Answers to the Most Common Questions About COVID-19 Vaccines

February 2021

If you’re eligible for the COVID-19 vaccine—or if you expect to be eligible soon—having questions is normal. Read on for answers to some of the most common questions about the new COVID-19 vaccines.

What is the vaccine approval process?

Like all vaccines that have received FDA approval, the COVID-19 vaccines were first tested in three sets of clinical trials. The vaccines’ Phase 1 trials involved 20 to 100 volunteers, and these initial trials were followed by Phase 2 trials that involved several hundreds of people. After determining the best dose and confirming the vaccine’s safety, researchers conducted Phase 3 trials with tens of thousands of people.

After enough data was collected from the vaccines’ Phase 3 trials, the pharmaceutical companies that developed the vaccines submitted this data to the FDA. First, a subcommittee of independent (unpaid) volunteer experts reviewed the data and offered recommendations to the FDA about whether to approve the vaccines. Then, the FDA reviewed the data and decided whether to approve the vaccines or to authorize them under an emergency use authorization (EUA). The FDA can use an EUA to make a drug available to the general population more quickly—but only if enough data has confirmed its safety and effectiveness.

Once the FDA had approved or authorized the vaccines, the CDC offered recommendations about who should receive the vaccine. Finally, the approved vaccines’ distribution began.

Are the COVID-19 vaccines safe?

The COVID-19 vaccines authorized by the FDA are the safest way to reduce your risk of contracting COVID-19. These vaccines were tested on more than 70,000 people in clinical trials, and since this extensive testing, these vaccines have been administered to tens of millions of Americans, resulting in no deaths or severe lasting injuries. A small proportion of people with histories of severe allergic reactions have experienced anaphylactic reactions to the vaccines. These people recovered with treatment. However, people who are allergic to the COVID-19 vaccines’ ingredients, including polyethylene glycol (PEG) and polysorbate, should not receive these vaccines.

What are mRNA vaccines, and how do they differ from conventional vaccines?

Many vaccines contain parts of the viruses or bacteria that cause disease. Other vaccines contain whole viruses or bacteria that have been deactivated or weakened so that they cannot cause illness. These weakened, dead or partial pathogens then stimulate the immune system to build a defense against diseases.

In contrast, the COVID-19 vaccines by Pfizer and Moderna are both mRNA vaccines, which do not contain any pathogens at all. Instead, mRNA vaccines contain a tiny strip of genetic material (called “messenger RNA”) that contains instructions about how to make a single protein from the SARS-CoV-2 virus that caused COVID-19. When people receive these vaccines, this genetic material enters some of their bodies’ cells. These cells “read” the material’s instructions and create the necessary protein. Then, when the immune system encounters this protein, it builds antibodies to protect against infection. These antibodies can then effectively fight off the SARS-CoV-2 virus when vaccinated people are exposed. The affected cells stop making the protective protein after the mRNA from the vaccine degrades, which happens relatively quickly.

What can I expect at my vaccination appointment?

Different vaccination centers may operate a little differently, but they share a few protocols in common. If you needed to book an appointment in advance to receive a COVID-19 vaccine, the vaccine administrator will confirm at your appointment whether you’re the person the appointment was booked for. Sometimes, showing government ID is necessary for this verification, but not all vaccination centers require IDs. The vaccine administrator will ask a couple of health-related questions, such as whether you have any allergies to the ingredients in the vaccine, whether you’re pregnant or whether you’re immune-compromised. (Pregnant people can still choose to receive the COVID-19 vaccine, but centers will often still ask about pregnancy status.)

You will then receive a printed copy of the “COVID-19 Vaccine Emergency Use Authorization (EUA) Fact Sheet for Recipients and Caregivers.” This document describes the vaccine that you will be receiving, what side effects might occur, when you need to receive your second dose and whom to call if you experience any health concerns after vaccination.

Next, the vaccine administrator will ask which arm you want to receive the injection into. After you expose your bare arm, the administrator will rub an area of your arm with alcohol and then use a syringe to inject the vaccine. After the injection, the administrator will cover the injection site with a small bandage and ask you to remain in the center’s waiting area for 15 to 30 minutes to make sure you don’t have an allergic reaction.  This way, in the unlikely event that you do have an allergic reaction, you will be able to receive prompt and effective care from the center’s experienced staff. If you need a second vaccine dose, the vaccine administrator will explain the procedure for booking an appointment to receive your next dose.

Do the COVID-19 vaccines have any side effects?

All vaccines, including the COVID-19 vaccines, can have side effects. Vaccine side effects are usually reactions to the vaccine, and they’re a normal part of getting vaccinated. They occur because the immune system is responding to the vaccine—which means the vaccine is working.

The most common side effects are redness, swelling, or soreness at the injection site, which most people experience. Your arm will probably be sore for a few days after receiving a vaccine.

Other possible side effects of the Pfizer and Moderna COVID-19 vaccines include headache, nausea, muscle stiffness, soreness and muscle or joint aches. More than half of people who participated in these vaccines’ clinical trials experienced at least one of these side effects, and around half of participants experienced headache or fatigue after receiving one or both doses of either COVID-19 vaccine. A smaller proportion of participants experienced fever, chills, diarrhea or vomiting.

In the clinical trials and among the general population, some people have reported experiencing more intense side effects after receiving the second dose of the Pfizer or Moderna COVID-19 vaccine. It’s a good idea to plan ahead for your second dose appointment and expect that you may experience a fever, chills, headache, diarrhea, vomiting or fatigue after receiving your second dose. You may need to rest for one or two days after this second dose, but any side effects will most likely resolve in one to three days. No long-term side effects have occurred from either COVID-19 vaccine.

Can a COVID-19 vaccine make me sick with COVID-19?

No. Since the mRNA vaccines against COVID-19 do not contain any pieces of the virus that causes the disease, the COVID-19 vaccines cannot give you COVID-19. You may feel unwell for a day or two after receiving the vaccine because of the possible side effects discussed above, but you cannot contract COVID- 19 from the vaccines.

Once I’ve gotten the COVID-19 vaccine, when will my immunity start to protect me?

You will begin to receive a small amount of protection from either of the COVID-19 vaccines one to two weeks after receiving your first dose. To gain full protection from the Pfizer and Moderna mRNA vaccines, however, you need to have received both doses. The first dose of the Pfizer vaccine is about 53% effective at preventing COVID-19, and the first dose of the Moderna vaccine is about 70% effective at preventing COVID-19. The second dose of the Pfizer vaccine is administered three weeks after the first dose, and people who have received both doses of the Pfizer vaccine have 95% protection against COVID-19 one week after receiving the second dose. The second dose of the Moderna vaccine is administered four weeks after the first dose, and people who have received both doses of the Moderna vaccine have 95% protection against COVID-19 two weeks after receiving the second dose.

What percentage of people need to get vaccinated in order to develop herd immunity against COVID-19?

Herd immunity occurs when enough people in a population are immune to a disease for the disease to stop spreading throughout the population. Herd immunity happens because too few people are susceptible to infection for transmission to continue. Determining exactly how many people need to be vaccinated in order to reach COVID-19 herd immunity is difficult because of multiple factors. Firstly, vaccinated people and people who have recovered from COVID-19 experience different levels of immunity, and researchers don’t know how long this protection lasts—whether months or years—among these two groups.

Secondly, herd immunity also depends on how effective the available vaccines are. The two COVID-19 mRNA vaccines are about 95% effective, but other vaccines will soon be available with different levels of effectiveness. Experts at the National Institutes of Health have estimated that 85% of people or more must be vaccinated in order to reach herd immunity, but that number is only an estimate based on incomplete information. Scientists will need more time to know for sure when herd immunity can be achieved.

Do I still need to wear a mask and socially distance after receiving a COVID-19 vaccine?

Yes, at least for now, you will still need to wear a mask and socially distance after receiving a COVID-19 vaccine. The CDC recommends that people who receive a COVID-19 vaccine wear a mask and practice social distancing in public. Scientists are still learning how well the COVID-19 vaccines prevent infection among people who have received the vaccine. (The vaccine definitely prevents disease—that is, the development of COVID-19 symptoms—but infection refers to the virus’s ability to enter bodies and begin replicating, even if the immune system destroys the virus before it can cause any damage.)

Scientists have not yet determined for certain whether the vaccine can prevent a vaccinated person from taking in the virus and then passing it on to someone else. However, recent evidence suggests a good chance that the vaccine does prevent virus transmission from vaccinated people to unvaccinated people. Nonetheless, until scientists can confirm whether the vaccine prevents transmission, safety requires that vaccinated people continue wearing masks and practicing social distancing in public.

One thing you won’t necessarily have to do after you’ve received both doses of a COVID-19 vaccine is quarantine after exposure to someone with COVID-19, according to February 2021 guidelines from the CDC. These guidelines state that the quarantine requirement doesn’t apply to people who have no COVID-19 symptoms and who received their second COVID-19 vaccine dose at least two weeks (but no more than three months) before exposure.

Where can I go to learn more about the COVID-19 vaccine and stay up to date on important vaccine information?

For more information and the latest updates on the COVID-19 vaccines, please visit the CDC’s vaccination page.

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