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Key Takeaways

While two COVID-19 vaccines are now authorized for use in the United States, not everyone is eager to get in line once their priority group is called. According to a Kaiser Family Foundation survey published on December 15, about a quarter (27%) of the public are vaccine hesitant,﻿﻿ saying they "probably or definitely would not get a COVID-19 vaccine even if it were available for free and deemed safe by scientists."

The survey found that those who were hesitant to get a COVID-19 vaccine were worried about multiple things, including possible side effects (59%), lack of trust in the government to ensure the vaccines' safety and effectiveness (55%), and concerns that the vaccine is too new (53%).

To address these concerns, Verywell spoke with infectious disease experts from all over the country.

Concern #1: The vaccine was developed so fast. It must have been rushed and therefore isn't safe.

The speed at which the Pfizer-BioNTech and Moderna vaccines were both developed and authorized is a stark contrast compared to how long other human disease vaccines have taken to create. The mumps vaccine, which held the previous record, took about four years. However, scientists can point to a few different reasons why the COVID-19 vaccines have a different timeline.

A lot of events conspired to make the vaccines available so quickly, but it wasn't because of any shortcuts, Timothy Brewer, MD, an epidemiologist and professor of medicine at UCLA's Fielding School of Public Health, tells Verywell.

Brewer points to the fact that both vaccines made by Pfizer and Moderna are mRNA vaccines, and the mRNA platform was already in development (though not net used in a human vaccine). By using a small part of the virus's genetic code (RNA), an mRNA vaccine shows cells how to make a coronavirus protein that our immune system can recognize and then respond to. After someone gets vaccinated, if they're exposed to COVID-19, their body will already have the antibodies to fight it off and keep it from entering cells.

"People have to realize that the vaccines are actually developed based on years and years of experience," Brewer says. "For example, the Moderna vaccine took the platform (mRNA) that they had for a MERS (Middle East respiratory syndrome) vaccine, and they swapped out the MERs genetic code and swapped in the SARS-CoV-2 genetic code. So, these vaccines didn't come out of nowhere; they came out of years of research and advances in technology."

Is mRNA Safe?

Brewer says that the timeline behind the actual manufacturing of the vaccines had a huge impact on how quickly they could be deployed after authorization from the Food and Drug Administration (FDA).

"Usually, companies don't commit to manufacturing vaccines until they've been approved. And governments or healthcare systems don't buy vaccines until they've been approved," Brewer says. "But what was really sped up under Operation Warp Speed was the commitment to purchase millions of doses of vaccine before they had been approved. That allowed manufacturing to rev up, even while the vaccines were going through the study and approval process."

Concern #2: The government and health agencies have been giving mixed messages about COVID-19 since the beginning. Why trust them about a vaccine?

A large complaint among Americans is the contradictory messages from the CDC and healthcare officials like Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID) at the National Institutes of Health (NIH). For example, in an interview on March 8, 2020, Fauci said "there's no reason to be walking around with a mask." But by April 3, the CDC updated its previous advice and recommended people wear face coverings "in public settings when around people outside their household, especially when social distancing measures are difficult to maintain."

Rupali Limaye, MD, an associate scientist at Johns Hopkins Bloomberg School of Public Health, tells Verywell that part of the issue regarding these messages is that they had to change. And that's because science-and the virus-is always evolving.

"The great news is that as we learn about the virus, we can update recommendations to reflect that latest data, which is what happened with the mask wearing example," Limaye says. "People saw that as. 'Oh, public health entities don't know what they're talking about.' But my argument is: This is us showing you that science is working; that we're making progress; that we're learning about it and able to use evidence to tell you what to do next."

Bruce Hirsch, MD, an infectious disease expert at Northwell Health in New York, says that at some point, people need to put their faith in organizations like the FDA and CDC.

"I think the objections to the vaccine are understandable because the vaccine is brand new, but it's not entirely intellectually consistent with some of the other actions that we take and the other risks that we take day in and day out of our lives," Hirsch says.

When you go to the grocery store, you trust that the food you're buying doesn't contain salmonella and won't make you sick. When you drive a car, you trust that your vehicle has passed industry safety regulations and won't malfunction. But there are-and will-be rare occasions when that head of lettuce gives you salmonella, and the car you drive breaks down, Hirsch says.

"Unless you're growing your own food and living off the grid, you have established some trust into societal norms," he says, citing FDA-authorized vaccines as societal norms.

Concern #3:The side effects of the vaccines haven't been observed long enough.

In clinical trial data presented to the FDA by Pfizer, the vaccine was well-tolerated in approximately 44,000 participants with no serious safety concerns. Side effects were mild, including fatigue and headache in fewer than 3% of participants.﻿﻿

Moderna presented similar findings. The company reported that the majority of side effects among its 30,000 vaccine clinical trial participants were mild or moderate. After the first dose, 2.7% of participants reported pain at the injection site.﻿﻿

The FDA says there is a "remote chance" that the Pfizer vaccine could cause a severe allergic reaction, which would occur within minutes or up to an hour after receiving the vaccine.﻿﻿

Robert Jacobson, MD, a Mayo Clinic pediatrician in the department of Community Pediatric and Adolescent Medicine, tells Verywell he doesn't know of any vaccines that have caused side effects more than six weeks after being administered.

"We have a track record with vaccines not causing long-lasting harm that shows up months later," Jacobson says. "We also have a process in place to watch for those just in case. But it would be very different from what science has taught us about how these vaccines behave; it would be incredibly unusual to see [delayed effects]."

Jacobson adds that mRNA vaccines quickly deteriorate inside the body.

"It's not like there's a lasting vaccine that stays with the person; it actually disappears in a matter of hours to days," he says. "In fact, that's why we need two doses."

What This Means For You

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Experts Address 3 Major COVID-19 Vaccine Concerns Among Americans

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While two COVID-19 vaccines are now authorized for use in the United States, not everyone is eager to get in line once their priority group is called. According to a Kaiser Family Foundation survey published on December 15, about a quarter (27%) of the public are vaccine hesitant,﻿﻿ saying they "probably or definitely would not get a COVID-19 vaccine even if it were available for free and deemed safe by scientists."

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A Closer Look Into the COVID-19 Vaccine & Symptoms

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A Closer Look Into the COVID-19 Vaccine & Symptoms

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Side effects are just a sign that protection is kicking in as it should.

Getty / The Atlantic

At about 2 a.m. on Thursday morning, I woke to find my husband shivering beside me. For hours, he had been tossing in bed, exhausted but unable to sleep, nursing chills, a fever, and an agonizingly sore left arm. His teeth chattered. His forehead was freckled with sweat. And as I lay next to him, cinching blanket after blanket around his arms, I felt an immense sense of relief. All this misery was a sign that the immune cells in his body had been riled up by the second shot of a COVID-19 vaccine, and were well on their way to guarding him from future disease.

Side effects are a natural part of the vaccination process, as my colleague Sarah Zhang has written. Not everyone will experience them. But the two COVID-19 vaccines cleared for emergency use in the United States, made by Pfizer/BioNTech and Moderna, already have reputations for raising the hackles of the immune system: In both companies' clinical trials, at least a third of the volunteers ended up with symptoms such as headaches and fatigue; fevers like my husband's were less common.

Dose No. 2 is more likely to pack a punch-in large part because the effects of the second shot build iteratively on the first. My husband, who's a neurologist at Yale New Haven Hospital, is one of many who had a worse experience with his second shot than his first.

Recommended Reading

But much like any other learning process, in this one repetition is key. When hit with the second injection, the immune system recognizes the onslaught, and starts to take it even more seriously. The body's encore act, uncomfortable though it might be, is evidence that the immune system is solidifying its defenses against the virus.

"By the second vaccine, it's already amped up and ready to go," Jasmine Marcelin, an infectious-disease physician at the University of Nebraska Medical Center, told me. Fortunately, side effects resolve quickly, whereas COVID-19 can bring on debilitating, months-long symptoms and has killed more than 2 million people.

When the immune system detects a virus, it will dispatch cells and molecules to memorize its features so it can be fought off more swiftly in the future. Vaccines impart these same lessons without involving the disease-causing pathogen itself-the immunological equivalent of training wheels or water wings.

The Pfizer and Moderna vaccines accomplish that pedagogy via a genetic molecule called mRNA that's naturally found in human cells. Once delivered into the upper arm, the mRNA instructs the body's own cells to produce a coronavirus protein called spike-a molecule that elicits powerful, infection-fighting antibody responses in people battling COVID-19.
Read: What's the use of a pretty good vaccine?
To ensure safe passage of mRNA into cells, the vaccine makers swathed the molecules in greasy bubbles called lipid nanoparticles. These strange, fatty spheres don't resemble anything naturally present in the body, and they trip the sensors of a cavalry of fast-acting immune cells, called innate immune cells, that patrol the body for foreign matter. Once they spot the nanoparticles, these cells dispatch molecular alarms called cytokines that recruit other immune cells to the site of injection. Marshaling these reinforcements is important, but the influx of cells and molecules makes the upper arm swollen and sore. The congregating cells spew out more cytokines still, flooding the rest of the body with signals that can seed system-wide symptoms such as fever and fatigue.

"It's the body's knee-jerk reaction to an infection," or something that looks like it, Mark Slifka, a vaccine expert and an immunologist at Oregon Health and Science University, told me. "Let's spray the area down with antiviral cytokines, which also happen to be inflammatory."

The mRNA itself might also tickle a reaction out of the immune system, simply because of how unusual it looks. "All of a sudden, you have a lot of new RNA that the cell didn't make," says Donna Farber, an immunologist at Columbia University, who got her second shot of Moderna's vaccine last month, with very few side effects.

The provocative nature of mRNA might help explain why Moderna's shot, which contains three times as much of the genetic material as Pfizer's, was linked to more side effects in clinical trials.

The innate immune system acts fast. But its actions aren't very long-lived or discerning: These cells just clobber anything that looks a little weird. Within a day or two of the injection, they start to lose steam. Cytokine production sputters; side effects start to fade. Around this time, innate immune cells start to pass the baton to another division of the immune system, called adaptive immunity, which includes sniperlike molecules and cells, such as antibodies and T cells, that will launch an attack on specific pathogens if they try to infect the body again.

T cells and B cells, the cells that make antibodies, need several days to study the spike's features before they can respond. But by the time the second injection rolls around, adaptive cells are raring to go, and far faster to react. Some of these cells have even been lingering at the site of injection, out of suspicion that their target would return. Stimulated anew, these sentinel cells will blast out their own cytokines, layering on an extra wave of inflammation. In some people, like my beleaguered neurologist husband, these complex reactions can manifest in fevers, aches, and prolonged exhaustion.

My husband had side effects after his first dose too: a headache, some fatigue, a touch of dizziness-all of which I can safely blame on his innate immune system. Those same innate responses return for another round of inflammation after the second shot. But the ruckus raised after the second injection might be a double whammy: The expected innate cells might be further egged on and amplified by a less sluggish surge of adaptive cells, concentrated near where the needle goes in.

"With the second dose, now everything is responding within that same short time period," Marion Pepper, an immunologist at the University of Washington, told me.

Pepper described her first shot of Pfizer's vaccine as "a piece of cake." The second injection saddled her with flu-like symptoms, tougher to take. But the side effects also signify that both branches of the immune system are being engaged as they should-cementing the memory of the coronavirus's spike protein in some of the body's most powerful cells.

That's a big part of why vaccine boosts are so important, Slifka said. Although the first shot stimulates both innate and adaptive immunity, the second injection reminds B and T cells that the threat of the coronavirus cannot be taken lightly, and ensures that the sharpest and strongest immune players will be used for any subsequent response.

"They're asking, 'Why is this happening 21 or 28 days later? I thought we took care of this four weeks ago,'" Slifka said.
Read: The long haul of vaccine results is just beginning.
Marcelin, the Nebraska infectious-disease physician, also experienced rougher symptoms after her second shot of Pfizer's vaccine. By the time she went to bed that evening, she was gripped with miserable muscle aches and chills. It took a couple more days before "I felt like myself," Marcelin recalled.

The side effects didn't faze her, though. She's now about three weeks out from her second dose-past the point when the vaccine's full protective effects are expected to kick in. "I would do it again," she says. "It was definitely worth it."

People shouldn't be perturbed by a lack of vaccine side effects either. As our bodies churn through new information, "some people's immune systems are louder than others'," Marcelin said. But the quiet ones are still hard at work.

My husband's immune system certainly fell into the diva category. The night after his second shot, he pinwheeled between cold and hot, alternately bundling himself in blankets and tossing them away. The flux seemed to bleed a bit into his emotional valence too. After snoozing on the couch for several hours, he perked up and couldn't stop laughing at a picture of an orange cat curled up next to a box of similarly crescent-shaped croissants.

But within 24 hours of his shot, he was feeling well enough to run (yes, run) to work and finish an 11-hour shift. In a couple of weeks, he'll join the millions of other Americans who, thanks to a pair of injections, will be cloaked in an extra layer of armor against the coronavirus.

As he told me Wednesday night, shivering through the cushion of two comforters: "This is a million times better than getting COVID."

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The Second COVID-19 Shot Is a Rude Reawakening for Immune Cells

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Side effects are just a sign that protection is kicking in as it should. At about 2 a.m. on Thursday morning, I woke to find my husband shivering beside me. For hours, he had been tossing in bed, exhausted but unable to sleep, nursing chills, a fever, and an agonizingly sore left arm.

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Science & Data behind the COVID-19 Vaccine 

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Science & Data behind the COVID-19 Vaccine 

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Featured Experts:

Yes. The two mRNA vaccines, Pfizer and Moderna, authorized by the U.S. Food and Drug Administration (FDA) and recommended by the Centers for Disease Control and Prevention (CDC), are very safe and very good at preventing serious or fatal cases of COVID-19. The risk of serious side effects associated with these vaccines is very small.

Lisa Maragakis, M.D., M.P.H., senior director of infection prevention, and Gabor Kelen, M.D., director of the Johns Hopkins Office of Critical Event Preparedness and Response, answer questions about the safety of the COVID-19 vaccines.

COVID-19 Vaccine Safety - What We Know

The Pfizer and Moderna vaccines are strongly recommended as safe and effective at preventing serious illness or death from COVID-19.

From December 2020 to December 2021, about 470 million doses of COVID-19 vaccine have been given in the U.S.

Roughly 12 months of data, including data from tens of thousands of participants in clinical trials, show that the vaccines are safe and effective at preventing serious disease or death due to COVID-19.

Additional shots and boosters are also authorized for those who meet the guidelines.

mRNA technology used in the Pfizer and Moderna vaccines has been in development for over 15 years.

The Pfizer and Moderna vaccines received full FDA approval, which means more data show how well that particular vaccine works.

Due to potential blood clots following administration of the Johnson & Johnson vaccine, the CDC updated its recommendations in December 2021. The CDC noted that the two mRNA vaccines, from Pfizer and Moderna, are preferred over the Johnson & Johnson vaccine.

Are there risks or safety concerns regarding the COVID vaccines?

To help catch risks or safety concerns, the CDC and FDA monitor possible safety issues with the COVID-19 vaccines. This ongoing work has led to revised recommendations for the Johnson & Johnson vaccine and information about rare side effects tied to the Pfizer and Moderna vaccines.

Johnson & Johnson/Janssen. In December 2021, the CDC updated its recommendations and stated that either of the two mRNA vaccines is preferred over the Johnson & Johnson (J&J) COVID-19 vaccine. The CDC explained that the Pfizer and Moderna vaccines are preferred due to a greater risk for developing rare but severe blood clots, called thrombosis with thrombocytopenia syndrome (TTS), following administration of the J&J vaccine. The CDC noted that there are some situations when a J&J vaccine may still be used, if the recipient understands the risks and benefits of the J&J vaccine. Read about side effects related to the rare blood clot.

At Johns Hopkins Medicine, we offer Pfizer and Moderna vaccines throughout our vaccine clinics.

Pfizer and Moderna. After observing rare occurrences of myocarditis following the second injection of the Pfizer and Moderna COVID-19 vaccines, the CDC released information for the public. Most of these myocarditis events happened in teens and young adults, and the majority of the cases were mild and cleared up on their own. Read about side effects related to myocarditis.

This review process continues to monitor vaccine safety. Potential risks of COVID-19 vaccines are reviewed and weighed against the benefits of protection that the vaccines offer as well as the known, serious risk of harm due to COVID-19.

What if I got a J&J vaccine? Should I worry about blood clots?

If you received a Johnson & Johnson vaccine more than three weeks ago, you do not need to watch for side effects related to blood clots.

If you received the J&J vaccine within the last three weeks, you should watch for possible symptoms of TTS and get medical help immediately if you have any of the following:

Severe or persistent headaches or blurred vision

Shortness of breath

Chest pain

Leg swelling

Persistent abdominal pain

Easy bruising or tiny blood spots under the skin near the injection site

Is there a risk of an allergic reaction to a COVID vaccine?

According to the CDC, anyone who has a severe allergy (e.g., anaphylaxis) to any of the mRNA vaccine ingredients should not receive this vaccine. The CDC says people with allergies to certain foods, insects, latex and other common allergens can safely receive the COVID-19 vaccine. Those with a history of severe allergic reaction (anaphylaxis) to injectables or other vaccines should discuss the vaccination with their doctor, who can evaluate and assess their risk.

The COVID-19 Vaccines: What you need to know | Las vacunas anticovid-19: información imprescindible

How was the COVID-19 vaccine developed so quickly?

The relatively quick development of these vaccines does not mean safety measures were skipped. There are several reasons why the COVID-19 vaccines were developed faster than other vaccines:

The technologies used to develop the COVID-19 vaccines have been in development for years to prepare for outbreaks of infectious viruses. The manufacturing processes were ready very early in the COVID-19 pandemic.

Countries shared genetic information when it was available about SARS-CoV-2, the coronavirus that causes COVID-19, which gave vaccine developers an early start at finding a vaccine.

The testing processes for the vaccines didn't skip any steps, but the vaccine developers conducted some stages of the process simultaneously to gather as much data as quickly as possible.

Governments gave money to vaccine developers in advance, so the companies had resources they needed.

Some types of COVID-19 vaccines were created using messenger RNA (mRNA), a new technology that allows a faster approach than the traditional way vaccines are made.

Social media enabled companies to reach out to and enroll study volunteers, and plenty of people wanted to help, so there were enough research participants to test the COVID-19 vaccines.

Because SARS-CoV-2 is so contagious and widespread, many volunteers who got the vaccines were exposed to the virus, and with so many exposures, it took a shorter time to see if the vaccines worked.

Companies began manufacturing vaccines ahead of their authorization or approval so some supplies would be ready if authorization occurred.

What steps are taken to help make sure the COVID-19 vaccines are safe?

Safety is always a top priority as federal agencies work with vaccine manufacturers and independent scientific organizations to develop, study, authorize and approve new vaccines. Here are some of the steps taken for COVID-19 vaccines, as well as other vaccines:

Careful testing. All vaccines go through clinical trials to test safety and effectiveness. For the COVID-19 vaccine, the FDA set high safety standards for vaccine developers to meet. This graphic from the National Institutes of Health shows the four phases a vaccine goes through before it is released to the public.

Authorization for emergency use. If a vaccine or medicine is needed to address an emergency situation such as the COVID-19 pandemic, once it is shown to be safe and effective, the FDA can grant it an emergency use authorization (EUA). An EUA allows a vaccine, treatment or medication to be used before the formal FDA approval.

Continuous monitoring for problems and side effects. Once a vaccine gets an EUA and is being given to people, the FDA and the CDC continue to watch carefully in case problems arise. Data on the vaccine's safety record accumulates over time, as more and more people who receive it report on their experience and any side effects. One important way to report adverse events after vaccination is through the Vaccine Adverse Event Reporting System.

Learn more from the CDC about the safety steps for the COVID-19 vaccine.

Demographics of the COVID-19 Vaccine Trials

Do I still have to wear a mask if I get a vaccine?

The CDC continues to monitor the spread of COVID-19 and makes recommendations for wearing face masks, both for those who are fully vaccinated as well as people who are not fully vaccinated.

The CDC also recommends that masks and physical distancing be required when going to a doctor's office, hospital or long-term care facility, including all Johns Hopkins hospitals, care centers and offices.

Johns Hopkins Medicine's mask safety guidelines have not changed, and we still require everyone to wear masks inside all of our facilities.

Can I get COVID-19 from the vaccine?

You cannot and will not get COVID-19 from any of the vaccines. The COVID-19 vaccines do not have any virus or other infectious material in them.

What about the safety of COVID-19 vaccination for diverse groups of people?

The FDA and other reviewers closely consider diverse populations and include them in the trials. The clinical trials for the first two COVID-19 vaccines included members of underrepresented minorities and older age groups, and people with conditions such as obesity, diabetes, and heart and respiratory conditions.

Does Johns Hopkins Medicine recommend that I get a COVID-19 vaccine?

Yes, we recommend that everyone who is eligible get vaccinated with one of the currently authorized mRNA COVID-19 vaccines: Pfizer or Moderna. We view both as highly effective in preventing serious disease, hospitalization and death from COVID-19 (including disease caused by the very contagious delta and omicron variants of the coronavirus), and we believe that their benefits outweigh their risks.

At Johns Hopkins Medicine, we offer Pfizer and Moderna vaccines throughout our vaccine clinics.

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Our experts answer some frequently asked questions about vaccine safety.

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Content Source: National Center for Immunization and Respiratory Diseases (NCIRD), Division of Viral Diseases

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