COVID-19 Vaccine FAQ: Safety, Side Effects, Efficacy

Nick Tate

December 17, 2020

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Pfizer began shipping the first batch of COVID-19 vaccines to all 50 states this week, capping months of anticipation, development, clinical trials, and regulatory reviews.

Amid efforts to fast-track the Pfizer/BioNTech vaccine, the FDA granted emergency use authorization for the vaccine -- delivered in two doses, 3 weeks apart -- deeming it safe and effective.

WebMD went to the experts for answers to a dozen frequently asked questions about the vaccine. Here's what you need to know.

How safe is the COVID-19 vaccine?

Paul Offit, MD, a member of the FDA advisory panel that recommended the vaccine's use, says rigorous clinical trials of the shot identified no safety concerns, despite its sped-up production.

"These vaccines were subjected to large phase III clinical trials," says Offit, a vaccine expert at Children's Hospital of Philadelphia. "Regarding safety, there was an insistence by the FDA that at least tens of thousands of people be observed for 2 months after the final dose to make sure that there were no uncommon side effects."

Offit says the fast-tracking of the vaccine was mostly a result of the upfront financing the federal government provided, so no shortcuts were taken in verifying its safety.

"The government essentially paid for overlapping phenomena, which is to say the vaccines were mass-produced without knowing if they worked or were safe yet, so they overlapped the phase III trials with the mass production, which would never normally happen," he says.

Gigi Gronvall, MD, an immunologist and infectious disease specialist with the Johns Hopkins Center for Health Security, agrees and says the vaccine still underwent all of the safety checks involved in any clinical trial.

"It has been tested as other vaccines and drugs have been. There were no things done differently, but the financial part made things happen faster," says Gronvall, an associate professor at the Johns Hopkins Bloomberg School of Public Health.

How concerned should we be about adverse reactions being reported?

There is a remote chance the vaccine could cause an allergic reaction in some people within an hour of receiving the shot, the FDA says. But such reactions are likely to be mild and not life-threatening.

British regulators stoked concerns about such risks last week, after two U.K. health care workers had allergic reactions to the Pfizer vaccine. Two health care workers in Alaska also reported severe allergic reactions.

As a result, British health authorities say they will continue to investigate the two cases and monitor the vaccination program for other cases. But they also say that serious reactions to the vaccine are "very rare."

During a meeting of an FDA advisory panel last week, experts pressed company officials on the question of allergic reactions, in light of the British cases.

Pfizer officials say there were no cases of serious allergic reactions in the clinical trial of its safety and efficacy involving nearly 44,000 participants.

The FDA panel decided against recommending that people with a history of allergic reactions to vaccines avoid the Pfizer shot — a position later echoed by the CDC.

But in greenlighting the vaccine, the FDA is requiring Pfizer to increase its monitoring for allergic reactions and submit follow-up data. That decision cleared the way for the first vaccinations to begin this week.

Offit hails the FDA decision and says follow-up studies of the vaccine's safety will put to rest any lingering concerns.

"The benefits of that vaccine far outweigh what now are just considered to be theoretical risks," he says.

He also says the British authorities' stance on potential allergic reactions and overreaction could needlessly scare many people away from a desperately needed vaccine in the middle of a raging pandemic.

Since the Pfizer vaccine program began, six severe allergic reactions have been reported in the U.S. out of 272,001 doses administered, according to the most recent Centers for Disease Control and Prevention report.

Scientists investigating the reactions say the culprit may be the compound polyethylene glycol (PEG), which is part of the fatty envelope that surrounds the mRNA, the main ingredient in the vaccine.

In addition, four cases of Bell's palsy, a type of temporary facial paralysis, were reported in people who received the vaccine. This is not more than would be expected in the general population, however.

In a statement, Pfizer said it "will closely monitor all reports suggestive of serious allergic reactions following vaccination." The company said its prescribing information includes a warning that "appropriate medical treatment and supervision should always be readily available in case of a rare anaphylactic event following the administration of the vaccine."

Should you get the vaccine if you have had allergic reactions to vaccines, medicines, or foods in the past?

The short answer is: Yes. But with a caveat.

The CDC on Sunday said people who have had severe reactions to vaccines or injectable drugs can still get the Pfizer vaccine for COVID-19. But they should discuss the risks with their doctors and be monitored for 30 minutes after getting the shot.

That recommendation, issued Dec. 13, reverses an initial CDC proposal one day earlier that would have advised those with a history of reactions not to be vaccinated. During a follow-up webinar with doctors, CDC officials said patients who have had severe reactions should take precautions, but they did not recommend they avoid the COVID-19 shot.

"My understanding of those (U.K.) cases is the people involved had had a history of hospitalization for allergic reactions," Gronvall says. "But there were a lot of people amongst the people who participated in the vaccine's clinical trials who had allergies to one thing or another. So, it's important to make that distinction."

It's also unclear what caused the allergic reactions in the U.K. workers, and it's possible the vaccine was not the culprit.

Gronvall says any adverse reactions will be monitored as the vaccine rolls out nationwide. She says follow-up research should help dispel concerns about rare allergic reactions among some people who use EpiPens — devices that inject the drug epinephrine to counteract a severe allergic reaction, known as anaphylaxis.

"I have already seen it — that people who carry EpiPens are concerned about whether they should get the vaccine," she says. "But this is data that the companies have indicated [some] of the people who participated in the clinical trials…were people who carry EpiPens.

"On balance, there's not a restriction here for people who have allergies. And I would say the risks of COVID-19 are pretty high right now," Gronvall says.

Offit says anaphylactic reactions to vaccines are very rare and easily treatable with epinephrine. That makes them much easier to manage than a severe case of COVID-19.

"You're always asked to wait around after a vaccine for 15 minutes because about one out of every million people who get a vaccine can have a severe allergic reaction," he says. "The only difference here is if you have had a severe allergic reaction in the past to an injectable medical product, then you wait around for 30 minutes, instead of 15."

Offit says there's no reason for concern for people with allergies to foods, beestings, or other medicines, noting studies have found many people who have allergies have received multiple vaccines without problems.

Can the vaccine cause other side effects?

Yes. The FDA says common side effects, in the clinical trial involving some 44,000 people, included pain where they got the shot, fatigue, headache, chills, fever, and joint and muscle pain. But these are all described as temporary, or transient.

The symptoms were generally mild or moderate, and happened more frequently after the second dose.

"People should also know that less serious adverse reactions — side effects — are pretty common with this vaccine, so it's not going to be like the flu shot, where you might just have a sore arm," Gronvall says.

"For a lot of people, they'll be pretty tired, they might have a small fever, and they can have headaches. And all of these reactions are normal, and that's the result of your body getting ready to be able to fight off COVID."

Why are side effects are worse on the second dose of the vaccine?

"It does appear that people have more effects after the second dose than the first," Gronvall says. "The reason is…as the vaccine gets your body ready to fight the disease, some of the things your body does to fight the virus makes you feel a little under the weather.

"That's because the vaccine is like a training program for your immune system. A lot of what your body does -- the [immune system] cells that are stimulated, the antibody production that's underway — all that stuff can make you feel…not so great for a day or 2."

Although these side effects can be unpleasant, they are temporary and an indication the vaccine is working.

"So, people should know that and…not feel that there's something wrong," Gronvall says. "That's my concern — that people will be surprised."

One other thing: "Older people tend to have fewer side effects than people in the 18-to-55 range, possibly because…younger people have more active immune systems," she says.

How effective is the vaccine, and is it well-tolerated by people who've received it?

In Pfizer's large clinical trial, the two doses of the vaccine were shown to be 95% effective at preventing COVID-19 cases, according to results published in The New England Journal of Medicine .

That's more than twice the effectiveness of the annual flu shot, which typically carries a 40% efficacy rate.

The clinical data also suggest the vaccine may prevent COVID-19 after just the first dose alone — with a 52% efficacy rate.

The vaccine was also fairly well-tolerated by people who took part in the study.

Although side effects of the vaccine are common — with most people in the study having reactions where they got the shot, headaches, and other symptoms — most were temporary and not serious.

Are there risks associated with vaccines made from genetic material called mRNA?

Not at all, the experts say.

Here's why: The Pfizer vaccine uses a snippet of genetic material called messenger RNA -- mRNA for short -- that doesn't contain the virus itself, but rather a genetic code associated with the coronavirus that triggers the body's immune response to fight it.

The mRNA essentially teaches human cells to make something resembling the so-called spike protein found on the surface of the coronavirus. This prompts the body's natural defenses to destroy it, if the vaccinated person is later exposed to the real virus.

"Part of what makes mRNA so delicate, and difficult to [use] logistically — requiring these ultra-cold conditions — is that it's pretty delicate," Gronvall says. "So once it gets into your cells, protein is made from it, and then the mRNA degrades pretty rapidly."

The CDC says mRNA vaccines are relatively new but have been around for years and found to be safe. The COVID-19 vaccine, for instance, cannot give someone the virus and cannot affect or interact with our DNA or change our genes in any way.

"The COVID-19 mRNA…vaccines are being held to the same rigorous safety and effectiveness standards as all other types of vaccines in the United States," according to an online CDC explainer.

A second COVID-19 vaccine, made by Moderna, also uses mRNA and has received FDA approval. The first doses of the vaccine, which contains PEG, began Dec. 21; A doctor in Boston experienced a severe allergic reaction after receiving the Moderna vaccine on Christmas Eve. The doctor is also allergic to shellfish and brought his own EpiPen to the vaccination, and he recovered quickly after using it. It is the first known severe reaction to the Moderna vaccine.

Ten other vaccines are also now in late-stage trials across the globe, using various other methods.

Researchers have been studying and working with mRNA vaccines for decades — for flu, Zika, rabies, and cytomegalovirus (CMV). Cancer researchers have also used mRNA to trigger the immune system to target specific tumor cells.

Does the vaccine pose any special concern regarding autoimmune disorders?

In a word: No.

Gronvall says there is no scientific evidence that mRNA can lead to immune system disorders or autoimmune diseases.

"There are a range of syndromes, even if they're very rare, that different sentinel [tracking] systems are looking for to see if there are any added risks for the vaccines, and those systems will be going on for years," she says.

"But as of now, the data doesn't say that an autoimmune risk is something that we need to be concerned about."

She adds that the vaccine is safe for people who have autoimmune disorders or those who are being treated with other immunotherapies — for allergies, cancer, arthritis, or other conditions.

"There are no contraindications at this point for people with those conditions," she says.

Betty Diamond, MD, an immunologist and rheumatologist at the Feinstein Institutes for Medical Research of Northwell Health, agrees that there's no evidence that mRNA vaccines cause autoimmune disease or may cause problems for people with such conditions.

"At the moment, there's every reason to suggest that people with autoimmune diseases ought to get either of these vaccines when they get rolled out," she says.

What about potential long-term side effects?

Offit says there is no evidence that the Pfizer vaccine can cause long-term side effects, noting that most would have likely appeared during the Pfizer study.

But he says the follow-up studies will be keeping a close eye out for that possibility.

"I can't name you a long-term side effect," he says. "Vaccines can cause serious rare side effects…that's true. But that all occurs within 6 weeks of a dose."

How is the government going to track potential side effects?

The FDA and CDC have created several programs for tracking any adverse reactions to the vaccine.

Health authorities are encouraging anyone who has a bad reaction to the vaccine to file a report with the Vaccine Adverse Event Reporting System (VAERS) or call 800-822-7967.

This national system collects and analyzes such reports to help federal health authorities monitor the safety of vaccines.

Health care professionals are also required to track and report reactions to the vaccine under the FDA's emergency use authorization of the shot.

In addition, CDC officials are using a new smartphone-based tool called v-safe to check in on people after they get the vaccine. If you enroll in the v-safe program, you will receive text messages directing you to surveys to report any problems or adverse reactions.

Several other CDC and FDA programs are designed to monitor acute-care centers, long-term care facilities, and large health insurance databases.

Gronvall says these programs should provide more reassurance over time that the vaccine is safe and effective.

"People should know that there is a system in place to capture this data…to figure out if any problem people have is because of the vaccine," she says. "So, there's going to be a lot of data to help people feel less concerned about it."

What don't we know about the vaccine?

It is still unclear if the vaccine makes you less contagious to others, if you contracted the virus before getting the shot and/or are asymptomatic.

"There are going to be studies that are planned on that. Hopefully, those studies will be done early next year, and then we'll know more," Offit says.

The FDA also says it's unclear whether the vaccine is effective and safe for children younger than 16.

In addition, more information is needed to determine how long the vaccine's protection lasts and whether you'll need to get one annually, like the flu shot.

Researchers also need to know more about the safety and effectiveness in pregnant and breastfeeding women, Gronvall says.

"That's the biggest areas where we need to gather more data — in pregnant women," she says. "They will need to be enrolled in clinical trials to be able to study them more carefully, even though the guidance was that they are not prohibited from getting the vaccine."

Offit agrees, noting pregnant women were included in the vaccine trial.

"Right now, the CDC currently says if you are pregnant, you may still get this vaccine; if you're breastfeeding, you may still get this vaccine," he says. "But there were about two dozen women in the Pfizer trial…who were pregnant at the time, and [with] the vaccine there was no untoward effect on the pregnancy."

So, can we stop wearing masks now?

Nope. Not yet.

Until scientists can determine if the vaccine makes you less contagious to others, we all still need to mask up, keep our distance from others, wash our hands often, and use other smart prevention strategies, experts say.

"For now, those who get the vaccine should assume that they might develop an asymptomatic infection and be contagious to others" Offit says. "So, at least for the moment, they should still wear a mask."

Gronvall adds: "The bottom line is you should wear a mask until we know more."


Paul Offit, MD, vaccine expert, Children's Hospital of Philadelphia.

Gigi Gronvall, MD, immunologist and infectious disease specialist, Johns Hopkins Center for Health Security.

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