By Andrea Kane

As the United States inches closer to authorizing a Covid-19 vaccine many people may now let themselves start wondering what it will feel like to get it.

Is it going to be like the flu vaccine? Will it be more painful? And what about side-effects?

The two front-runners for getting an emergency use authorization from the US Food and Drug Administration -- Pfizer/BioNTech and Moderna -- use new mRNA technology. No US-licensed vaccine has ever used it, although researchers have been studying it for decades, against infections like flu, rabies and Zika, and even for some types of cancer.

The way these mRNA vaccines work is that they give our body the instructions, in the form of messenger RNA, for making a little piece of this particular coronavirus (SARS-CoV-2) -- specifically the spike protein. When our body gets these instructions, it starts producing the spike protein. That in turn triggers our immune system, which recognizes the spike protein as "foreign," to make antibodies against it. So when we get infected with the real virus, our body is already prepared to fight it.

These vaccines require two doses: one to prime the body, and then a few weeks later, a second shot to boost the response. Study results show that Pfizer/BioNTech and Moderna's vaccines are each in the neighborhood of 95% effective.

But because the technology is so new for a vaccine, it has raised many questions and some concerns among those for whom it's intended.

One participant in the Moderna trial said getting vaccinated was "definitely not a walk in the park" but he would certainly do it over again.

Yasir Batalvi, a 24-year-old recent college graduate living in the Boston area, said he originally signed up to join a trial on the NIH website back in early July because he felt moved to do something to help during the pandemic.

"I felt so helpless. I mean, this pandemic really has affected everybody's lives so significantly. And it's not just lives, you know, it's livelihoods," Batalvi told Dr. Sanjay Gupta. "And so I signed up because I kind of just wanted to do what I could. And I didn't think I was necessarily going to get picked. But I got a call, eventually, in September. And then by mid-October, I was enrolled."

He was a bit nervous rolling up his sleeve, especially when he was given a 22-page consent form to sign. But he said he felt like he was doing a public service.

"I think just because coronavirus has been such a significant disruption to our lives, that I decided that it was what I had to do. It felt like civic duty," he said. "Because I think mass scale vaccination is really the only realistic way out of the pandemic that we're in."

So what did it feel like?

"The actual injection felt, at first, just like a flu shot, which is basically just a little pinch in the side of your arm," Batalvi said. "Once I left the hospital, that evening, the stiffness got a little bit worse. It was definitely manageable, but you kind of don't really feel like moving your arm too far above your shoulder. But the side effects are pretty localized. I mean, it's just in the muscle in your arm. And that's about it. It doesn't really affect anything else and you feel fine."

That was after the first dose. But the second dose was different.

"I actually had some pretty significant symptoms after I got the second dose. Once I got the second dose, I was fine while I was in the hospital. But that evening was rough. I mean, I developed a low-grade fever, and fatigue and chills," Batalvi said. He said he was out for that day and evening, but he "felt ready to go by the next morning."

He said he called the study doctors to let them know about his symptoms. They weren't alarmed and told him he shouldn't be either.

Feeling under the weather does not mean that you got Covid-19 from the vaccine -- in fact, experts say having this kind of reaction shows that your body is responding the way it should, and it should not deter anyone from getting vaccinated or going back for their second dose.

"That means your immune response is working for you. You should feel good about that," said vaccine expert Dr. Paul Offit of the Children's Hospital of Philadelphia. "And [there] shouldn't really be any difficulty coming back for that second shot, knowing that you're now in a much better position to fight off this awful virus, which has killed more than 250,000 people and can cause a lot of long term effects."

Infectious disease expert Dr. Anthony Fauci told Facebook CEO Mark Zuckerberg the same thing on Monday.

"What the body is telling you by that response is that it's responding well to the injection," he said.

"When you get an injection of the vaccine, you induce a response. In some people, they don't feel anything," he said. "Others feel an ache in the arm. Some may feel an ache in the arm and kind of a little chilly feeling, almost like you have a flu-like syndrome, and in a minority of people, they get a fever."

Fauci, the director of the National Institute of Allergy and Infectious Diseases, said "almost all of this goes away within 24 or at the most, 48 hours" and added that it's important to be honest with people about the side effects they may experience.

Operation Warp Speed Chief Scientific Adviser Moncef Slaoui said Monday that approximately 10 to 15% of immunized study subjects will develop "quite noticeable side effects."

"Most people will have much less noticeable side effects. That frankly -- in comparison to a 95% protection against an infection that can be deadly or significantly debilitating -- I think is an appropriate balance," he said.

Side effect like the ones Batalvi experienced should not be confused with safety issues. Any vaccine maker looking for FDA approval or authorization has to show two months' worth of safety data after the second dose is given -- because that's when most serious safety issues have occurred in earlier trials. On that front, it's so far, so good for both the Moderna and Pfizer vaccines. But only time will tell if any serious safety events show up in the next couple of years.

"While we know that the predictable 90, 95% of side effects that happened within two months after immunization are actually really good for the two vaccines that have been filed now, the Moderna and Pfizer vaccines, we don't have the experience for a year or two years, and we're going to learn as we go," Slaoui said.

Batalvi doesn't know for sure if he got the active vaccine or a placebo. But based on his experience, he said he can take a good guess.

"You know it's a double blinded, randomized study. So neither I nor the study doctors nor Moderna knows whether I've gotten the vaccine or not. But I'm confident based on the side effects that that I got the real deal," he said.

Batalvi said he looks forward to this pandemic ending so that his family can meet his sister's new twins -- a boy and a girl -- born earlier in the week.

"I hope once this vaccine comes out, people feel confident taking it. I mean, I'm right here: I took the vaccine -- it was all right. I think we can get through this," he said.