But arguably, the most important part is here: in the human trials.

Where the vaccine is tested on real people, in three main phases, starting with just a small group of people, and ultimately testing a group of thousands.

This is where scientists confirm the effectiveness and safety of the vaccine on a large scale. Before that, scientists scrutinize its effectiveness.

But before any of that, a vaccine first has to pass this crucial phase, where scientists study the side effects.

In a trial you call it "adverse events." But that doesn't.... that's kind of overstating it.

Dr. Kirsten Lyke led a Phase I trial for the Covid-19 vaccine made by Pfizer and BioNTech.

She says side effects are a possibility with any vaccine.

In general, it's nothing to fear. It's our own immune systems kicking in and doing what it's supposed to do.

The most common flu vaccine this year, for example, comes with a chance for many normal side effects: pain, fatigue, headaches.

But many of the new Covid-19 vaccines are more likely to cause these kinds of side effects than you might be used to, particularly after the second dose.

Most people will feel a little pain in their arm. Many will be tired and get headaches.

I mean, obviously, having no side effects is desirable.

But I think with the spike protein of the coronavirus, it really does elicit some side effects.

But that's totally normal.

And once you understand why vaccine side effects happen, you might actually be happy to get them.

First, we have to talk about this: your immune system, which is a huge network of different cells and proteins in your body.

You've got things like white blood cells that fight the invading virus or bacteria, communication cells that organize the response, and antibodies that search for and identify the enemy.

When, say, a virus attacks your body, your immune system attacks back.

Your body increases blood flow to get more of these battle cells in circulation.

Your temperature might go up, too, as one of the tactics your body has to help kill the invaders.

And after your white blood cells destroy the virus, they produce antibodies that will identify the virus should it reappear in the future, and remember how to fight it.

They're how you gain immunity.

This response is actually what gives you a lot of the symptoms you feel when you catch, for example, the common cold.

But the cold virus doesn't give you a fever, or a runny nose, or body aches — your immune system does while fighting the virus.

And triggering this system, without actually getting you sick, is how vaccines work.

Most vaccines are made up of a weakened or dead pathogen, or a portion of one.

Or, in some of the new Covid-19 vaccines, the genetic code of a portion of one, either in the form of DNA, or what's called messenger RNA, along with minor ingredients to keep it stable.

It's harmless, but when your immune system detects it, it responds just like it's a real danger.

It attacks the intruder, and creates those memory antibodies to be able to fight it again in the future.

Vaccines are designed to give you the same immunity as if you would fought off the real virus, and some of the new Covid-19 ones do this particularly well.

The messenger RNA vaccines are quite good at stimulating your immune system. That's why you have ninety-five percent efficacy.

That's right — a 95% chance of being protected against Covid-19. That makes them some of the most effective vaccines.

But that also means they're really good at activating your immune system.

Which means your body will increase blood flow to where that vaccine is, which is why pain at the injection site is so common.

Your body might even think, "Better turn up the heat!", and then you get a fever, or the chills.

So experts emphasize that we should look at most side effects as a good thing: It means the vaccine is working.

I have a friend, actually, who participated in one of the Pfizer trials, the early trials, and, you know, so he could have gotten either placebo or vaccine.

After the second dose, the next day he woke up and he had... he was sort of fatigued and he had low grade fever. And he looked at his wife and said, "Yes!! I got the vaccine!!"

See, that's the right attitude.

When we talk about these common reactions to vaccines, like fever and fatigue, we're mostly talking about the mild-to-moderate ones.

These are the lowest of the side effect categories that health regulators use, the kind you get over in a day or two.

Then there's severe side effects — the type that might make you call in sick to work or call a doctor.

These were rare in the clinical trials for the first two studied vaccines to become available, with the exception of some severe fatigue and muscle pain on the second dose.

And that's because if these vaccines were dangerous, they'd never reach the public in the first place.

We really don't accept any sort of permanent, serious harm from a vaccine, nor should we.

News stories that imply otherwise can be scary, but they get more attention than they probably deserve.

Like this one, about a serious allergic reaction... in someone with a history of serious allergic reactions.

Or this one, about someone dying after getting a second dose of the vaccine... when, it turned out, other factors had caused his death.

In fact, no deaths have been reported from the millions of doses that have been given out.

And the controlled studies with thousands of people found the same thing: No deaths from the vaccine.

But maybe more importantly, the studies also recorded zero deaths from Covid-19, and zero hospitalizations from Covid-19.

These vaccines aren't just safe — they're life savers.

You should be skeptical of anything you put in your body, including vaccines.

But once you've seen the data, and you see that there wasn't a serious side effect before approval, and hasn't been a serious side effect post-approval, then I think you should be convinced.

Basically, you want to reduce where the virus can go.

And if you immunize as many people as possible, that pool of people that it can transmit to becomes less and less and less and less.

Vaccines are the way out of the Covid-19 pandemic.

And, like with all vaccines, many of us who get it will also feel a little... meh... for a day or two.

But the scientists who have studied these vaccines — who have seen the side effects — are some of the most eager to get it.

I couldn't wait to get this vaccine, are you kidding me? I mean, two doses of vaccine gives me a 95% chance of being protected.

We would be the first ones to not want to take it if we felt it was unsafe, right?

So, um, take it from your friendly vaccine person!