- Most vaccinated people are protected from COVID-19 even if they don’t have side effects. Only about 50% of people vaccinated with either the Pfizer or Moderna vaccines experienced side effects other than arm pain, while 95% were protected from COVID-19 infection.
- Less than half of people who received the Johnson & Johnson vaccine developed side effects besides arm pain, while up to 74% were protected from COVID-19 infection.
- Common vaccine side effects are arm pain, headache, fatigue, and muscle aches, and these effects are more likely to occur after the second dose.
- Women and younger people are more likely to experience symptoms.
The COVID-19 vaccines have been shown to be very effective, but they can also cause a few temporary side effects. Some people develop side effects after the first dose, while others don’t develop them until after the second dose, and still others don’t develop them at all.
Some people may be more likely to have side effects, but it’s not possible to know exactly how any one person will react to the vaccine. We’ll go over common side effects, what they mean, and why some people are more likely to experience them than others.
What are the most common side effects of the COVID-19 vaccines?
The most common side effects of the vaccines include arm pain, headache, fatigue, fever, chills, and body aches. Pain or redness at the injection site is known as a local reaction, while side effects occurring elsewhere in the body are known as a systemic reaction.
Although all of these effects are common, it does not mean that every person will experience all of them. Some people may experience only arm pain or headache, while others may have different symptoms or even none at all. We simply can’t tell ahead of time how someone will react to the vaccine.
Are the side effects the same for the Pfizer, Moderna, and Johnson & Johnson vaccines?
The Pfizer and Moderna vaccines are both mRNA vaccines, meaning they work in the same way to stimulate your immune system. Because they are the same type of vaccine, their common side effects are similar. According to the Centers for Disease Control and Prevention’s Vaccine Adverse Events Reporting System (VAERS), the Moderna vaccine may be slightly more likely to cause common side effects like headache, fever, chills, pain, and nausea.
Serious side effects, such as a severe allergic reaction, are fortunately very rare for both vaccines. In the VAERS data, there were 5 serious allergic reactions per million among people who received the Pfizer vaccine and 3 per million among those who received the Moderna vaccine.
The Johnson & Johnson vaccine is a carrier vaccine, not an mRNA vaccine. This means it is a different type of vaccine and works to stimulate your immune system in a different way. Although it can still cause some of the same side effects as the Pfizer and Moderna vaccines, the symptoms may be milder.
Does the first dose of the COVID-19 vaccine have stronger side effects than the second dose?
No, for most people it is actually the opposite. The second dose of the COVID-19 vaccine is more likely to cause stronger side effects.
The side effects are due to your immune system’s response to the vaccine. The first dose stimulates the immune system and teaches it to recognize the coronavirus, and the second dose acts like a booster. Once your immune system responds to the first dose, the second dose helps your body mount a stronger response. This second stronger response is what helps protect you from coronavirus infection once you are fully vaccinated.
Should I be worried if I didn’t have side effects from the vaccine?
Just because you didn’t have side effects from the vaccine doesn’t mean that the vaccine isn’t working.
In fact, in the clinical trials for the Pfizer and Moderna vaccines, only about half of the participants developed side effects other than arm pain, which was more common. In the clinical trial for the Johnson & Johnson vaccine, less than half of participants had side effects other than arm pain.
Tests of how well the vaccines work, however, showed that the Pfizer and Moderna vaccines both had an approximately 95% efficacy rate. The Johnson & Johnson vaccine had 66% efficacy overall and 74% efficacy in the United States. These results mean that the vaccines protect most people even if they don’t develop any side effects.
Does everyone who gets the COVID-19 vaccines experience side effects?
No, not everyone who gets the COVID-19 vaccines has side effects. In the clinical trials of both Pfizer and Moderna, the majority of participants experienced pain at the injection site, but only about 50% of the participants had any other side effects after the first dose.
The chance of experiencing side effects after the second dose was higher for both vaccines, especially the Moderna vaccine. Some of the side effects after the second dose of the Moderna vaccine, such as fatigue, headache, and muscle aches, occurred in close to 60% of participants. But even though more people developed side effects with the second dose of both vaccines, some people did not have any side effects at all.
For the Johnson & Johnson vaccine, less than half of participants had side effects other than arm pain.
Is there a way to know if the COVID-19 vaccine worked or not?
Unfortunately, there is not an easy way to tell if the vaccine worked because, as we discussed earlier, not everyone develops side effects and the vaccine has been shown to be effective even in people who had no side effects at all. The good news is that because the vaccines are highly effective, it’s very likely that they work even if you don’t have any reaction to them.
Antibody tests are blood tests that can tell you if your immune system responded to exposure to COVID-19, either through infection or vaccination. But currently these tests are not recommended after vaccination because they are unlikely to show the full response of your immune system.
Some people may not respond to a vaccine, but the chance of that happening is low for most people. One group who may be less likely to respond is people who are immunocompromised either because of an underlying disease or an immune-suppressing treatment. The COVID-19 vaccine is safe for people undergoing treatment that suppresses the immune system, but it may be necessary to stop the treatment temporarily or delay vaccination until treatment is completed so that the vaccine can work effectively.
Why are some people more likely to experience side effects than others?
Vaccine side effects occur because of how your immune system responds to the vaccine. According to the VAERS data, women and people between the ages of 18 to 49 are more likely to experience side effects.
Side effects are more likely to occur in women because women tend to have stronger immune responses than men due to biological differences. Similarly, because immune system function can decrease with age, younger people may be more likely to experience side effects than older people.
People whose immune systems were previously exposed to COVID-19 through infection may also be more likely to have side effects. In a study of 40,000 people who received the Pfizer vaccine in the UK, those that had had COVID-19 previously were nearly twice as likely to have systemic side effects after vaccination.
The bottom line
The COVID-19 vaccine is able to prevent infection in 95% of people who receive the Pfizer or Moderna vaccine and in 66% to 74% of people who receive the Johnson & Johnson vaccine. This is considered to be very effective. By comparison, only about 50% of people who are vaccinated experience side effects besides arm pain. Women, younger people, and those that have previously had COVID-19 may be more likely to have side effects.