From the WebMD Archives

This story was updated March 5, 2021.

Dec. 16, 2020 -- As more shipments of one COVID-19 vaccine roll out, everyone has questions. Among them:

What is a vaccine?

A vaccine is a substance that stimulates your immune system to make antibodies -- blood proteins produced in response to a foreign substance -- as it would if you were exposed to the actual disease. After vaccination, you develop immunity to the disease, so you are protected from getting sick if you get infected.

Are eggs involved in the making of the COVID vaccines?

Unlike some other vaccines, the COVID-19 vaccines viewed as frontrunners do not use eggs to make them.

How do the vaccines work?

The vaccines from Pfizer and Moderna use a technique known as mRNA, or messenger RNA. These vaccines "give instructions for our cells to make a harmless piece of what is called the 'spike protein,' " according to the CDC. This protein is found on the surface of the coronavirus that causes COVID-19.

Once these vaccine instructions, or mRNA, are injected, your cells use it to make the spike protein; then the instructions are broken down and eliminated. The protein piece is displayed on the cell surface, triggering our immune system to make antibodies against it, just as it would if it were exposed to the real coronavirus that causes COVID-19. In this way, the body learns how to protect itself when and if the real virus shows up.

The mRNA vaccines don't use the live virus that causes COVID-19, nor does the mRNA get into the cell's nucleus, which is where our DNA (genetic material) is stored.

The vaccine from Janssen/Johnson & Johnson is made using a type of virus called adenovirus type 26, or Ad26. The Ad26 delivers a piece of the DNA, or genetic material, used to make the spike protein, so the person can temporarily make this protein and teach the immune system to react against the coronavirus. Ad26 is modified so it can't make the person sick..

How many doses do you need?

One dose for the J&J vaccine, and two doses  for the vaccines from Moderna and Pfizer.

What is the interval between doses?

For the Moderna vaccines, the two doses are given 4  to 6 weeks apart. For Pfizer's vaccine, the two doses are given 3 to 6 weeks apart. The U.S. government and the manufacturers have partnered to make sure there are enough doses available for everyone to get two. 

What happens if you don't take the second dose?

Protection is assumed to be less. In data that Moderna submitted to the FDA before its Dec. 17 review for its request for emergency use authorization, for instance, its analysis suggested that the first dose provides protection from getting COVID-19, but the data did not allow for a “firm conclusion," the FDA says. Both the Pfizer and Moderna vaccines are believed to be around 50% effective after just one dose. 

After the required doses, how long until it takes effect and provides protection?

That happens “about a week after the second dose," says Naor Bar-Zeev, PhD, an associate professor of international health and vaccinology and deputy director of the International Vaccine Access Center at Johns Hopkins Bloomberg School of Public Health. Once the second dose kicks in, both the Pfizer and Moderna vaccines have shown in studies to be about 95% effective. The J&J effectiveness data was based on 28 days after the inoculation. 

How well do the vaccines work?

Overall, the Pfizer and Moderna vaccines are about 95% effective.Overall, the J&J vaccine was found 72% effective in preventing moderate-to-severe COVID-19 in U.S. studies.   

How long does the protection last?

Because the vaccines are new, this is not yet known for sure. Based on other viruses that are similar to the coronavirus that causes COVID-19, the COVID-19 vaccines that are shown to be highly effective might protect people for a few years, says Paul Offit, MD, director of the Vaccine Education Center and a professor of pediatrics at Children's Hospital of Philadelphia. That's an educated guess based on his expertise and known facts about the virus that causes COVID-19.

What about side effects?

People should expect to have some side effects, similar to what some people report after getting a flu vaccine, according to experts meeting recently with the CDC. These experts said to expect temporary side effects such as soreness in your arm where you got the shot, fatigue, body aches, and perhaps a fever.

At least two people getting the Pfizer vaccine in the U.K. had severe allergic reactions, promoting the British government to tell those prone to anaphylaxis-like reactions not to take it.

As for serious side effects, it's too soon to know about rare side effects, says Bar-Zeev. That is because phase III trials do not supply enough information about rare side effects. That will become evident, if it occurs, as more people are vaccinated. Even so, Bar-Zeev says it's not a reason to avoid the vaccine.

As millions of doses are distributed, if there are very rare side effects, they are expected to show up in a very short time frame, Offit says.

After the FDA's emergency authorization (EUA) is granted, are the vaccines still tracked?

Yes. The FDA expects the manufacturers to continue their clinical trials to find out more about how safe and effective they are, and pursue full FDA approval or licensure. The EUA, which is different from FDA approval, is based on the FDA's evaluation of available evidence, assessing risks and benefits. It issues the EUA if the benefit-risk balance is favorable.

Do the COVID vaccines not only keep the person from getting sick, but also from spreading the virus if exposed?

That is not yet known. As more data and monitoring are done, experts will be able to find out if a vaccinated person, if exposed to the virus, can still spread it even if they don't get the disease themselves, says Bar-Zeev of Johns Hopkins.

Are the vaccines free?

Yes, for patients, but the health care providers will bill insurance companies, Medicaid and Medicare, or tap federal funds for the uninsured. In one estimate, the cost per dose was $37 for Moderna's vaccine, $20 for Pfizer's, and $10 for J&J.

Will it be possible to choose which vaccine you prefer?

In general, it does not matter, since once a vaccine gets the FDA's emergency use authorization (EUA), they all work. And even as more vaccines become authorized and available, you may have only one choice.

If a vaccine needs two doses, can you switch to another vaccine for the second one?

No. Experts advise staying with the same vaccine for both. That's true even for the Moderna and Pfizer vaccines, which use the same general approach yet are different.

How much of the population is likely to get vaccinated?

It's not possible to say, although surveys have cited a problem with “'vaccine hesitancy," even among health care workers. But that may change. "I think this will be a desired vaccine," Offit says.

How much of the population needs to be vaccinated for so-called herd immunity?

"Herd protection is not a goal of the initial rounds of vaccine deployment," Bar-Zeev says. "Only once population-wide vaccination is a reality would herd protection be even considered."

After I get vaccinated, do I still have to wear a mask?

Yes. Even after vaccination increases, say Bar-Zeev and other experts, preventive behaviors will still be needed. "The ability to reduce transmission will require not just high vaccine uptake, but ongoing social distancing and masks," he says. And herd protection may require high rates of vaccination in groups that are themselves at low risk, he says, “so [that] raises ethical questions."

When can we expect to get vaccinated?

On March 2, President Joe Biden announced that he had ramped up production, vowing there will be enough vaccine doses so every U.S. adult could be immunized by May.  

Show Sources

Paul Offit, MD, director, Vaccine Education Center, and professor of pediatrics, Children's Hospital of Philadelphia; professor of vaccinology, Perelman School of Medicine, University of Pennsylvania.

CDC: "Vaccines: The Basics," "When Vaccine is Limited, Who Gets Vaccinated First?" "Understanding How COVID-19 Vaccines Work," "Meeting of the Advisory Committee on Immunization Practices (ACIP) November 23," "Understanding mRNA COVID-19 Vaccines."

JAMA: "COVID-19 and mRNA Vaccines -- First Large Test for a New Approach."

The Lancet: "Oxford-AstraZeneca COVID-19 vaccine efficacy."

Facebook interview: Mark Zuckerberg and Anthony Fauci, MD, Nov. 30, 2020.

FDA: "Vaccines and Related Biological Products Advisory Committee Meeting, Dec. 17, 2020."

Naor Bar-Zeev, PhD, associate professor of international health and vaccinology and deputy director, International Vaccine Access Center, Johns Hopkins Bloomberg School of Public Health, Baltimore. "The Cost Per Jab of Covid-19 Vaccine Candidates."

AstraZeneca: "COVID-19 vaccine AZD1222 is better tolerated in older adults than younger adults with similar immune responses across all age groups."

The Lancet: "Safety and immunogenicity of ChAdOx1nCoV-19 in a prime-boost regimen in young and old adults (COV002) in a single-blind, randomized, controlled, phase 2/3 trial."

Moderna: "Moderna's COVID-19 Vaccine Candidate Meets its Primary Efficacy Endpoint in the First Interim Analysis of the Phase 3 COVE Study." "Covid-19: Moderna vaccine is nearly 95% effective, trial involving high risk and elderly people shows."

CBS News: "UK warns against giving Pfizer vaccine to people prone to severe allergic reactions."

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