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

Coronavirus 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 very strong immunity to COVID-19. It looks like immunity will last a while, but researchers need to follow immune levels over time. At this point, we're not sure if immunity will last a year or 10 years, or if there will be a need for a booster shot at some point.

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.

Both COVID-19 vaccine 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. The timing of the second shot is not strictly critical. The CDC advises that you can receive the second COVID-19 vaccine up to 42 days (or 6 weeks) after the first dose.

Pfizer/BioNTech announced that a preliminary study shows their COVID-19 vaccine is effective against the mutation in the two new variants. Experts also believe the Moderna vaccines will also be effective against the new coronavirus strains. 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

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 16 years old and up. 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.

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 the clinical trials show that the COVID-19 vaccine was 95% effective for people across all subgroups. These include racial and ethnic minorities, participants 65 years old and older, and those with one or more of the following conditions:

• obesity
• diabetes
• hypertension
• chronic cardiopulmonary diseases.

One group that was not studied was children.

Children were not included in any of the coronavirus vaccine clinical trials. Neither were pregnant women. Pfizer and Moderna have started studying the vaccine in children ages 12 to 18. However, there haven't been any studies on younger children.

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.

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's no evidence that COVID-19 vaccines have any effect on fertility. 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

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.

There is one caution after getting the COVID-19 vaccine, which also relates to other vaccines: Wait at least 14 days after your second COVID-19 vaccine dose to get a flu shot – if you haven’t gotten one already – or any other vaccination.

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.

Studies have not yet produced evidence either way about whether a vaccinated person can spread COVID-19. In a recent FDA review of Pfizer’s data, the agency listed transmission of the virus as an unknown and said further review is needed.

Both the Pfizer and Moderna trials tested whether their vaccines prevented their participants from getting sick, not whether it prevented them from passing on the virus to others. That's why health experts say people who were vaccinated should still wear masks and socially distance. It’s also why public health experts hope at least 70% of the population will get vaccinated to reach herd immunity.

Experts recommend getting the vaccine even if you’ve had COVID-19. There isn’t enough information about whether there's natural immunity from being sick or how long it may last, according to the CDC. Experts also recommend waiting until 90 days after all symptoms have disappeared before getting your first shot. You will have some antibodies from your body’s fight with the disease. The waiting period will 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.

Experts say people who get a COVID-19 vaccine should wait at least two weeks after their second dose before getting a flu shot or any other vaccination.. After two weeks, any other vaccine is safe with either the Pfizer or Moderna coronavirus vaccines. Infectious disease experts add that it's never been more important to get your flu shot than this year with COVID-19 surging.

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.

Yes. Our safety protocols continue to be important to help prevent transmission of the virus. The COVID-19 vaccine is one more way we can stay safe. You should still continue to mask, maintain physical distance, practice hand hygiene and wear eye protection. The vaccine is not 100% effective and not everyone will be vaccinated for a while. In addition, there will be some people who will choose not to be vaccinated.

Experts say it will take several months to get enough people vaccinated to end the current COVID-19 pandemic restrictions. Vaccination efforts across the region and the country with the Pfizer and Moderna vaccines are still ramping up. UC Davis Health is playing a big part in Northern California's effort to vaccinate everyone.

UC Davis Health research teams, the UC Davis School of Medicine, clinicians and more than 200 participants in the Pfizer clinical trial at UC Davis Health also played key roles in bringing the first vaccine against COVID-19 to reality. Now UC Davis Health is currently conducting clinical trials for a vaccine candidate from Novavax. Learn more about the Novavax clinical trial.

We will still need to wear masks or face coverings and maintain social distancing for the foreseeable future. We can stop these measures when we achieve some level of herd immunity. That's going to take at least 70% of the population to be immune. Right now, less than 10% of people in the U.S. have been infected with COVID-19. WIth vaccine production and distribution still ramping up, we are hopeful that we can achieve herd immunity through immunization by the middle or end of this year.

Yes. The FBI, Department of Health and Human Services (HHS) and Centers for Medicare and Medicaid Services (CMS) are warning people about several emerging scams related to COVID-19 vaccines. As COVID-19 vaccine distribution gets underway, be on the lookout for these signs of potential fraud schemes, according to the FBI:

• You are asked to pay out of pocket to get a vaccine.
• You are asked to pay to put your name on a vaccine waiting list or to get early access to the vaccine.
• Advertisements for vaccines through social media platforms, email, phone calls, online or from unsolicited/unknown sources.
• Marketers offering to sell or ship doses of the COVID-19 vaccine for payment.

If you believe you have been a victim of COVID-19 fraud, please report it to:

• FBI at 1-800-CALL-FBI or tips.fbi.gov
• HHS at 1-800-HHS-TIPS or tips.hhs.gov
• CMS/Medicare: 1-800-MEDICARE

Get more information on these scams and how you can protect yourself and your family

More than 11,000 UC Davis Health employees and students have been vaccinated against COVID-19. UC Davis Health’s rate of vaccination is far ahead of most other health systems in California. At this time, the only group of employees not yet vaccine eligible are those working exclusively remotely.

UC Davis Health will approach COVID-19 vaccination for health care workers similarly to how we have handled flu immunization before 2020. Health care workers will be strongly encouraged to get the vaccine but can sign a declination form for any reason including a religious or medical reason. This is a UC-wide policy. All UCs will have the same choice regarding vaccinations and signing a consent form or a declination form. If an employee declines, they must wear personal protective equipment as required by UC Davis Health to lessen risk to patients and other health care workers whenever they are at any UC Health facility.

The same type of policy will apply to other UC Davis Health employees. They will be strongly encouraged to get the COVID-19 vaccine but can sign a declination form for any reason including a religious or medical reason.