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Earlier this month, the number of people in the United States who have died from COVID-19 passed 675,000, eclipsing fatalities from the 1918 influenza pandemic and making this the deadliest outbreak in recent American history

Vaccines are our most powerful tool for bringing the novel coronavirus to heel. There are currently three vaccines against COVID-19 available in the U.S. The Pfizer-BioNTech vaccine was fully approved for people aged 16 and up by the Food and Drug Administration in August (the shots also have emergency use authorization for 12- to 15-year-olds and may soon become available for younger kids). The vaccines developed by Moderna and Johnson & Johnson have received emergency use authorization.

As of this week, more than 77 percent of adults in the United States have received at least one dose of these vaccines. However, some Americans are still unsure about being vaccinated. We’ve asked vaccine experts to address some common questions about how the vaccines work and their safety. Here’s what they have to say.

Should I be worried about how quickly these vaccines were developed?

There are two types of COVID-19 vaccines used in the US. Those from Pfizer and Moderna are mRNA vaccines, which use snippets of genetic material from the novel coronavirus, SARS-CoV-2, to train the immune system. The Johnson & Johnson shot is what’s known as a viral vector vaccine. It uses a different virus (the “vector”) that’s been engineered to deliver coronavirus genes. Both of these technologies have been under development for decades

“The global community came together and said, ‘we need to have ways to make vaccines quickly for emerging viruses,’” says Kawsar Talaat, an associate professor at the Johns Hopkins Bloomberg School of Public Health who researches vaccine candidates for pandemic influenza, COVID-19, and other diseases. “The coronavirus pandemic just sped up the process by infusing it with a tremendous amount of money, but certainly they didn’t come out of nothing.”

Two other members of the coronavirus family—the original SARS virus and the virus responsible for Middle East Respiratory Syndrome (MERS)—have caused worrying outbreaks within the past 20 years. Several research groups were therefore already working on vaccines for coronaviruses when the COVID-19 pandemic began, Talaat says, and were quickly able to apply this knowledge to developing and testing vaccine candidates for SARS-CoV-2. These included Moderna and the University of Oxford, which partnered with AstraZeneca to develop a vector-based COVID-19 vaccine that’s now used in several countries.

Was the vaccine tested as rigorously as other vaccines in the past?

When COVID-19 struck, researchers around the world put aside their other work to focus on developing a vaccine.   

“No corners were cut; all of the normal stages of vaccine development were followed,” Talaat says. “The only difference is that because there was a lot of money available, things happened at a much faster pace than they could have with more limited funding.”

This meant that vaccine developers could start planning later-phase clinical trials while early ones were still ongoing. Additionally, COVID-19 vaccine candidates were pushed to the front of the line for the Food and Drug Administration’s review process, cutting down on months of wait time.

The novel coronavirus’s wildfire spread also meant that researchers didn’t have to wait very long to find out which of the vaccine candidates undergoing clinical trials could ward off COVID-19. “It was really easy to test to see if they worked or not, because so many people who weren’t vaccinated were getting sick,” says Megan Ranney, an emergency physician and associate dean of strategy and innovation at the Brown University School of Public Health.

How safe are the vaccines and how do we know?

To determine how effective and safe a vaccine candidate is, researchers first test it in animals. If it performs well, the vaccine is moved into three rounds of increasingly large clinical trials in people, ranging in size from a few dozen to thousands of volunteers. Independent data and safety monitoring boards oversee this entire process, explains Kate Hopkins, the director of Research, Vaccine Acceptance & Demand at the Sabin Vaccine Institute in Washington, D.C. 

Even after a vaccine is licensed, health authorities continue to monitor the safety of the vaccine. “We can catch really rare side effects really early and address them and identify who is potentially at higher risk and inform people about those,” Talaat says.

Most side effects happen within a couple days after a person gets vaccinated. For COVID-19 vaccines, this can mean short-lived symptoms such as fever, chills, or soreness in the arm that received the injection. Serious allergic reactions to COVID-19 vaccines are rare and treatable; anaphylaxis has occurred in approximately two to five people per million who were vaccinated in the United States. There have also been very rare reports of a blood-clotting disorder associated with the J&J vaccine within several weeks of vaccination, mostly in adult women under the age of 50, and of inflammation of the heart muscle or surrounding tissue, particularly in male adolescents and young adults. 

More than 183 million people in the United States alone have been fully vaccinated against COVID-19 since mid-December last year, and no long-term side effects have been detected, Ranney says. 

“There are some medicines for which side effects won’t show up until months or years later because you’re taking them every day,” she says. With “a vaccine, you inject it, it helps your body create an immune response, and then it’s gone from your body; it doesn’t continue to change anything.”

Will the mRNA vaccine change my DNA?

Nope. All the available COVID-19 vaccines are designed to help our immune systems target the spike-shaped protein on the surface of the virus, which helps the virus attach to and invade our cells. The mRNA vaccines contain the genetic code in the form of messenger RNA (hence the name mRNA vaccine) for the spike protein. 

The mRNA is encased in little fat bubbles called lipid nanoparticles that help it enter our cells, Talaat says. Once inside the cells, the “protein-making factories,” or ribosomes, use these instructions to build copies of the spike protein. These little spikes trigger an immune response, but they aren’t the whole virus and cannot make us sick.

The mRNA is very delicate and breaks down quickly in the body. “mrNA is a very short-lived molecule; it’s gone within a very brief period of time,” Talaat says. “It doesn’t touch our DNA and it can’t change our DNA in any way.”

I’ve already had COVID-19. Shouldn’t my body be immune now?

People who’ve already had COVID-19 do develop some immunity to the virus, but they’re generally not as well protected as people who are vaccinated, Talaat says. 

Last month, researchers reported that COVID-19 survivors in Kentucky who remained unvaccinated were more than twice as likely as vaccinated survivors to become reinfected. In fact, COVID-19 survivors who later receive the vaccine may even develop a particularly robust immune response.

Another thing to keep in mind is that some people mount a stronger immune response than others after having COVID-19, and it’s hard to know who will fall into which group. Additionally, if you received monoclonal antibodies or convalescent plasma as a treatment for COVID-19, it might have interfered with your body’s ability to produce a lasting immune response, Talaat says. 

“It’s better to be vaccinated—to be safe and to make sure that you do have a good immune response against the virus,” she says.

Will the vaccine cause male or female infertility?

There’s no evidence that the vaccines have any impact on fertility

“We have now done studies of the COVID vaccine in pregnant women, in women of childbearing age, and in men, and have shown that it in no way changes any aspect of fertility, sperm counts, or pregnancy outcomes,” Ranney says.

Meanwhile, she notes, pregnant people face a heightened risk of severe illness from COVID-19, and the infection has been linked to decreased sperm counts and increased risk of preterm births.

I’ve never had the flu, and I’ve never gotten a flu shot. Am I healthy enough to not need the vaccine?

Your chances of getting COVID-19 are much higher than your chances of getting the flu, Talaat says. “The way the pandemic is circulating, I think at some point we’re all either going to have been infected or have been vaccinated, [and] sometimes both,” she says.

Compared to the flu, COVID-19 is also more likely to cause severe illness and death and spreads more rapidly, Hopkins says. In both cases, though, vaccines reduce the risk of becoming seriously sick or dying.

“Our healthcare system is quite at its breaking point, and anything we can do to keep ourselves healthy and not have to go see the doctor and not have to end up in the hospital is a good thing,” Talaat says.

Do young people get severe COVID-19?

Unfortunately, yes. While the risk of severe COVID-19 does increase with age, Hopkins says, young people can become seriously ill too. Age also isn’t the only risk factor; people with certain conditions such as diabetes and cancer are also more vulnerable to COVID-19.    

Most kids who catch COVID-19 don’t become seriously ill. However, weekly rates of COVID-19 hospitalizations in children 11 years old and younger are on the rise.

Young people can also develop long COVID, with symptoms such as fatigue and breathing problems lingering for weeks or months.

They have medications to treat COVID-19 now, so is it really a big deal if I get sick?

A number of treatments are being used to combat COVID-19, including monoclonal antibodies and the antiviral drug remdesivir. But these medicines aren’t perfect, Hopkins says. They’re also most effective when given early in the disease and aren’t available everywhere. 

“While the effects of some treatments have clearly shown improved outcomes, they are not universally effective in treating COVID-19 and preventing severe disease or death,” Hopkins says.

Why do I need a booster shot if the vaccines supposedly work?

Last week, the Centers for Disease Control and Prevention recommended booster doses of Pfizer’s COVID-19 vaccine for certain groups, including adults 65 and older, those especially vulnerable to becoming severely ill from COVID-19, and those who live or work in settings where they have a high risk of being exposed to the novel coronavirus. 

There isn’t strong evidence that booster shots are needed for the general public at this time, however. Breakthrough infections seem to be happening more commonly in older people, Ranney says. 

“So those are the people for whom we’re saying they need a booster dose, because their immune systems aren’t as strong to begin with,” she says. “But their chance of getting really sick or even dying with those breakthrough infections is still dramatically lower thanks to the vaccines than it was before the vaccines.”

People are getting breakthrough infections, so does the vaccine even work?

No vaccine is 100 percent effective at preventing every infection, Hopkins says, and measures such as social distancing and masking up will still play an important role in thwarting the spread of COVID-19. But that doesn’t mean that the shots aren’t doing their job.

Breakthrough infections are still uncommon, even with the rise of the highly contagious delta variant of the virus, The New York Times recently reported. And when vaccinated people do catch COVID-19, their cases tend to be much milder on average and less likely to lead to long COVID compared with those in unvaccinated people. Estimates vary, but one recent report from the CDC indicates that vaccinated people were about five times less likely to be infected and more than 10 times less likely to be hospitalized or die from COVID-19 than those who were unvaccinated. 

“My ER used to be full of old people really sick with COVID and a few young ones,” Ranney says. “Now almost the only people who are coming into my ER sick with COVID are people who did not get the vaccine.”

There’s evidence that COVID-19 vaccines are reducing asymptomatic infections and transmission, even in the face of the delta variant. This means that getting vaccinated can also protect those around you, Talaat says.

“Even if you don’t think that your own personal risk of COVID is that high, think about the people in your life who are important to you, and are any of them at higher risk for COVID?” she says. “Wouldn’t you want to do everything you can to protect them?”

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