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SciCheck’s COVID-19/Vaccination Project

Q&A on COVID-19 Vaccines

This article is available in both English and Español

Editor’s note: We are no longer updating this article, which was last revised in July 2021. For up-to-date information about the COVID-19 vaccines authorized in the United States, please see our articles on the Pfizer/BioNTech, Moderna, Johnson & Johnson and Novavax vaccines. 

Three COVID-19 vaccines are now authorized in the U.S. In this story, we answer some common questions about the shots.

For more information about specific vaccines, we’ve created individual pages for COVID-19 vaccines that are authorized.

The first vaccine that received the Food and Drug Administration’s green light was Pfizer/BioNTech’s two-dose mRNA vaccine, which was authorized via an emergency use authorization, or EUA, on Dec. 11. (For more, please see “A Guide to Pfizer/BioNTech’s COVID-19 Vaccine.”)

Close behind was Moderna’s vaccine, which also uses an mRNA platform and got the same nod on Dec. 18. A day earlier, the FDA advisory committee voted 20-0, with one abstention, that the vaccine’s benefits outweigh the risks for use in people 18 years of age and older. Moderna announced on Nov. 30 that its product had a 94% efficacy in preventing symptomatic disease in its phase 3 trial. (See “A Guide to Moderna’s COVID-19 Vaccine” for more.)

On Feb. 27, the one-shot vaccine from Janssen Biotech Inc., a Johnson & Johnson pharmaceutical company, was authorized by the FDA. Unlike the first two vaccines authorized in the U.S., the J&J vaccine uses adenovirus viral vector technology. (See “A Guide to Johnson & Johnson’s COVID-19 Vaccine.”)

AstraZeneca announced on March 25 that its vaccine had 76% efficacy against symptomatic disease in its U.S. phase 3 trial, and it expected to file for FDA authorization in “the coming weeks,” although it still has not done so as of mid-July.

In June, Novavax released results from a phase 3 trial in the U.S. and Mexico testing its protein subunit vaccine, finding an overall efficacy of 90.4% against symptomatic COVID-19 and full protection from moderate to severe disease. The company has indicated that it might not file for authorization until the end of September.

When can I get a vaccine, and how is it being distributed?

Access to a vaccine will depend on where you live and whether you’re at high risk of exposure to the coronavirus or of developing a severe case of COVID-19 if infected.

Given the limited early supply, the first COVID-19 vaccines are slated primarily for health care workers and nursing home residents. The federal government is doling out doses to states and other jurisdictions based on population, and letting those local governments decide on the specifics of prioritization within their borders.

Although states have leeway to make alternate decisions, most are sticking to the guidance from the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices, which concluded on an interim basis that the first vaccines (phase 1a) should go to the nation’s 21 million health care personnel and 3 million residents of long-term care facilities.

Health care workers are not only at increased risk of getting COVID-19, but also are needed to keep hospitals running during the pandemic. Those in nursing homes and similar facilities have higher rates of death or severe COVID-19 than other Americans, due to their age, preexisting conditions and shared living conditions.

The next prioritized groups (phase 1b), as recommended by ACIP in a Dec. 20 meetinginclude those 75 years of age and older and front-line essential workers, such as first responders, educators, and agricultural, correctional, postal, public transit, manufacturing and grocery store employees. Combined, ACIP estimates this phase includes about 49 million people.

Following that, ACIP suggests vaccinating people between the ages of 65 and 74, anyone younger who has a high-risk medical condition, and the remaining essential workers (phase 1c) — groups that together account for around 129 million Americans.

All of this means vaccine doses are unlikely to become available to the general public beyond those groups for at least several months after the initial rollout.

Officials with Operation Warp Speed, the U.S. government’s effort to fast-track vaccine development and manufacturing, said on Dec. 14 that between Pfizer/BioNTech and Moderna, they expect to have enough doses to vaccinate 20 million people by the end of the year and 100 million people by the end of March. That would cover everyone in phase 1a and 1b and a portion of those in phase 1c.

According to then-Health and Human Services Secretary Alex Azar, the U.S. government signed contracts to purchase enough vaccines to allow any American who wants to get vaccinated to do so by the end of June 2021, although that assumes the various candidate vaccines are authorized and companies are able to deliver the promised doses. 

Other experts caution that these timelines may be optimistic and will be in flux, as it’s not unusual to run into manufacturing hiccups as vaccine production is ramped up.

“It’s really hard to mass produce vaccines,” Dr. Paul A. Offit, a vaccine expert at the Children’s Hospital of Philadelphia, told us, adding that the delicate lipid-wrapped mRNA vaccines have never been scaled up before. “Hopefully there won’t be glitches.”

The early distribution efforts have not been as speedy as initially hoped. As of Jan. 6, only 17.3 million doses had been distributed, with just 5.3 million going into arms, according to a CDC tally.

But the pace of vaccination has increased substantially over the following months, with the U.S. administering more than 3 million doses on some days in late March.

The Biden administration in February increased orders from Pfizer and Moderna, the only two companies with authorized vaccines at that point, and announced some of the supply would be delivered faster than originally expected. On March 2, President Joe Biden announced there would be enough vaccine doses for every U.S. adult by the end of May.

About a week later, Biden called on states to make all adults eligible for vaccination by May 1, and by late March, most states had announced they would reach that goal earlier.

How effective are the vaccines, and what do they protect against?

The details will be specific to each vaccine, but the Pfizer/BioNTech, Moderna and Johnson & Johnson vaccines are very effective at preventing symptomatic disease.

According to the results of the phase 3 trials, the Pfizer/BioNTech and Moderna vaccines had an efficacy of 94% or higher, which approximately means your risk of getting sick is cut by 94% or more if you are vaccinated. That’s a very good result — although it’s important to keep in mind that the effectiveness of the vaccines outside the controlled setting of a clinical trial is typically somewhat lower. A study by the Centers for Disease Control and Prevention found these two vaccines were 90% effective in real-world conditions, two weeks after the second dose, and 80% effective two weeks after the first dose. The study monitored 3,950 health care personnel, first responders and other essential workers for 13 weeks.  

Data from the Moderna trial also demonstrate that the vaccine protects against severe COVID-19. The Pfizer/BioNTech vaccine likely does as well, although the trial accumulated fewer severe cases, limiting the conclusions that can be drawn on that point.

Johnson & Johnson’s phase 3 trial began in late September, nearly two months after the Pfizer and Moderna trials and shortly before a new variant of the coronavirus emerged in South Africa. Globally, J&J reported an efficacy of 66.1% in preventing moderate to severe COVID-19 and an efficacy of 85.4% in preventing severe or critical COVID-19. But efficacy was higher in the U.S. population (72% efficacy in preventing moderate to severe disease and 85.9% efficacy in preventing severe or critical disease) than in South Africa (64% and 81.7% efficacy in preventing moderate to severe or severe/critical disease, respectively).

The clinical trials evaluated whether people developed symptoms, and subsequently tested positive for the coronavirus, or SARS-CoV-2. What is still unknown is whether these vaccines protect against infection with the coronavirus, and if not, whether they reduce or prevent viral transmission. The shots might only protect against disease, which could mean vaccinated people who are infected but not showing symptoms could still pass the virus on to others.

However, on March 8, the CDC said that “a growing body of evidence suggests that fully vaccinated people are less likely to have asymptomatic infection and potentially less likely to transmit SARS-CoV-2 to others.”

As National Institute of Allergy and Infectious Diseases Director Dr. Anthony Fauci explained in a December interview, it’s possible for a vaccine to block transmission even if it doesn’t prevent infection.

“The level of immune response that protected you from clinically recognizable disease would be enough to lower the titer of the virus in your nasopharynx such that you would not be infective,” he said of that scenario. “So you could be infected, but not have enough virus in your nasopharynx to transmit it.”

If I get vaccinated, can I return to my normal activities without precautions?

Not entirely, at least not yet. In guidance for the fully vaccinated, the Centers for Disease Control and Prevention said fully vaccinated people “no longer need to wear a mask or physically distance,” except in health care settings, correctional facilities and homeless shelters. The CDC also noted that its guidance does not apply in settings where masks and social distancing are “required by federal, state, local, tribal, or territorial laws, rules, and regulations, including local business and workplace guidance.” The CDC also continues to require face masks “on all planes, buses, trains, and other forms of public transportation.”

How long will a person be protected if vaccinated?

Scientists don’t know how long vaccination will last, but this is something that the clinical trials, which are continuing, will try to answer.

Dr. Peter Marks, the head of the FDA’s Center for Biologics Evaluation and Research, which is responsible for vaccines, told JAMA on Dec. 14 that based on the people who were immunized earliest in the trials, it appears there’s a duration of protection against disease of at least several months.

“It’s probably not just two months,” he said. “It’s probably at least probably on the order of four to six months. The question is, will it reach out to a year? Hopefully yes, but I think we’ll have those data in the not too distant future — and we’ll have real data, because of the people who are being followed on the trial.”

Offit anticipates the protection might last years, although he cautions that that’s just a guess.

“I think the thing that was most encouraging, at least looking at the Pfizer and Moderna data, is they induce T cell responses, which means there’s decent T cell memory,” he said, adding that that suggests there would also be decent B cell memory.

Those memory responses would mean that even if a person no longer has circulating antibodies, they’d still be able to mount a rapid response if they encountered the virus and could likely avoid falling ill.

It may turn out that people will need to receive an additional shot periodically to boost the immune system to extend the duration of protection. But Offit doesn’t think this would need to be so frequent as every year.  

What side effects should you expect if you get a COVID-19 vaccine?

Reports suggest that many of the coronavirus vaccines will be more unpleasant to get than a typical shot.

Trial data from Pfizer/BioNTech and Moderna show that many people experience pain at the site of injection, fatigue, joint or muscle pain, headache, chills or fever after receiving the jabs. These reactions are more likely after the second dose, given several weeks after the first, and more common and severe in younger people.

Of the two vaccines, Moderna’s vaccine may produce a stronger reaction, as more people reported some of these symptoms and to a greater degree than in the Pfizer trial, although it’s hard to compare the two directly. 

Offit said these reactions were not dangerous — and are actually good signs of the vaccine working — but some people might end up missing a day of work or two.

Harvard immunologist Barry Bloom said in a press call that the difference could come down to the different lipid formulations used in each vaccine or the fact that the Moderna vaccine includes more mRNA per dose than the Pfizer/BioNTech shot.

As for the J&J shot, side effects reported in the clinical trial include injection site pain, headache, fatigue and myalgia (muscle pain or ache), generally lasting a day or two post-vaccination. These side effects were more common in 18- to 59-year-old participants.

No serious safety concerns were reported in any of the trials for the three vaccines.

Since the rollout of the Pfizer/BioNTech and Moderna vaccines, a small number of people in the U.S. have had serious allergic reactions following receipt of the shots. Some allergic reactions are to be expected with any vaccine, although these are typically very rare.

On Feb. 26, J&J said it had received one report of anaphylaxis, a life-threatening allergic reaction, in South Africa.

As of March 22, the CDC said that anaphylaxis was rare: There were 2 to 5 reported cases per million people vaccinated. “This kind of allergic reaction almost always occurs within 30 minutes after vaccination,” the CDC said, adding that vaccine providers have medicines on hand to immediately treat any such reaction.

To make sure serious allergic reactions can be identified and treated, all people receiving a vaccine should be observed for 15 minutes after getting a shot, the CDC says, and anyone who has experienced anaphylaxis or had any kind of immediate allergic reaction to any vaccine or injection in the past should be monitored for a half hour.

People who have had a serious allergic reaction to a previous dose or one of the vaccine ingredients should not be immunized. Also, those who shouldn’t receive one type of COVID-19 vaccine should be monitored for 30 minutes after receiving a different type of vaccine.

In late April, the CDC and the FDA  added warnings to fact sheets for the J&J vaccine about a suggested increased risk of rare blood clots combined with low levels of blood platelets, a condition called thrombosis with thrombocytopenia syndrome. These rare cases hadn’t been found among the more commonly administered Pfizer/BioNTech and Moderna vaccines, which use a different type of vaccine technology.

The CDC says of the J&J vaccine that “women younger than 50 years old especially should be aware of the rare risk … and that other COVID-19 vaccines are available where this risk has not been seen.” As of July 12, the CDC and FDA had confirmed the condition in 38 cases, after more than 12.8 million doses of the J&J vaccine. There have been three deaths linked to the blood clotting condition, as of May 7, according to the CDC.

In July, the FDA also warned of an observed increased risk of the neurological disorder Guillain-Barré Syndrome associated with the J&J vaccine. The warning came after preliminary reports of 100 cases including one reported death, but it isn’t known whether the death or the cases were caused by the vaccine.

There is emerging evidence that the Pfizer/BioNTech and Moderna mRNA vaccines may very rarely cause inflammation of the heart muscle (myocarditis) or of the surrounding lining (pericarditis), particularly in young men. In late June, after a review of safety monitoring data, the CDC’s Advisory Committee on Immunization Practices concluded there is a “likely association” between COVID-19 mRNA vaccination and the two conditions in teens and young adults.

As of July 12, the agency says there have been 1,047 preliminary reports of either condition in people 30 years old or younger following immunization with any COVID-19 vaccine. The bulk of the reports, which are through the Vaccine Adverse Event Reporting System and do not necessarily mean the vaccine caused the problem, are with the Pfizer and Moderna vaccines and are more common among males and after the second dose.

Health officials have emphasized that the potential vaccine-related myocarditis and pericarditis cases are rare and the benefits of vaccination still outweigh the risks. Early evidence, the committee said, suggests these myocarditis cases are less severe than typical ones. The CDC has also noted that most patients who were treated “responded well to treatment and rest and quickly felt better.”

A review of clinical information has not demonstrated that the vaccines have caused any deaths, the CDC said, although there is “a plausible causal relationship” between the J&J vaccine and a few fatal cases of thrombosis with thrombocytopenia syndrome.

Did the speed of vaccine development compromise on safety?

No. As Fauci has explained, the unprecedented speed of the COVID-19 vaccines was due to multiple factors, including past investments in mRNA vaccines, and did not require skimping on safety.

“The speed of it is a reflection of the extraordinary scientific advances that have been made in platform technologies for vaccines. It was not at the expense of safety,” he said. “In addition, an enormous amount of money — hundreds of millions, if not billions of dollars — were put into the development and the production of vaccine doses, so that they would be ready to give to people as soon as the vaccine was shown to be safe and effective.”

As a technology, one of the boons of mRNA vaccines (the design of the Pfizer/BioNTech and Moderna vaccines) is their speed — in terms of design, a plausible candidate can be cooked up just knowing the genetic sequence of the virus and an idea of which protein to target.

In this case, scientists already knew from past experience with SARS and MERS, which are other diseases caused by coronaviruses, that the spike protein the virus uses to enter cells was likely the right one. And much of the work had already been done in determining how best to ferry the RNA into cells.

Although no mRNA vaccine has ever received FDA approval before, this type of vaccine has been studied for years, including in people in some early-phase clinical trials.

With Johnson & Johnson, the company had experience working with adenovirus-based vaccines, including an Ebola vaccine that gained approval in Europe in July.

The testing of the vaccines was also sped up by consolidating some of the early testing stages. As Icahn School of Medicine at Mount Sinai virologist Florian Krammer has written in the journal Nature, the trials “were designed such that clinical phases are overlapping and trial starts are staggered, with initial phase I/II trials followed by rapid progression to phase III trials after interim analysis of the phase I/II data.”

Frequently, these steps are only completed sequentially, largely because companies don’t want to proceed with expensive trials until there’s a sign that the vaccine will work. But with the government taking on the financial risk in many cases, that pressure was removed.

“It is very important to point out that moving forward at financial risk is the main factor that has enabled the accelerated development of SARS-CoV-2 vaccine candidates,” Krammer wrote, “and no corners have been or should be cut in terms of safety evaluation.”

The phase 3 randomized controlled clinical trials, which each enrolled upwards of 30,000 people, were also hastened because of the severity of the coronavirus outbreak in the U.S. With half of people receiving a placebo and the other half getting the vaccine, how long a trial takes is dependent on how rapidly COVID-19 cases accumulate.

And of course, production capacity was sped along by starting manufacturing even before knowing whether the vaccines would be successful. Here, another plus for the mRNA vaccines is that their components are often less time-consuming to produce than traditional vaccines, which require production of protein or virus in cells or eggs.

Why are the vaccines being authorized on an emergency basis, and how is that different from full approval?

Given the threat of the ongoing pandemic, the FDA is expediting its review of COVID-19 vaccines. Those authorized so far applied for EUAs rather than the standard full licensure, which is known as a biologics license application, or BLA.

An EUA typically is a less stringent standard — regulators must determine that the product “may be effective” and the “known and potential benefits outweigh the known and potential risks.” But knowing that a vaccine will go into millions of healthy people, the FDA set up special, more rigorous requirements for a COVID-19 EUA for vaccines that resemble the requirements for a BLA.

In particular, the FDA required “at least one well-designed Phase 3 clinical trial that demonstrates the vaccine’s safety and efficacy in a clear and compelling manner” and said it expected vaccine companies to continue to collect data to pursue full FDA-approval. The agency also wanted to see at least two months of follow-up data on half or more of the participants.

That’s less than the minimum of six months that’s usually needed for a BLA, but in terms of safety, there is little reason to think more time would uncover any concerns.

“When vaccines cause serious and occasionally fatal adverse events, they invariably occur within six weeks,” said Offit, who serves on the FDA’s advisory committee.

It’s important to be clear that no vaccine or medical product is 100% safe, and so while large phase 3 clinical trials of the first three authorized vaccines have not revealed any safety concerns, it’s possible rare adverse events could crop up as the vaccine is moved into millions of people.

“I think you have to be humble and be open-minded to the fact that there may be a serious side effect,” said Offit.

But, as he told JAMA, this is always true when deploying a new vaccine, which is why surveillance systems are in place — and any theoretical risk of a vaccine must be balanced against the risk of contracting SARS-CoV-2.

“The choice not to get a vaccine here is not a risk-free choice. It’s a choice to take a different risk,” he said. “The term safety in the medical world means that the benefits outweigh what are at this point theoretical risks.”

If there is a rare serious adverse event, Offit told us, the good news is that it will be detected rapidly and usually within the first several million doses. As we noted, the CDC and FDA have identified 32 cases of a rare blood clotting condition among the more than 10.2 million J&J vaccines administered. They recommended a pause in the use of the vaccine on April 13 while they investigated the first six cases identified.   

How can I report a potential safety issue?

The CDC and FDA have assembled multiple surveillance systems to monitor the safety of COVID-19 vaccines as they are taken by more and more people. Several of these systems draw on participation from the public, and people are encouraged to use them.

The first is the previously existing Vaccine Adverse Event Reporting System. Experts will comb through the submissions to identify and follow-up on adverse reactions that might be due to the vaccine.

The CDC is also rolling out a new smartphone-based tool called v-safe for COVID-19 vaccines. If you register for the program, you will be asked via text message how you are feeling every day for a week after vaccination and less frequently after that. If your answers indicate a potential problem, the system will flag your responses and someone will contact you for more information.

How much will a COVID-19 vaccine cost?

For the duration of the pandemic, no one should have to pay anything for a COVID-19 vaccine or its administration.

Karyn Schwartz, a senior fellow with the Kaiser Family Foundation with expertise in health insurance, told FactCheck.org that the vaccines themselves are being provided by government free of charge and there will be no out-of-pocket costs for patients due to various changes to health insurance rules, such as through the Coronavirus Aid, Relief, and Economic Security, or CARES, Act, and the Families First Coronavirus Response Act. 

Vaccine providers are allowed to bill insurance companies for administering the vaccine, however, so Schwartz said people should not be alarmed if asked to provide their health insurance information.

“Insurers will be picking up some of the administration costs,” she said. “But it won’t be like so much else in health care where you think it’s covered by your insurance and then it’s not and you get a bill.”

At some point, the government will stop purchasing doses, and COVID-19 vaccines will likely become like other shots that are covered by insurance — and some of the protections in place will go away, especially if you are uninsured, Schwartz said.

Unless the law is changed, though, she added, COVID-19 vaccines for Medicare enrollees will always be covered under Part B with no cost-sharing.

Will I have a choice in the specific vaccine I receive?

No, at least not to start. While there are three vaccines available now and more potentially coming soon, supplies will still be limited for months, and it’s unlikely you will be able to choose which one you get.

But if doses become plentiful, Offit said that could change, although he estimated that might not happen until the end of 2021.

Could a COVID-19 vaccine become mandatory?

It’s possible that in the future, when COVID-19 vaccines are no longer in short supply, that states or certain workplaces might require individuals to be vaccinated. 

As legal and public health expert Joanne Rosen of Johns Hopkins University has explained, the legal precedent for states to make vaccinations compulsory goes back to a 1905 Supreme Court case, Jacobson v. Massachusetts, in which a man challenged an ordinance requiring all adults to be vaccinated against smallpox or to pay a $5 fine. The court sided with the state, finding that the smallpox inoculation requirement was a reasonable regulation to protect public health.

Employers are also allowed to require their workers to get a vaccine, but only if vaccination is reasonably related to a person’s job duties. That typically includes places such as hospitals or nursing homes, which often require their staff to get flu shots each year.

The federal government cannot issue a vaccine mandate, Rosen told us, although it can provide financial incentives to nudge states to enact mandates. 

She said she would not be surprised if some workplaces do introduce COVID-19 mandates, but was less confident that states would, at least in the shorter term.

“I don’t know if states will enact COVID-19 vaccine mandates or will opt, instead, to strongly encourage people to get vaccinated and roll out public education campaigns,” she said.

For more on this issue, see our Ask SciCheck item “Exploring the Legality of COVID-19 Vaccine Mandates.”

Editor’s note: SciCheck’s COVID-19/Vaccination Project is made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over our editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation. The goal of the project is to increase exposure to accurate information about COVID-19 and vaccines, while decreasing the impact of misinformation.