomatic disease caused by the delta variant, a study out of the United Kingdom found.
"[Vaccines] sort of train your body to fight off COVID, and if you have two training sessions, it's better than one training session,” Landon says. “If you only get one dose of a two-dose vaccine, then when it comes to the variants, especially, you're fighting with one arm tied behind your back.”
Side effects are better alternative to disease
Some, but not all, vaccine recipients have reported mild-to-moderate side effects after their shots, and these symptoms are more common after the second dose than the first. But experts say fear of short-term side effects shouldn't keep people from completing the vaccine series — especially considering the alternative.
"I'd rather put up with that day or two of feeling bad than the real possibility of having a fatal disease,” Rupp said. To date, more than 606,000 Americans have died from COVID-19.
Plus, if your first vaccine packed a punch, it's unlikely the second one will, as well, Landon adds. (In fact, if you experience side effects after your first dose, it could mean you previously had COVID-19. The reaction is your body recognizing the virus’ spike protein and activating an immune response.)
If you do experience side effects, over-the-counter medications such as ibuprofen, acetaminophen, aspirin or antihistamines can help with most post-vaccine pain or discomfort. And keep in mind: “It disappears as quickly as it comes on,” says Ross Kedl, professor of immunology and microbiology at the University of Colorado Anschutz School of Medicine.
"That's you being drunk with immunity … It's all due to immune-related molecules that have gone way over the top in an effort to make sure that the immunity is kicked off in the right gear,” he adds.
If concerns over side effects, allergies or adverse reactions are keeping you from getting your second shot, talk to your health care provider. “They should take it seriously, and they also should be able to make a recommendation that's right for you,” Rasmussen said.
It's not too late to get your second dose
Already missed your second shot? Experts say it's not too late to go back and get it — even if you're beyond the CDC's recommended six-week window. (Keep in mind, too, that some countries, including the U.K, have been administering the mRNA doses 12 weeks apart to stretch out their supply.)
"It would be better to get that booster dose, even if it's a little bit later, than forgoing it altogether,” Rupp says.
It's currently not advised to mix vaccines — where you take one dose from Moderna and one dose from Pfizer — so if your pharmacy or vaccine site doesn't stock the vaccine you need to complete your series, take a beat and reschedule your appointment for a site that does. “And we are getting to the point now where it should be easier for people to schedule vaccine shots in most places,” Rasmussen said.
The CDC says jurisdictions can work with providers to prioritize administering second doses over initiating first doses and to “promote the importance of receiving a second dose for achieving maximum vaccine effectiveness.” Rasmussen says federal and state governments should also work to ensure that people who lack transportation to vaccine sites or the skills needed to navigate online appointments can easily access their second doses.
"The reasons for going back and getting that second dose of the vaccine are really threefold,” Rupp says. “They're doing this to protect themselves; … they can prevent spreading that infection to their family and to their loved ones; and then, third, and also very important, we're doing it for our community and our country. This is the way we really get ourselves through this pandemic."
Editor’s Note: This article, originally published April 28, 2021, has been updated to reflect new information.
Rachel Nania writes about health care and health policy for AARP. Previously she was a reporter and editor for WTOP Radio in Washington, D.C. A recipient of a Gracie Award and a regional Edward R. Murrow Award, she also participated in a dementia fellowship with the National Press Foundation.