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Vaccination is a vital tool for stopping the spread of the novel coronavirus (SARS-CoV-2).

According to the World Health Organization (WHO), there are at least 13 different COVID-19 vaccines currently in use around the world, including the Pfizer-BioNTech vaccine and the AstraZeneca vaccine — many countries across the world have authorized or approved one or both of these vaccines for use.

Read on to learn how these two vaccines work, how effective they are, and other critical information you need to know if you’re deciding which one to get or simply want to learn more.

Other namesBNT162b2, ComirnatyOxford, ChAdOx1, AZD1222, Vaxzevria
TypemRNA vaccineadenovirus vector vaccine
Doses2 (21 days apart)2 (28-84 days apart)
EfficacyUp to 95 percentApprox. 70.4 percent
US AuthorizationDecember 11, 2020 (16 and older)
May 10, 2021 (12-15 years old)
Not authorized
WHO AuthorizationDecember 31, 2020 (18 and older)February 15, 2021 (18 and older)

The Pfizer-BioNTech and AstraZeneca COVID-19 vaccines each work in different ways. The Pfizer-BioNTech vaccine uses mRNA technology, while the AstraZeneca vaccine uses an adenovirus vector.

Below, we’ll discuss the mechanisms used in each vaccine meant to help protect you from becoming ill with COVID-19.


The Pfizer-BioNTech vaccine is often referred to by other names in scientific literature, including:

  • BNT162b2
  • Comirnaty

This vaccine is given in two doses. The second dose is given 21 days (3 weeks) after the first dose.

The Pfizer-BioNTech vaccine uses mRNA technology that scientists have been working on for many years. Some of the earliest successful mRNA vaccine clinical trial results were published in 2008.

The Pfizer-BioNTech vaccine works through the following steps:

  1. After you receive the vaccine, the mRNA that it contains is taken up by nearby cells.
  2. Once mRNA is inside of a cell, the mRNA remains outside of the nucleus and doesn’t directly impact your DNA.
  3. The mRNA in the vaccine provides instructions to the cell on how to produce the spike protein that’s found on the surface of SARS-CoV-2. The virus uses this protein to bind to and enter cells before it begins reproducing and spreading the virus throughout your cells.
  4. Using the information provided by the vaccine mRNA, the cell makes spike protein. When this process has been completed, the mRNA is then destroyed.
  5. The spike proteins that the cell has produced are then displayed on the cell surface.
  6. Immune cells in your body can now recognize the spike protein as a foreign substance and work to build an immune response against it.
  7. Your immune system can now produce antibodies and other immune cells that specifically recognize the SARS-CoV-2 spike protein. These tools can help to protect you from becoming sick if you’re exposed to the novel coronavirus.

The mRNA used in the vaccine makes the Pfizer-BioNTech vaccine less stable in storage than other types of vaccines. Because of this, it needs to be kept at ultra-cold temperatures between -112°F (-80°C) to -76°F (-60°C) once it’s been prepared for injection.

Once it’s been diluted and prepared for use, the vaccine remains stable at room temperature for up to 6 hours. After this period ends, it must be discarded and can no longer be injected.


The AstraZeneca (AZ) vaccine is also known by several other names in scientific papers, including:

  • ChAdOx1
  • AZD1222
  • Vaxzevria

Similar to many other COVID-19 vaccines, the AstraZeneca vaccine is given in two doses. The second dose is given between 28 to 84 days (4 to 12 weeks) after the first dose.

The AstraZeneca vaccine is delivered using an adenovirus vector. This is the same type of technology that the Johnson & Johnson COVID-19 vaccine uses.

This type of vaccine uses a deactivated adenovirus that’s been modified to deliver the vaccine contents into a host cell. Once the contents enter the cell, the adenovirus breaks down. This means that it can’t cause disease in humans or interact with our DNA.

Here’s how the AstraZeneca vaccine works:

  1. After the vaccine is injected, the modified adenovirus can enter nearby cells and release its genetic material. This genetic material has instructions on how to make the spike protein found on the surface of SARS-CoV-2.
  2. Your cells use the information provided by the adenovirus’s genetic material to make the spike protein.
  3. When the spike protein is made, it can travel to the cell surface and be displayed there.
  4. Immune system cells notice the spike protein and recognize it as a foreign substance. This triggers the production of an immune response.
  5. Your immune system generates antibodies and immune cells that specifically recognize this new spike protein. If you’re exposed to the novel coronavirus, the antibodies and cells can help prevent you from becoming ill.

The AstraZeneca vaccine is stable at higher temperatures than the Pfizer-BioNTech vaccine, able to be refrigerated between 35°F (2°C) and 46°F (8°C) rather than stored in an ultra-cold freezer.

Once a vial is opened, it can be kept at room temperature for up to 6 hours. It can no longer be injected after this period and must be discarded.

Will boosters be needed?

In August 2021, the FDA approved the use of Pfizer or Moderna booster vaccination doses to be given to people with weakened immune systems — specifically those who’ve undergone solid organ transplants.

Research on how long the immunity generated from any COVID-19 vaccine will last is active and ongoing.

An April 2021 press release from Pfizer notes that protection from the Pfizer-BioNTech vaccine lasts at least 6 months. Similar information regarding the AstraZeneca vaccine has yet to be released.

It’s likely that a vaccine booster will be needed at some point in the future. A COVID-19 vaccine booster could be beneficial in helping reinforce your immunity and to protect against coronavirus variants.

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After vaccination, it takes your body time to build up immunity. This means that you’re considered to be fully vaccinated when it’s been at least 2 weeks since your second dose of either the Pfizer-BioNTech or the AstraZeneca vaccine. You’re not considered immune right after your second dose.

How is vaccine efficacy calculated?

Just how effective these vaccines are is determined by a calculation called vaccine efficacy. Vaccine efficacy measures the percent reduction of a disease in people who’ve been vaccinated compared to those who haven’t.

For example, if a study reports a vaccine efficacy of 80 percent for a vaccine against a certain virus, this means that the risk of vaccinated individuals becoming ill with that virus is decreased by 80 percent.

Efficacy numbers can vary among studies of the same vaccine due to factors like study size, how long the study lasts, and even the health of the participants involved in the study.

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Vaccine efficacy is carefully reviewed by the Food and Drug Administration (FDA) when making the decision to grant emergency use authorization (EUA). Both the Pfizer and AstraZeneca vaccines have been given EUA in the United States and numerous other countries.

Let’s examine vaccine efficacy data from large-scale clinical trials for these two vaccines.


The efficacy of the Pfizer-BioNTech vaccine was evaluated in a phase 3 clinical trial with 43,448 participants. Each participant was given two injections of either the vaccine or a placebo spaced 21 days (3 weeks) apart.

Researchers calculated vaccine efficacy 7 days after trial participants had received their second injections. At this point, the efficacy of the Pfizer-BioNTech vaccine was found to be 95 percent.

Some other data that was reported from this clinical trial includes:

  • COVID-19 after second dose
    • At least 7 days after their second dose, 8 people in the vaccine group contracted COVID-19.
    • In the 7 days after getting their second injection, 162 people in the placebo group got COVID-19.
  • COVID-19 between doses
    • Only 39 cases of COVID-19 were observed in the vaccine group between the first and second doses. This means that vaccine efficacy during this period is about 52 percent.
    • 82 cases of COVID-19 were reported between the first and second injections in the placebo group.
  • Severe COVID-19
    • After participants received their first injection, only 1 case of severe COVID-19 was reported in the vaccine group. 9 cases were reported in the placebo group.
  • COVID-19 deaths
    • No deaths from COVID-19 were reported in either the vaccine or the placebo group.


The efficacy of the AstraZeneca vaccine was assessed in four clinical trials carried out in the UK, Brazil, and South Africa. All four trials combined included 23,848 participants.

Participants were given two injections of either the AZ vaccine or a control injection containing no protection against COVID-19. Depending on the participant’s location, injections were spaced between 4 and 12 weeks apart.

Researchers evaluated vaccine efficacy 14 days (2 weeks) after the second dose and found that:

  • Vaccine efficacy in participants who received two standard doses of the AZ vaccine was 62.1 percent.
  • Vaccine efficacy in participants who received a low dose followed by a standard dose was 90 percent.
  • Vaccine efficacy across both groups was calculated to be 70.4 percent.

Some other findings in the study included:

  • COVID-19 hospitalization: 10 people in the control group (who didn’t get the AstraZeneca vaccine) were hospitalized with COVID-19 in the 21 days after the first injection.
  • Severe COVID-19 or COVID-19 death: Of the 10 people from the control group who were hospitalized, 2 people had COVID-19 categorized as severe. Of these 2 people, 1 died from COVID-19.

Clinical trials of the AstraZeneca vaccine are also underway in the United States, Peru, and Chile. In late March 2021, AstraZeneca announced that an interim analysis of 32,449 trial participants found that the vaccine had an efficacy of 79 percent.

Shortly after this interim analysis was released, the National Institute of Allergy and Infectious Disease raised concerns about the data used for this calculation. AstraZeneca reviewed the data and corrected the vaccine efficacy to 76 percent.

Should I get vaccinated if I’ve already had COVID-19?

Get a COVID-19 vaccine even if you’ve already had COVID-19 and recovered.

It’s unknown how long an individual’s natural immunity to COVID-19 lasts after an infection. It’s possible to contract the virus and become ill more than once.

Some people who’ve recovered from COVID-19 should wait before getting vaccinated. The Centers for Disease Control and Prevention (CDC) recommends waiting 90 days after a COVID-19 infection if your treatment involves monoclonal antibodies or convalescent plasma.

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To be approved for large-scale use (even emergency use), a vaccine must be assessed by the FDA as both safe and effective.

All vaccines come with some risks or potential side effects. But the benefits of a vaccine must always outweigh its potential risks.


The CDC states that those who have an underlying health condition can get a COVID-19 vaccine as long as they haven’t had a previous serious allergic reaction to the vaccine or any of its ingredients.

This includes people who have a weakened immune system or are pregnant or breastfeeding. Specific safety data for these populations is limited, but increasing evidence indicates that vaccination poses minimal risk.

All of the ingredients used in the Pfizer-BioNTech vaccine are also listed in an FDA fact sheet meant to be used by both people giving and receiving the vaccine.


The European Medicines Agency (EMA) notes the following related to the safety of the AstraZeneca vaccine:

  • People with allergic reactions to previous vaccines may still receive the AstraZeneca vaccine unless they’ve had a previous allergic reaction to the AstraZeneca vaccine itself or one of its ingredients.
  • People with weakened immune systems may be vaccinated with the AstraZeneca vaccine. No safety concerns have been reported. But their immune response to COVID-19 following vaccination may be weaker than that of someone with a stronger immune system.
  • People who are pregnant or breastfeeding can get the AstraZeneca vaccine. Data on the use of the vaccine in this group is limited. Talk with a doctor before vaccination if you have any concerns.
  • People who’ve had a blood clot along with a low platelet count due to a previous dose of the AstraZeneca vaccine should NOT receive this vaccine.

Refer to the vaccine’s official summary of product characteristics to see the different ingredients used in the AZ vaccine and decide if you may have an allergy to one of its ingredients.

All vaccines can have side effects. Let’s look at the potential side effects of the Pfizer-BioNTech and AstraZeneca vaccines.


There are several side effects that are possible after receiving the Pfizer-BioNTech vaccine, including:

These side effects are typically mild to moderate in intensity and go away in a few days. Systemic side effects like fever or aches and pains are more common after the second dose.

Anaphylaxis is a severe allergic reaction that’s a rare but serious side effect of the Pfizer-BioNTech vaccine. Some symptoms of anaphylaxis to watch out for include:

  • trouble breathing
  • swelling in the face and throat
  • rapid heartbeat
  • rash
  • feeling dizzy or weak

The clinical trial of the Pfizer-BioNTech vaccine reported just 4 other serious but rare side effects related to the vaccine, including:

  • shoulder injury related to vaccination
  • lymphadenopathy
  • paresthesia in the right leg
  • paroxysmal ventricular arrhythmia

No deaths from getting the vaccine or the placebo were reported during the trial.


The product summary for the AstraZeneca vaccine classifies side effects into several categories.

Some side effects are considered common and impact about 1 in 10 people who get the AstraZeneca vaccine, like:

  • injection site reactions, including:
    • pain
    • swelling
    • redness
    • bruising
  • fatigue
  • feeling unwell (malaise)
  • fever
  • chills
  • muscle or joint pain
  • headache
  • nausea or vomiting
  • diarrhea
  • low platelet levels (thrombocytopenia)

Other side effects are uncommon and affect about 1 in 100 people, including:

Most side effects above are mild to moderate and go away in a few days. As with the Pfizer-BioNTech vaccine, it’s possible to experience a severe allergic reaction called anaphylaxis after receiving the AstraZeneca vaccine.

Like the Johnson & Johnson vaccine approved for emergency use in the United States, the AstraZeneca vaccine is also associated with a risk of a very rare clotting condition called thrombosis with thrombocytopenia syndrome (TTS).

After reviewing the data on these clots, the EMA determined that the health benefits of the AstraZeneca vaccine outweighed the potential risks of TTS. The EMA estimated that TTS occurred in less than 1 in 10,000 people.

In the clinical trials for the AstraZeneca vaccine, two instances of severe side effects were found to be related to the vaccine, including:

Additionally, one death reported in the vaccine group wasn’t found to be related to COVID-19 or the AstraZeneca vaccine.

Viruses mutate constantly. In many cases, mutations are harmful to the virus and prevent it from finishing its life cycle. But in other cases, a new mutation can provide the virus with an advantage.

A large number of variants of the new coronavirus have been detected so far. Some are considered “variants of concern” — this means that they may:

  • be spread more easily
  • cause more severe illness
  • evade diagnostic tests (resulting in false negative results)
  • be unaffected by the immune system, even after vaccines or monoclonal antibody treatments

A few examples of novel coronavirus variants include:

  • B.1.1.7: First identified in the United Kingdom. It’s more transmissible than the original strain of the novel coronavirus and may potentially cause more severe illness.
  • B.1.351: First detected in South Africa. Like B.1.1.7, it’s also more transmissible. It also appears largely unaffected by the immune system, even in those who’ve been vaccinated.
  • P.1: First detected in Brazil and Japan. Similar to B.1.351, it also seems to be able to evade the immune system.
  • B.1.617.2: First detected during an outbreak in India early in 2021. It’s also known as the Delta variant and may be more easily spread than other variants.

Research into how effective the different COVID-19 vaccines are against these variants is ongoing. Read on to learn what we know so far.


An April 2021 study tested serum, the part of the blood containing antibodies, from people who’d been vaccinated with the Pfizer-BioNTech vaccine against test viruses with spike proteins from different COVID-19 variants.

Researchers found that the serum neutralized both the B.1.1.7 and the P.1 test viruses at a similar rate to the original strain of the novel coronavirus. They also noted that neutralization of B.1.351 test viruses was lower but still robust.

A July 2021 study from Qatar looked at vaccine efficacy in both vaccinated and unvaccinated individuals with confirmed cases of COVID-19. At the time of the study, most COVID-19 in Qatar was due to either the B.1.1.7 or the B.1.351 variant.

Researchers found that the vaccine efficacy against these variants was quite high for people who were fully vaccinated with the Pfizer-BioNTech vaccine:

  • B.1.1.7 variant: 89.5 percent
  • B.1.351 variant: 75 percent
  • Severe, critical, or fatal illness due to either variant: 100 percent

Research from the United Kingdom has also found that the Pfizer-BioNTech vaccine is effective against the B.1.1.7 and B.1.617.2 “Delta” variants, finding the following efficacy:

  • B.1.1.7 variant: 93 percent
  • B.1.617.2 variant: 88 percent


A March 2021 study assessed protection against the B.1.1.7 variant in 8,534 individuals following a second dose of the AstraZeneca vaccine or a control injection. Researchers found that:

  • 520 participants developed COVID-19 after their second injection. Only 173 (33.3 percent) were in the group that received the vaccine.
  • After sequencing 147 samples from symptomatic infections, 35 percent of infections were found to be caused by B.1.1.7.
  • Vaccine efficacy was calculated to be:
    • 70.4 percent for symptomatic COVID-19 caused by the B.1.1.7 variant
    • 81.5 percent for COVID-19 not caused by B.1.1.7

The same research from the United Kingdom mentioned earlier also found that individuals that were fully vaccinated with the AstraZeneca vaccine showed a vaccine efficacy of:

  • B.1.1.7 variant: 66 percent
  • B.1.617.2 variant: 60 percent

A different March 2021 study looked into the efficacy of the AZ vaccine versus placebo against the B.1.351 variant in South Africa. Vaccine efficacy 14 days after the second injection was found to be low, at about 21.9 percent overall for the B.1.351 variant in both people receiving the vaccine and those receiving the placebo.

During the pandemic, the federal government of the United States is providing authorized COVID-19 vaccines free of charge. This is regardless of a person’s immigration status or health insurance coverage.

If you go to get a COVID-19 vaccine during the pandemic, you can’t be charged for:

  • the vaccine
  • any copays, coinsurance, or administration fees
  • fees for an office visit if the only service rendered was COVID-19 vaccination

Because the AstraZeneca vaccine isn’t yet authorized in the United States, it’s not included in this program. But the Pfizer-BioNTech vaccine is due to its emergency use authorization.

Other countries, like the United Kingdom and countries in the European Union, also provide the Pfizer-BioNTech and AstraZeneca vaccines free of charge. Countries who participate in the COVAX program may also be able to provide you with one of these vaccines for free.

COVID-19 vaccination will likely continue after the pandemic as either first-time vaccinations or booster shots. This means that the vaccines may come at a higher cost since both are made by for-profit pharmaceutical companies.

The exact details of when the pandemic may end or what the costs of the vaccines may be have yet to be confirmed.

COVID-19 vaccines used in the United States, including the Pfizer-BioNTech vaccine, are currently authorized under an emergency use authorization (EUA). EUAs are issued by the FDA, but aren’t the same thing as full FDA approval.

An EUA is used by the FDA to allow the use of unapproved medical products during a public health emergency, like a pandemic. The FDA must carefully review a product before granting an EUA:

  1. Product submission: A company seeking an EUA for a product needs to submit results from a phase 3 clinical trial to the FDA with both safety and efficacy data. This trial can still be in progress, but must meet specific milestones set by the FDA.
  2. Internal review: Once the submission is received, FDA scientists carefully go over clinical trial data.
  3. External advisory panel review: During the internal review, an external advisory panel also reviews and discusses the data provided in the submission. This panel includes scientists and public health experts.
  4. The decision: The FDA considers the feedback from the internal review and the external advisory panel review. When deciding to grant an EUA, the FDA must conclude that the benefits of a product outweigh its risks.

The AstraZeneca vaccine isn’t authorized for emergency use in the United States and has yet to submit for an EUA.

Now let’s look at some specific authorization details for these two vaccines. Additional authorizations or approvals may be added on an ongoing basis.


The FDA authorized the Pfizer-BioNTech vaccine for emergency use in individuals aged 16 and older on December 11, 2020. It was also authorized on May 10, 2021 for use in adolescents between the ages of 12 and 15.

The World Health Organization (WHO) has also listed the Pfizer-BioNTech vaccine for emergency use. Many other countries worldwide have also either authorized or approved the vaccine:


The AstraZeneca vaccine has yet to be authorized for emergency use in the United States by the FDA.

On February 15, 2021, the WHO listed the two versions of the AZ vaccine for emergency use in adults aged 18 and over.

Similar to the Pfizer-BioNTech vaccine, many countries around the world have either authorized or approved the AZ vaccine.

Let’s take a closer look at the namesake companies who’ve produced the Pfizer-BioNTech and AstraZeneca vaccines.


Pfizer is an American pharmaceutical company headquartered in New York City. They develop new drugs and vaccines for a variety of different health conditions.

A few examples of Pfizer’s more well-known drugs or vaccines include:

  • Enbrel (etanercept), used to treat rheumatoid arthritis and psoriatic arthritis
  • Lipitor (atorvastatin), used to lower cholesterol levels
  • Lyrica (pregabalin), used to treat nerve pain
  • Prevnar 13, a type of pneumococcal vaccine
  • Viagra (sildenafil), used to treat erectile dysfunction

The COVID-19 vaccine was developed in collaboration with the German biotechnology company BioNTech. This collaboration came out of a previous agreement between the two companies to develop a flu vaccine using mRNA technology.


AstraZeneca is a British and Swedish pharmaceutical company headquartered in Cambridge, United Kingdom. Their main therapy areas include oncology and conditions that impact the respiratory system, cardiovascular system, and kidneys.

Some examples of AstraZeneca products include:

  • Crestor (rosuvastatin), used to lower cholesterol levels
  • FluMist, a live intranasal flu vaccine
  • Nexium (esomeprazole), used to treated gastroesophageal reflux disease (GERD)
  • Symbicort (budesonide/formoterol), used to treat asthma and chronic obstructive pulmonary disease (COPD)
  • Tagrisso (osimertinib), used to treat non-small-cell lung cancer

AstraZeneca developed their COVID-19 vaccine in collaboration with scientists at the University of Oxford. That’s why you’ll sometimes see this vaccine referred to as the Oxford/AstraZeneca vaccine.

To best protect yourself from COVID-19 and stop the spread of the novel coronavirus, it’s important to get vaccinated. The best vaccine to get is typically the one that’s the soonest available to you.

If you live in a country where both the Pfizer-BioNTech and AZ vaccines are authorized or approved, you may need to decide which one to get if both options are offered to you. As you decide, you may want to consider the potential risks or side effects associated with each one.

For example, you may have concerns about TTS and choose to get the Pfizer-BioNTech vaccine instead of the AstraZeneca vaccine.

Talk with a doctor if you’re unsure which one’s better or safer for you. They’re an important resource for information and can cover various topics with you, like:

  • potential benefits of vaccination
  • safety and efficacy evidence currently available
  • potential risks of becoming ill with COVID-19, particularly if you have an underlying condition or are pregnant
  • side effects associated with vaccination and what to do if you experience them
  • how to find a vaccination site close to you

No two vaccines are alike

Keep in mind that it isn’t possible to directly compare these two vaccines because their clinical trials:

  • used different study protocols, methods, and endpoints
  • were performed in different geographic locations across the globe
  • had different groups of study participants
  • occurred at different times in the pandemic

This is important to consider when weighing the benefits, risks, and efficacy of each vaccine. Talk with a doctor or a specialist with experience in vaccines, like an immunologist or epidemiologist, if you have any questions about how these vaccines were tested and authorized.

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The Pfizer-BioNTech and AZ vaccines are both important tools for protecting you against COVID-19 and ending the global pandemic. The best vaccine to get is usually the one you can get as soon as possible.

Neither vaccine is inherently better than the other. But one may be safer for you or more easily available depending on your overall health and location.

Talk with a doctor if you have any questions or concerns related to COVID-19 vaccination.