Key takeaways: 

  • There are currently three authorized COVID-19 vaccines available in the U.S. — two mRNA vaccines (Pfizer and Moderna) and one viral vector vaccine (Johnson & Johnson).
  • The three COVID-19 vaccines cause similar side effects, including mild flu-like symptoms and injection-site reactions. Severe reactions to the vaccines are very rare.
  • Each vaccine provides very strong protection against severe illness, hospitalization, and death from COVID-19, but scientists are still trying to find out if they can prevent us from spreading the virus as well.

Over the past year, the world has experienced a deadly pandemic — COVID-19. In the face of such a challenge, pharmaceutical companies around the world quickly started developing vaccines to slow the spread of the virus and to bring an end to the coronavirus pandemic. So far, three of these vaccines have been authorized for use in the United States — PfizerModerna, and Johnson & Johnson.

Here we’ll compare the three COVID-19 vaccines and the differences in how they work, their effectiveness, and what side effects you can potentially expect from each.

How do the COVID-19 vaccines work?

Vaccines work by triggering the cells in your body to make antibodies — memories created in response to an infection — against a certain virus or bacteria. This means that if you are exposed to the germ in the future, you’ll be better prepared to fight it.

The processes and technology used to make vaccines vary. And while several different types of COVID-19 vaccines are being researched, we’ll cover the two types currently available in the U.S. — mRNA and viral vector vaccines.

mRNA vaccines

The mRNA vaccines — Pfizer and Moderna — work by using a technology called messenger RNA (mRNA). You can think of mRNA as a blueprint that is delivered to cells in your body. The cells in your body already use mRNA as instructions to make proteins that they need, but the mRNA contained in the vaccines is specific to the coronavirus that causes COVID-19.

How do mRNA vaccines cause an immune response?

When you receive a dose of an mRNA vaccine, your cells get the blueprint for making part of the coronavirus, called the spike protein. Spike proteins are found on the surface of the coronavirus, and they help the virus enter and infect your cells. They’re only one small part of the virus, so it’s impossible to get COVID-19 from the vaccine itself.

Once your immune cells notice the spike proteins, they will start to make antibodies against it, allowing your body to build its immunity. In other words, your body will already know how to fight off the coronavirus if you’re ever exposed in the future.

What happens to the mRNA in my body?

After your body receives the spike protein blueprints, the cell breaks down the mRNA. But while it’s there, it only interacts with the part of the cell responsible for making proteins (called ribosomes), and it never enters the nucleus of your cells — which contains your DNA. Therefore, it will not affect your DNA.

Is mRNA technology safe?

Although these vaccines were developed faster than most others have been, it doesn’t mean that they’re unsafe. Both mRNA vaccines have been shown to be safe and effective during clinical trials, and this still holds true after over 300 million doses have been administered in the U.S.

And while the mRNA technology is relatively new to the market, it’s been researched for decades. In fact, the use of mRNA vaccines has been gaining interest among scientists in recent years because they are quicker to develop than traditional vaccines. Plus, their structure allows them to be easily updated, making them adaptable to new viral strains or variants.

Viral vector vaccines

Viral vector vaccines — like the Johnson & Johnson vaccine — use a harmless virus (called a viral vector) to deliver a message to your cells. They have been used in the past for treating certain cancers and for an Ebola vaccine. But in the case of the COVID-19 vaccines, they deliver instructions for making the coronavirus spike protein.

How do viral vector vaccines cause an immune response?

When you receive a dose of the viral vector COVID-19 vaccine, the harmless virus enters your cells and gives the message to start producing spike proteins. They work similarly to the mRNA vaccines in that after the viral vector prompts your cells to make spike proteins, your immune system will start making antibodies against them. This helps to create protection against the coronavirus.

Can the virus in the vaccine make me sick?

The Johnson & Johnson vaccine uses an inactivated adenovirus — a common cold virus that’s been engineered so that it can’t make you sick — as its viral vector. It’s a less common adenovirus (called adenovirus type 26) since using one that most people have been exposed to could result in the vaccine being less effective. And like with the mRNA vaccines, it’s impossible to get COVID-19 from the viral vector vaccine.

How effective are the COVID-19 vaccines?

While it’s easy to get caught up in the clinical trial efficacy numbers, the bottom line is that each of the COVID-19 vaccines has been shown to be very effective, especially at preventing severe illness resulting in hospitalization or death.

Next, we’ll review how the vaccines performed in clinical trials, how effective they are in the real world, and when you can expect to be fully protected after getting vaccinated.


The Pfizer COVID-19 vaccine requires two doses, given 3 weeks (21 days) apart. If needed, the second dose can be given up to 6 weeks (42 days) after the first dose. Immune response varies from person to person, but you are considered to be fully protected 2 weeks after completion of the second dose.

In clinical trials, the Pfizer vaccine was shown to be 95% effective at preventing symptomatic COVID-19 infections in people ages 16 and older. The vaccine was also recently tested and authorized for use in adolescents ages 12 to 15, showing it to be 100% effective against the disease.

Real-world effectiveness data is also encouraging. The mRNA vaccines have been shown to be 94% effective at preventing hospitalization from COVID-19 in people aged 65 and older, which is a group considered to be at higher risk for developing severe illness.


The Moderna COVID-19 vaccine also requires two doses, but they’re given 4 weeks (28 days) apart. Similarly, you’ll need to wait at least 2 weeks after the second shot to get the full benefit. And like the Pfizer vaccine, you’re able to get the second dose up to 6 weeks after the first dose, if needed.

In clinical trials, the Moderna vaccine was shown to be around 94% effective at preventing symptomatic COVID-19 infections in people ages 18 and older who received both doses.

As described above, the mRNA vaccines have been shown to be highly effective in older adults, according to real-world data. Additionally, a study in healthcare workers showed that the mRNA vaccines are 90% effective 2 weeks after the second shot. This means that they’re offering real-world protection similar to what was shown in clinical trials.

Johnson & Johnson

Unlike the mRNA vaccines, the Johnson & Johnson COVID-19 vaccine only requires one dose, making it a more convenient option for many people.

It may seem as though that the Johnson & Johnson vaccine is significantly less effective than the mRNA vaccines, but it’s important to know that they are difficult to compare head-to-head. A number of factors could contribute to the differences, like what the studies were measuring and the prevalence of coronavirus variants where the vaccines were being tested.

That said, the Johnson & Johnson vaccine was 66% effective at preventing symptomatic COVID-19 14 days after the vaccination during clinical trials. And most importantly, it was 85% effective at preventing severe disease (hospitalization and death) at least 28 days after the vaccination.

What’s more, real-world data from a study that has yet to be peer reviewed showed that the vaccine was about 76% effective at preventing COVID-19.

Why do the Pfizer and Moderna vaccines require two doses?

The mRNA vaccines require two doses in order to build the maximum immune response. There are two types of cells involved in forming immunity — B cells and T cells.

B cells are the first to form after a vaccine is given, and their job is to make antibodies in the first few weeks. However, the amount of antibodies in your body may start to go down quickly if you don’t get your second dose.

But once your body sees the spike protein again from your second shot, your immune system recognizes it from the first time and is able to make more T cells — targeted immune cells that last longer — and mature B cells, which make even stronger antibodies. This means better and longer protection against the coronavirus.

You may be wondering if one shot will give you enough protection if you’re partially vaccinated. However, there are still a lot of unknowns about how long you’ll be protected if you skip your second shot, so it’s still recommended to get both.

Why do the Pfizer and Moderna vaccines have different waiting periods between the first and second shot?

The time interval between the first and second shot is determined by the data collected from their clinical trials. The companies have published their safety and efficacy results based on these specified intervals. Pfizer’s published data is based on a 21-day interval between doses, while Moderna’s is based on a 28-day interval.

Who can get the COVID-19 vaccines?

If you have certain types of allergies — to the vaccine ingredients, other vaccines, or other injections — the CDC has recommendations for whether or not you should get the shot. But for most other people, it’s recommended that you get vaccinated. And while there are some general guidelines across all COVID-19 vaccines, the biggest difference is which age groups can get each vaccine.


All three COVID-19 vaccines can be given to people who are at least 18 years old. The Moderna and Johnson & Johnson vaccines are currently only authorized for use in this age group.


The Pfizer vaccine, on the other hand, was recently authorized for anyone at least 12 years old. Moderna also has adolescent vaccine trials underway, with interim findings of 96% efficacy in people ages 12 to 17. Johnson & Johnson is still relatively early in the process of testing their vaccine in younger people.

But for younger children under 12 years old, we’ll still need to wait a while before vaccines are available. Both Pfizer and Moderna are in the process of testing their vaccines in this group — even as young as 6 months old.

Do all three vaccines have the same side effects?

Most vaccines have the potential to cause minor side effects, and the COVID-19 vaccines are no exception. Each of the vaccines cause similar side effects to each other.

Due to the high-profile nature of the coronavirus pandemic, you may have some concerns about side effects that you’ve heard about in the news. Let’s break down what you should expect.

Common side effects

All three COVID-19 vaccines have the potential to cause a reaction in the injection arm including:

  • Soreness
  • Redness
  • Swelling at injection site or of lymph nodes

They can also cause mild flu-like symptoms, such as:

  • Headache
  • Fatigue
  • Body aches
  • Mild fever
  • Chills
  • Nausea

The side effects tend to be mild to moderate in severity and only last for a few days. And if you’re receiving an mRNA vaccine, the side effects tend to be stronger after the second shot.

It’s important to remember that people can react to the vaccines differently — some may react strongly, whereas others won’t react at all. And if you don’t experience any side effects, that doesn’t mean that it isn’t working.

Serious side effects

Overall, all three of the COVID-19 vaccines have been well-tolerated with temporary side effects. However, more serious side effects can sometimes happen. Here’s a few that you should know.

Severe allergic reactions

severe allergic reaction after a COVID-19 vaccine is extremely rare, but if you do have one after the first dose, the CDC recommends that you not get the second dose.

A severe allergic reaction, also known as anaphylaxis, requires the use of epinephrine and possibly a trip to the hospital. The CDC recommends that everyone who receives a COVID-19 vaccine should be monitored for 15 minutes afterward — this is when most severe allergic reactions tend to happen.

But if you have a history of anaphylaxis from any cause, or if you’ve had an immediate reaction after a different vaccine or injectable medication, you should be monitored for 30 minutes.

Severe arm pain

The CDC has received rare reports of “COVID arm” after receiving the first shot of a COVID-19 vaccine. This is a red, swollen, itchy, and potentially painful rash on the injected arm that can sometimes cover a large area. It can occur a few days to a week after your vaccination.

It’s still recommended to get the second dose if you experience this, but let your vaccinator know when you go for your second shot.

Johnson & Johnson vaccine blood clot risks

There have been reports of a rare but serious blood clotting disorder after receiving the Johnson & Johnson vaccine. After a brief pause and expert review, the CDC and FDA agree that this vaccine is still safe to use.

However, if you receive the Johnson & Johnson vaccine, you should be on the lookout for the following symptoms for 3 weeks after your shot:

  • Shortness of breath
  • Blurred vision
  • Severe headache that won’t go away
  • Leg swelling
  • Chest pain
  • Abdominal pain that won’t go away
  • Bruising or blood spots under the skin away from the injection site

The bottom line

COVID-19 vaccinations are now available and free of charge to anyone in the U.S. who wants one. The Pfizer vaccine is available for anyone at least 12 years old, and the Moderna and Johnson & Johnson vaccines can be given to anyone at least 18 years old.

Overall, all three vaccines effectively prevent serious illness, hospitalization, and death from COVID-19. And the most commonly reported side effects are mild to moderate arm pain and flu-like symptoms that last for a few days.

If you’re unsure if a COVID-19 vaccine is right for you, then talk to your healthcare provider to get more information. And if you’re having trouble finding a vaccine in your area, you can search for a vaccine provider here.