How the COVID-19 vaccine works, potential side effects and more
Updated Sept. 28, 2021

Vaccine makers have said their COVID-19 vaccines do provide protection against the Delta variant. If infection does occur after a person is fully vaccinated, the illness shouldn’t be severe. Experts say that if you're fully vaccinated, it's very unlikely that you will be hospitalized or will die. In fact, people who get many of these breakthrough infections from mutations like the Delta variant will be asymptomatic.

Pfizer/BioNTech announced that a preliminary study shows their COVID-19 vaccine is effective against the mutation in the two variants from South Africa and the UK. Experts also believe the Moderna vaccines will also be effective against these coronavirus strains. Both of these vaccines prove to be about 85% effective agains the Delta variant. Johnson & Johnson's vaccine is about 60% effective against the Delta variant. The CDC says that so far, studies suggest that antibodies built as a result of the current authorized vaccines recognize these variants. More research is ongoing.

Like with other vaccines, we don’t have just one antibody that forms, we have multiple and different antibodies. The antibodies attack the spike protein in several areas. If one part changes, there are other areas we can have an immune response against. Antibodies from people previously infected with other COVID-19 strains appear to have effectively neutralized the new variants.

In addition, Pfizer/BioNTech and Moderna said they can change the vaccine to adapt to new COVID-19 strains in as little as six weeks.

Learn more about the new COVID-19 strains from a UC Davis Health infectious disease expert

According to the CDC and FDA, the following groups of people are eligible for COVID-19 vaccine boosters or third doses:

• People with compromised immune systems age 12 and older: The CDC recommends that “people with moderately to severely compromised immune systems receive an additional dose of mRNA COVID-19 vaccine at least 28 days after a second dose” of Pfizer or Moderna COVID-19 vaccine.

Additionally, only those in the following groups who received the Pfizer vaccine are eligible 6 months after their second dose:

• People age 65 and older
• Residents of a long-term care facility
• People ages 18 to 64 deemed "high risk" of severe COVID-19: This includes people with underlying medical conditions.
• People who may have increased exposure to the virus because of their jobs: This includes health care workers, first responders and educators.

Boosters for people who received two doses of Moderna or a single dose of Johnson & Johnson vaccine have not been approved at this time, but may be in the future.

Learn more about COVID-19 booster shots and third doses from UC Davis Health

COVID-19 vaccines are still so new that more time is needed to study how long immunity lasts. Studies currently available show that people who were vaccinated had a strong immunity to COVID-19. It looks like immunity will last a while, but researchers need to follow immune levels over time.

The Pfizer, Moderna and Oxford/AstraZeneca coronavirus vaccines require two shots. After the initial vaccine dose, a second shot is given 3 to 4 weeks later. The Johnson & Johnson vaccine is one dose.

With the two-dose COVID-19 vaccines, both doses are needed to reach full immunity. The level of protection increases dramatically – from 52% to 95% – after the second shot. It's also unclear whether the lower level of immunity from just one shot would last long.

Although there is no data available on this, some health experts also caution that if enough people miss a second vaccine, their lower immunity level would allow the virus to survive. This could allow it to mutate even more and become vaccine resistant.

Both Pfizer/BioNTech and Moderna advised against delaying the second dose if at all possible. However, like with other vaccines, the second COVID-19 vaccination is a booster shot. This increases your level of protection by further teaching your body what the virus looks like, so it can respond quickly. As with other vaccinations, there’s no indication that a delay in receiving the booster shot reduces effectiveness. Delaying the second dose will delay full protection. However, people who receive the second dose at any time after the recommended date can be considered fully vaccinated.

These two COVID-19 vaccines are very similar. The main differences come in to play in relation to transporting and handling the vaccines. The Pfizer vaccine must be stored at -94° Fahrenheit. The Moderna vaccine needs to be stored at -4° Fahrenheit.

Both teach your immune system to destroy the coronavirus. Both use messenger RNA (mRNA) to instruct your body to build the spike proteins that are on the SARS-CoV-2 virus. Then your immune system kills it and remembers the protein so your body is ready if you're infected with COVID-19.

Both are very safe and very effective. The Pfizer vaccine is 95% effective against COVID-19, and Moderna’s vaccine is 94.1% effective. They have similar temporary side effects, and those reactions are stronger after the second shot for both.

There are two differences that impact the public:

• Pfizer’s vaccine is authorized for people ages 12 and older. Moderna’s is authorized for people ages 18 and older.
• The two Pfizer doses are given 21 days apart. The two Moderna doses are given 28 days apart.

Mixing COVID-19 vaccine doses from different manufacturers is not recommended. There is very little data on safety and immune responses with mixed vaccines.

More than 13 million Americans have received the Johnson & Johnson (Jannsen) vaccine. Johnson & Johnson has reported that its vaccine is effective against Delta. One recent study, which has not been peer-reviewed or published in a scientific journal, suggests that its vaccine is less effective against the Delta variant than other vaccines. This has prompted discussion over whether Johnson & Johnson recipients might also need a booster. But the first study to assess the vaccine against the Delta variant in the real world reported an efficacy of up to 71% against hospitalization and up to 95% against death.

The CDC strongly recommends against that. The COVID-19 vaccines “are not interchangeable” and the impact of switching has not been studied, according to the latest guidelines. However, the CDC said people may switch from one authorized COVID-19 vaccine in “exceptional situations in which the first-dose vaccine product cannot be determined or is no longer available.”

The findings from clinical trials show the Pfizer and Moderna COVID-19 vaccines are 95% effective, while the Johnson & Johnson vaccine is about 85% effective for people across all subgroups. These include racial and ethnic minorities and those with one or more of the following health conditions:

• obesity
• diabetes
• hypertension
• chronic cardiopulmonary diseases.

The CDC, the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) agree that the COVID-19 vaccines should be offered to pregnant women who are eligible for vaccination.

The Pfizer COVID-19 vaccine is the only one currently approved for use in children ages 12-17.

The Pfizer vaccine received emergency use authorization from the U.S. Food and Drug Administration (FDA) for children ages 12-17. Moderna has begun testing the COVID-19 vaccine in children as young as six months.

Pfizer is the only approved vaccine for children ages 12-17. For those who are under 18 years old, they will need to have a parent or legal guardian provide consent for treatment at their appointment. UC Davis Health accepts written or verbal consent. It's also advised that anyone under age 18 have someone drive them to and from their appointment. With any vaccine, there's an increased risk of fainting for children ages 11 to 18, according to the CDC.

That is an extraordinarily high rate of effectiveness, and it means the COVID-19 vaccines work very well. By comparison, flu vaccines are about 40% effective, on average.

Here’s where the number comes from: In the clinical trials, half the people got the coronavirus vaccine and half got the placebo. Researchers counted how many people got sick with COVID-19 after the vaccines reached their peak effectiveness. Among both groups in the Pfizer trial, 170 people total got COVID-19, but only eight of those received the vaccine. That’s about 5% of total cases. That’s where the 95% estimate comes from. Moderna’s numbers were similar.

More than 10 million people have gotten at least one dose of their coronavirus vaccine, according to the Centers for Disease Control and Prevention (CDC) tracker. The vast majority who talked about their COVID-19 vaccine side effects or reactions described them as similar to a flu shot.

The most common side effects are a sore arm and sometimes fever, chills, tiredness and headaches for a day or two, according to the CDC. As reported in both studies, the majority of reactions beyond a sore arm happen after the second dose. Those are all signs your body is building an immune response. Read more: COVID-19 vaccine clinical trial volunteers talk about living with the vaccine since August

In a study released in January, the CDC said the risk of anaphylaxis – a severe allergic reaction – is extremely low. Based on data from people who have received the first of the two recommended doses, only about one in every 90,000 people will experience this adverse reaction. The people most at risk of an adverse reaction are those who have had severe allergic reactions to other medications or food. There have been no deaths. On the other hand, the COVID-19 pandemic has killed more than one out of every 1,000 Americans.

There have been increased reports of cases of inflammation of the heart — called myocarditis andpericarditis — happening after COVID-19 vaccination in the United States. These reports are rare, given the number of vaccine doses administered, and have been particularly in adolescents and young adults. CDC and its partners are actively monitoring these reports by reviewing data and medical records to learn more about what happened and to see if there is any relationship to COVID-19 vaccination. Most patients who received care responded well to medicine and rest and quickly felt better.

Cases reported have occurred mostly in male adolescents and young adults age 16 years or older, more often after getting the second dose of one of the mRNA COVID-19 vaccines than after the first dose, and typically within several days after COVID-19 vaccination. Patients can usually return to their normal daily activities after their symptoms improve, and they should speak with their doctor about return to exercise or sports. CDC continues to recommend COVID-19 vaccination for everyone 12 years of age and older, given the greater risk of COVID-19 illness and related, possibly severe complications. The known and potential benefits of COVID-19 vaccination outweigh the known and potential risks, including the possible risk of myocarditis or pericarditis.

For children age 12 to 18, there is an increased risk of fainting after a COVID-19 vaccine, which is fairly common for adolescents after any vaccine, according to the CDC. For this reason, children age 12 and older are advised to have someone drive them to and from their vaccine appointment.

Learn more about what to expect after getting a COVID-19 vaccine

Learn about COVID-19 vaccine safety monitoring and why the vaccine is safe

It is normal to have a stronger reaction after the second dose because that's when your immune system really kicks into gear. That big immune response is what causes the stronger reaction. It's called a boosting phenomenon, and it builds an even greater ability to defeat COVID-19. Here's how it works:

When you get the first dose, your body learns to make antibodies to fight COVID-19. Then you get the second vaccine, and you already have the antibodies ready to go, so that shot kicks them into action. That more robust immune response accounts for the stronger reactions. Those reactions tell you the vaccine is working.

Although stronger reactions after the second dose are common, experts say some people may have little or no reaction.

To help with the stronger side effects, it's OK to take Tylenol or other over-the-counter pain relievers after your shot – but not before – to help ease discomfort. As always, you will want to consult with your physician.

Learn more about what to expect after getting a COVID-19 vaccine

Yes, it's still working. Everyone reacts a little differently to every vaccine. For some people, their reaction is no reaction. But your immune response is still building and the COVID-19 vaccine is still working.

We do not have long-term safety data of these COVID-19 vaccines since they have only been studied in humans for about 6 months. However, unexpected long-term safety issues have not been found in any licensed vaccines, even after decades of study. There are several vaccine safety monitoring systems in the U.S., and there will be expanded COVID-19 vaccine safety monitoring.

There is no way you can get COVID-19 from the vaccine. The novel coronavirus vaccine includes a small fragment of messanger RNA (mRNA) that encodes for a small portion of the protein. The mRNA can’t replicate, and it's very unstable so it doesn't last long. It can't cause COVID-19. None of the COVID-19 vaccines currently in development in the U.S. use the live virus that causes COVID-19.

There are only two cautions before getting a COVID-19 vaccine:

• Don’t medicate with over-the-counter pain relievers before coming in for your shot. We don’t know yet what impact that might have on your vaccine-induced antibody response.
• If you are getting a flu shot or another vaccination, do it at least 14 days before your first COVID-19 shot. If possible, a month before is preferable but not required.

Otherwise, eat and drink as normal and go about your daily activities.

After your vaccination, it’s OK to take over-the-counter pain relievers if you need them for a headache, mild fever or any other discomforts.

To be sure you don't have a serious reaction, you will have to wait 15 minutes before you leave the vaccination site (30 minutes if you have a history of strong allergic reactions). After that, you can go about your day as normal – maybe with a touch of moderation. Experts say it’s fine to have a glass of wine or any normal meal that evening. But it’s probably not the time to line up stiff drinks.

As for exercise later that day or the next, it’s fine to go for a run, walk, bike ride or anything you normally do, as long as you feel OK. Fatigue is a possible side effect after getting the COVID-19 vaccine, so just don't push it.

If you do plan to get an additional vaccine for something other than COVID-19, you may want to talk to your doctor first. However, the CDC advises that flu shots can be given at the same time as COVID-19 vaccines.

Learn more about what you should do after you get a vaccine from the CDC (pdf)

This is not entirely clear. Data released by the U.S. Food and Drug Administration (FDA) show that COVID-19 protection from the Pfizer/BioNTech vaccine was demonstrated in the clinical trials at about 14 days after the first shot. The FDA said some level of immunity may start sooner, but how much is not certain.

The trials also confirmed that the vaccine was 95% effective against COVID-19 seven to 14 days after the second dose.

COVID-19 infections that occur in those who have been vaccinated, known as breakthrough infections, may result in transmission to others. Most studies suggest a much lower rate of COVID-19 transmission that occur in those vaccinated compared to infections in unvaccinated people.

Experts recommend getting the vaccine even if you’ve had COVID-19. There isn’t enough information to say if you have natural immunity from being sick or how long it may last, according to the CDC. Experts recommend waiting at least until you've recovered from acute illness and no longer have to isolate (typically 2 weeks). They also suggest waiting about 90 days after you've recovered from COVID-19 (as reinfection is unlikely in the first months after infection). The waiting period can help keep your reactions to the COVID-19 vaccine more mild.

If you’ve been exposed to someone with suspected or confirmed COVID-19, you are not required to quarantine if all of the following are true, according to the CDC:

• You are fully vaccinated for COVID-19 and it’s been at least 2 weeks since your last dose (second dose in a two-dose series or only dose in a single-dose series).
• It’s been less than 3 months since your final dose in the series.
• You've remained asymptomatic since your current COVID-19 exposure.

Yes, the CDC advises that flu and COVID-19 vaccines (whether it's your inital shots or a booster/third dose) can be given at the same time. You do not have to wait any amount of time between the two vaccines.

If you plan on getting another vaccine around the same time as a COVID-19 vaccine, you should talk to your primary care physician before doing so.

Learn about how to schedule your flu vaccine with UC Davis Health

No. The coronavirus vaccine includes a portion of messenger RNA. It doesn't alter our DNA, and it doesn't stay with us or get transcribed forever. The vaccine has a limited life within us and then it's degraded.

The National Institutes of Health (NIH) is currently funding research studies to find out if COVID-19 vaccination can affect a woman’s menstrual cycle.

Learn more about COVID-19 vaccines and women's health

There's no evidence that COVID-19 vaccines have any effect on fertility. Furthermore, there is no scientific theory for how COVID-19 vaccines could cause fertility problems. The Vaccines and Related Biological Products Advisory Committee (VRBPAC) briefing for both Pfizer and Moderna detail the findings from the clinical trials about effectiveness, safety and side effects. There are no reports of any impact on fertility. You can read the 50-plus page briefings from VRBPAC to the FDA on both the Pfizer-BioNTech and Moderna vaccines.

Learn more about COVID-19 myths and facts

Many pregnant women have received COVID-19 vaccinations without any problems. The National Institutes of Health (NIH) has started a study on COVID-19 vaccines during pregnancy and postpartum. Pregnant women are at an increased risk for COVID-19 complications. Health experts believe the risks of  infection far outweigh the risks of getting vaccinated. The CDC, the American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) agree that the COVID-19 vaccines should be offered to pregnant women who are eligible.

Yes, the COVID vaccines are safe for breastfeeding women. Research so far suggests that there may be some immunity provided to babies through breast milk, but this is not yet proven.

The CDC recommends that if you're due for a mammogram and have been recently vaccinated for COVID-19, ask your doctor how long you should wait to get your mammogram. People who have received a COVID-19 vaccine can have swelling in the lymph nodes (lymphadenopathy) in the underarm near where they got the shot. This swelling is a normal sign that your body is building protection against COVID-19. However, it is possible that this swelling could cause a false reading on a mammogram. Some experts recommend getting your mammogram before being vaccinated or waiting four to six weeks after getting your vaccine.