Vaccines are pretty weird if you really think about it. At a base level, they're a way to trick your body into building up defenses against threats that haven't necessarily penetrated your biology yet. And that comes with a bit of unpredictability since human biology tends to vary.

So when vaccines like the currently FDA-authorized shots such as Pfizer/BioNTech's, Moderna's, and Johnson & Johnson's for COVID-19 are developed in about a year rather than the upwards of 10 years many vaccines take to develop, it isn't surprising that you may run into some common, and not-so-common, side effects. And it takes time to learn exactly what those are and who it affects.

Just what side effects may you experience? Here's what we know about everything from the strange to the mundane adverse events.

What are the most common COVID vaccine side effects?

It's important to note that, whatever side effects you may or may not feel from a given COVID vaccine, it's still significantly preferable to severe illness, hospitalization, death, or passing the coronavirus on to more vulnerable people.

There are instances wherein certain people may have severe reactions to the COVID vaccines from Pfizer/BioNTech, Moderna, and Johnson & Johnson. They all appear to present similar side effects and higher risks for a very rare group of people in general, according to available data.

For instance, the vaccines can lead to dangerous conditions such as anaphylaxis in a small portion of the population. That's also why after a vaccine is administered, your vaccination site will require you to stick around for at least 15 minutes to see if you suffer from severe side effects (and up to a half hour if you have a history of anaphylaxis).

"Anaphylaxis after COVID-19 vaccination is rare and occurred in approximately two to five people per million vaccinated in the United States," the Centers for Disease Control (CDC) notes.

The good news is, for the vast majority of people, side effects appear to be relatively mild and last anywhere from hours to a couple of days. "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," writes the agency.

That doesn't mean that they're pleasant. According to the CDC, the most common side effects from the currently authorized COVID vaccines are: arm pain at the injection site or throughout the arm where the shot was administered; swelling on the administration arm; redness; fatigue; headache and dizziness.

But then things can get strange for other parts of your body. For instance, for several hours or several days after receiving your first or second dose of the Pfizer or Moderna shots, or the single dose of the Johnson & Johnson vaccine, you may experience some fatigue, feel loopy, have a headache, experience muscle aches throughout various parts of your body, have chills, fever, or nausea. In essence, effects which resemble flu-like symptoms but tend to subside pretty quickly. For some, the reactions to a second dose of the Pfizer or Moderna vaccine may last longer and be more severe.

The CDC recommends that you use over-the-counter pain killers and standard medication to deal with these lower-grade symptoms or pain, but only after receiving your shot. Taking Tylenol or Advil or other pain killers before the administration of a dose may hamper the immune response the vaccine is made to elicit. And a wet, cool towel at the injection site can help, too.

What are the more rare COVID vaccine side effects?

These common side effects aren't really all that different from many other vaccines, though they may seem a bit more random. Redness, muscle aches, swelling, nausea, fever, and fatigue are all pretty normal reactions to even a traditional flu vaccine. Your body is building up an arsenal of defenses against an enemy it hasn't encountered. That internal manufacturing work can lead to inflammation or other strange reactions from your body as it adjusts.

But COVID vaccines can also come with some much stranger, though rarer, adverse events. For instance, there is evidence that after a second dose of the Pfizer or Moderna vaccines, they had a lymph node reaction which led to lumps developing in the collarbone or neck area. This may occur within a few days of vaccine administration, but also tends to subside within a few days with no lasting harm.

The Department of Health and Human Services (HHS) has a central registry mandated by federal law to keep track of all vaccine side effects called the Vaccine Adverse Event Reporting System, which is how we know about some of these rarer reactions. According to the system, many who suffered from more severe side effects such as anaphylaxis had a history of allergies.

There's also a strange gender dynamic here, preliminary research suggests: Women seem to bear the brunt of the rare-but-more-intense adverse events.

A CDC report from January 2021, the early days of the COVID immunization campaign, found that 78.7% of reported side effects (including more severe ones) were reported by women in the first month of the shots being doled out. The vast majority still only had mild symptoms that subsided within days, but it follows a trend of women, particularly younger women, having worse reactions to vaccines than men do.

The CDC and public health experts around the globe still have a clear message, though. Whatever side effects you might experience, it's still worth it to get a vaccine when you can.