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Last updated February 2, 2021. 6 minute read

How safe are the COVID-19 vaccines?

Studies have found the COVID-19 vaccines to be safe and effective. On top of that, the chances for a severe allergic reaction to occur are extremely rare. Read on for more about the COVID-19 vaccines, how they work, and what to expect after getting one.

Dr Chimene Richa Md Written by Chimene Richa, MD
Reviewed by Steve Silvestro, MD

Millions of doses of coronavirus disease 2019 (COVID-19) vaccines have been given to adults in the United States, and data so far indicates these vaccines are indeed safe.  

The two vaccines that have Emergency Use Authorization (EUA) from the U.S. Food and Drug Administration (FDA) are the Moderna vaccine and Pfizer-BioNTech vaccine (CDC, 2021a). Both vaccines underwent clinical trials involving tens of thousands of people, and after careful consideration, the FDA approved EUAs for the vaccines. 

But it doesn’t stop there. Scientists at the Centers for Disease Control and Prevention (CDC) continue to keep track of any adverse effects people experience after getting the vaccine.  There is also a voluntary tracker, called v-safe, that you can use to tell the CDC about any side effects (CDC, 2021b). Here’s more about the vaccines, how effective each is, and any potential side effects to watch out for. 


  • With millions of doses administered nationwide, the COVID-19 vaccines have so far been shown to be safe and effective.
  • The most common side effects of the vaccines are mild and include injection site soreness, headaches, and fatigue.
  • You cannot get sick with COVID-19 from the vaccine, and the risks of severe allergic reactions are very small.

How do the COVID-19 vaccines work?

The goal of the vaccines is to train your immune system to recognize the COVID-19 virus as quickly and efficiently as possible without you getting sick. 

The vaccines contain blueprints of genetic material that code for specific spike proteins that exist on the outside of the coronavirus. These spike proteins are also what gives the virus its signature crown shape. The COVID-19 vaccines then deliver these blueprints to your cells, where protein factories start to build the viral spike proteins. The vaccine itself doesn’t contain the code for any other part of the virus—just the spike proteins. 

Once your cells create the viral proteins, your immune system mounts an immune response in the form of antibodies. Antibodies can essentially be stored for future use so if you come into contact with the coronavirus, your immune system can use them to fight off the infection before you get sick. 

The vaccines currently in use require two doses to be effective. After you receive both doses, it can take 1–2 weeks (depending on which vaccine you get) until you achieve protection against COVID-19. Studies have shown that both vaccines are very effective at protecting you from COVID-19, with the Moderna vaccine 94.5% effective and the Pfizer-BioNTech 95% effective (FDA, 2020a; FDA, 2020b).

Side effects of the COVID-19 vaccines

So far, the side effects of both the Moderna and Pfizer-BioNTech vaccines are similar. Here are some common side effects people experience (CDC, 2021c):

These side effects can occur after either dose of the vaccine, but many people report side effects after the second dose. 

It’s important to note that these are not signs that you have COVID-19. Rather, they are proof your immune system is working against the virus. That being said, you can catch COVID-19 right after getting the vaccine as it takes weeks for your body to develop full protection—especially if your body hasn’t had enough time to build up immunity.

Severe allergic reactions

Rarely, the Moderna and Pfizer-BioNTech vaccines can cause severe allergic reactions (also called anaphylaxis), but this can occur with any vaccine or medication. 

Only five cases of anaphylaxis occur per one million doses of the Pfizer-BioNTech vaccine, and less than three cases per million doses of the Moderna vaccine. To put that in perspective, you have less than a 0.0005% chance of having a severe allergic reaction to the vaccine (CDC, 2021c). It’s so rare that you’re more likely to get sick from COVID-19 than have a severe allergic reaction (CDC, 2021e). 

Data has also found that 80% of people who experienced adverse reactions had a history of allergies or allergic reactions to drugs and foods in the past. Furthermore, 90% of anaphylaxis reactions occurred within 30 minutes of getting the vaccine (CDC, 2021c). So, if you’ve had an allergic reaction to something unrelated to vaccines or injectable therapies, then there’s no reason not to get a COVID-19 vaccine. 

In addition, those with allergies to things like food, pets, and environmental allergies don’t need to take special precautions before getting the vaccine. However, if you have a history of severe allergic reactions, you’ll likely be monitored for 30 minutes after getting the vaccine to watch for any concerning side effects (CDC, 2021d). 

Symptoms of anaphylaxis include trouble breathing, difficulty swallowing, hives, swelling of your mouth or throat, and widespread skin rash. If you think you might be having a severe allergic reaction to the vaccine, you should call 911.

Can you get COVID-19 from the vaccine?

To put it simply: the vaccines do not give you COVID-19. 

None of the COVID-19 vaccines in use today contain live viruses, so they can’t make you sick. The only thing the vaccines carry from COVID-19 virus are blueprints of genetic material needed to create spike proteins. This trains your immune system to recognize the virus should you become exposed in the future. To get COVID-19, you would need the entire virus particle to reproduce itself in your body, and the vaccines don’t carry whole virus particles. 

It’s common to experience side effects after receiving the vaccine, including soreness at the injection site (usually in your arm), headaches, and fatigue. But these are not signs of COVID-19—it just means your immune system is responding to the vaccine.

Scientists don’t know yet if people who’ve received a vaccine can still carry the virus and transmit it to others unknowingly. That’s why it’s important to continue to still wear a face mask, avoid large gatherings, practice social distancing, and wash your hands frequently to decrease the likelihood of spreading the virus.

Pregnancy and breastfeeding safety

None of the vaccines currently available have definitively studied vaccine safety in pregnant women. In fact, the clinical trial specifically excluded pregnant people from enrolling. During clinical trials of the Moderna and Pfizer-BioNTech vaccines, some participants became pregnant, however, no pregnancy-related side effects were reported. 

That being said, overall pregnancy risks and outcomes are unknown and more research is needed in this area (FDA, 2020a; FDA, 2020b). It’s also essential to remember that pregnant people with COVID-19 are more at risk for severe symptoms or even death compared to those who are not pregnant. They may also be at risk for other bad outcomes, such as delivering the baby too early or having a miscarriage (Wastnedge, 2020). Most experts do not believe the vaccines pose a significant threat to pregnant people because it doesn’t actually contain the live virus, and what viral genetic material does enter your system gets broken down by your cells. 

If you’re in one of the groups being recommended for the vaccine (like healthcare workers and those who are pregnant) you should speak with a healthcare provider beforehand about the risks and benefits of getting the vaccine. The CDC is not currently recommending pregnancy tests before vaccination, nor is it advising people to avoid getting pregnant after getting the vaccine (CDC, 2021d). 

Similarly, there have not been any studies on people who are breastfeeding. Again, because the COVID-19 vaccines don’t contain a live virus, it’s not likely to harm newborns. 

COVID-19 vaccines and infertility

There is currently no data linking the vaccine to infertility. 

You may have encountered false information saying that the spike proteins are similar to a protein needed for fertility. That would mean if your body forms antibodies against spike proteins, it also forms antibodies against the fertility protein, thereby causing infertility. But the two proteins are not alike enough for this to occur. 

If the vaccines did cause infertility, then many women who were exposed to COVID-19 would be infertile, and so far there is no evidence to suggest that this has occurred (Pfizer, 2021). And as we noted, several people in both the Moderna and Pfizer-BioNTech vaccine trials became pregnant, making it even more unlikely that the vaccine causes infertility. However, scientists are continuing to study the effects of the COVID-19 vaccines and pregnancy (FDA, 2020a; FDA, 2020b).