Common side effects include pain, redness and swelling where you got the shot, fatigue, headache, muscle or joint pain, chills, fever, nausea, and swollen lymph nodes. Most side effects are minor and subside in a day or two, but they may be more pronounced after the second vaccine dose.

Possible Side Effects After Getting a COVID-19 Vaccine (CDC)
COVID-19 vaccination will help protect you from getting COVID-19. You may have some side effects, which are normal signs that your body is building protection. These side effects may affect your ability to do daily activities, but they should go away in a few days. Some people have no side effects.

COVID-19 vaccines: Safety, side effects — and coincidence (Harvard Medical School)
COVID-19 is an unpredictable and potentially deadly disease. And the information we have about the effectiveness and safety of COVID-19 vaccinations is encouraging. Minor side effects should be expected; severe allergic reactions may rarely occur. Side effects from the vaccine are not reasons for most people to avoid vaccination.

What the Vaccine Side Effects Feel Like, According to Those Who’ve Gotten It (NY Times) Here is what some of the first Americans to be vaccinated against Covid-19 are saying about how they felt afterward, with some side effects but no second thoughts.

What the Vaccine’s Side Effects Feel Like (The Atlantic)
The COVID-19 vaccine will make some people feel sick. But they’re not—that’s the immune system doing its job. Both the Pfizer/BioNTech and Moderna vaccines are quite “reactogenic”—meaning they stimulate a strong immune response that can cause temporary but uncomfortable sore arms, fevers, chills, and headaches. In other words, getting them might suck a little, but it’s nowhere near as bad as COVID-19 itself.

The Second COVID-19 Shot Is a Rude Reawakening for Immune Cells (The Atlantic)
Side effects are just a sign that protection is kicking in as it should. Dose No. 2 is more likely to pack a punch—in large part because the effects of the second shot build iteratively on the first.

COVID-19 vaccine side effects; why the second dose could feel worse (Mayo Clinic)
When you come back with a second dose … your body is primed by that first dose of vaccine. The second vaccine dose goes into your body, starts to make that spike protein, and your antibodies jump on it and rev up your immune system response. It’s kind of like they’ve studied for the test. And it’s acing the test.

In rare cases, some people can experience anaphylaxis, a severe allergic reaction, in response to the vaccine. This is more common in people who have had severe reactions to other vaccines in the past, and is very rare overall – about 11 cases per million doses, according to CDC estimates. Vaccine providers can effectively respond to most cases of anaphylaxis with epinephrine (EpiPen©) on site, which is why people are asked to remain at the location for observation for 15 minutes after receiving the vaccine. There have been no reported deaths as a result of allergic reactions to the vaccine.

Information about COVID-19 Vaccines for People with Allergies (CDC)

• If you have had a severe allergic reaction or an immediate allergic reaction—even if it was not severe—to any ingredient in an mRNA COVID-19 vaccine, you should not get either of the currently available mRNA COVID-19 vaccines (Pfizer-BioNTech and Moderna).
• If you have had a severe allergic reaction or an immediate allergic reaction to any ingredient in Johnson & Johnson’s Janssen (J&J/Janssen) COVID-19 vaccine, you should not get the J&J/Janssen vaccine.
• If you aren’t able to get one type of COVID-19 vaccine because you are allergic to an ingredient in that vaccine, ask your doctor if you should get a different type of COVID-19 vaccine
• PEG and polysorbate are closely related to each other. PEG is an ingredient in the mRNA vaccines, and polysorbate is an ingredient in the J&J/Janssen vaccine.
• If you are allergic to PEG, you should not get an mRNA COVID-19 vaccine. Ask your doctor if you can get the J&J/Janssen vaccine.
• If you are allergic to polysorbate, you should not get the J&J/Janssen COVID-19 vaccine. Ask your doctor if you can get an mRNA COVID-19 vaccine.
• All three vaccines do not contain Eggs, Preservatives or Latex.

COVID-19 Vaccine Reported Allergic Reactions (Allergy and Asthma Network)

CDC and FDA have recommended that use of the Johnson & Johnson vaccine resume in the United States, effective April 23, 2021. The CDC has determined that the benefits of the vaccine still significantly outweigh the risk.

However, women younger than 50 years old especially should be aware of the rare risk of blood clots with low platelets after vaccination, and that other COVID-19 vaccines (Pfizer and Moderna) are available where this risk has not been seen.

The Johnson & Johnson Vaccine and Blood Clots: What You Need to Know (Yale Medicine)
Focusing on the word “pause” is key to understanding what happened with the Johnson & Johnson vaccine. “Putting a pause on a new vaccine or medication is not unusual. Even after it has completed its clinical trials, a vaccine still must be monitored in the real world,” says Yale Medicine infectious diseases specialist Jaimie Meyer, MD, MS. “If there are serious adverse events, we need to stop, take a deep breath, collect and analyze the data—and really try to understand why this is happening. Then, we can decide whether we should proceed.” Transparency is also important, Dr. Meyer adds. “People should know that there are these events that have been observed—even if they are very rare events—so they can make informed choices,” she says.

How to Think About Johnson & Johnson Vaccine and Rare Side Effects (healthline)
The clots are rare but serious, and may be caused by an unusual immune reaction that triggers clotting.

CDC Guidance

Ensuring COVID-19 Vaccine Safety in the US (CDC)

Millions of people in the United States have received COVID-19 vaccines under the most intense safety monitoring in U.S. history.

Five ways that scientists are ensuring the safety of COVID-19 vaccines (GAVI)

As companies race to develop COVID-19 vaccines, some development processes have been run in parallel to stop the pandemic as quickly as possible. Yet safety remains paramount; now that we are on the brink of rolling out some of the vaccines that did well in phase 3 trials, we look into how researchers are ensuring a COVID-19 vaccine is as safe as possible.

COVID vaccines and safety: what the research says (Nature)

There is no question that the current vaccines are effective and safe. The risk of severe reaction to a COVID-19 jab, say researchers, is outweighed by the protection it offers against the deadly coronavirus. Nature looks at what scientists are learning about the frequency and nature of side effects as huge numbers of people report their reactions to physicians and through safety-monitoring systems, such as smartphone apps.