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Everything you need to know about how COVID vaccines work and their side effects

It's what scientists are calling the "light at the end of the tunnel," but is a vaccine for COVID-19 really safe?

SACRAMENTO COUNTY, Calif. — Pharmaceutical company AstraZeneca said Monday that late-stage trials showed its coronavirus vaccine was 90% effective. The news gives public health officials hope they may soon have access to a third vaccine - as two other drug makers, Pfizer and Moderna, have reported results showing their vaccines are almost 95% effective.

Scientists said that Pfizer may possibly begin distribution by the end of 2020. 

Pfizer and Moderna have the "same recipe," UC Davis Virologist Sam Díaz-Muñoz said. But this recipe is a first of its kind.

"It's the light at the end of the tunnel," said Díaz-Muñoz. "From my perspective, it's an absolute scientific triumph."

"This is absolutely revolutionary," said UC Davis Chair of Medicine, Dr. Tim Albertson. "We don't have vaccines that are built on this model. This is absolutely a unique model."

While it's a new model for a vaccine, the science that's been put behind it was actually well underway before COVID-19 even existed. 

COVID-19 is a type of coronavirus, other coronaviruses that have threatened us before include SARS-1 and MERS, but they didn't get to the state of a pandemic, like COVID-19. But scientists began studying these types of coronaviruses years ago, helping establish the research needed to a speedy vaccine today.

RELATED: First COVID immunizations could arrive in the U.S. on Dec. 12

"No one knew about them because they studied this obscure virus called coronavirus, and they just continued working on it," said Díaz-Muñoz. "This was close enough that when the information on SARS COVID 2 came up, they were ready."

Pfizer and Moderna are the two closest vaccines to FDA approval.

Dr. Albertson is currently the principal investigator for the Pfizer vaccine trial for UC Davis, monitoring around 230 people in that trial. So far, as of Monday he said only four have tested positive for coronavirus - and also noted that includes around the half of the trial participants that were given a placebo.

So, what exactly does the Pfizer and Moderna vaccine do once they're in your body? 

First of all, it's different than the flu shot. The vaccine doesn't contain the virus like a flu shot does, but instead has a type of code that can help your body recognize and fight off this virus.

"What they contain is MRNA, [so that] can't reproduce [COVID-19]. MRNA is a messenger, RNA, it produces a protein and that protein is what you're developing the antibodies to and that's what's protecting you from the virus," said Albertson.

"That's going to activate and unleash our immune system and putting that snippet of code into our cells and then saying, 'Hey cells! Manufacture this!'" said Díaz-Muñoz. "So, it's making our body manufacture this part and then our body reacts immunologically to it."

Both of the vaccines require two shots, around three to four weeks apart. But what should we expect when we get the vaccine? It's something Díaz-Muñoz is quite familiar with.

"I’m going to come out to you here, I’m a participant in the trial, in the Moderna trial. So I know from experience myself that the symptoms can be bad," Díaz-Muñoz said. "It's completely natural because [your body is] working overtime to make immune response."

Díaz-Muñoz said some people should be prepared for side effects of COVID-19 like fevers, chills and fatigue, but that's exactly what scientists expected.

"I just want people to be prepared and not freak out that we rushed into this and this is not known," said Díaz-Muñoz.

While we may have familiarity with initial side effects, a lingering concern of the vaccine is also long-term effects. Díaz-Muñoz said since it's a new vaccine, it's not 100% clear yet, but in all likelihood - it's far better than contracting COVID-19.

"Based on the science of this, we really don't anticipate side effects going forward long term, and I think what people have to think about is what do we know for certain? We know for certain that COVID kills. We know that," said Díaz-Muñoz. "So it's a balance between being protected from that with something that all of the data we have shows it's safe versus taking your chances."

For those anxious or concerned about a vaccine, Díaz-Muñoz recommends reaching out to scientists like him -- or those who've taken the vaccine.

But as we get closer and closer to that vaccine, scientists are still incredibly worried about the spread of COVID-19 during the holidays. 

"I don't think there's any question there's a light at the end of the tunnel," said Albertson. "The only question is can we get to the end of the tunnel to see the light?"

"It's going to be a dark winter. This is going to be the worst part of the pandemic," said Díaz-Muñoz. "Protect yourself, protect your loved ones so we can get to the end of the tunnel and see the light... and then we're going to have a massive party."

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