Why is a second dose of the COVID-19 vaccine necessary?

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The Food and Drug Administration (FDA) will decide whether to grant approval this week for a COVID-19 vaccine, but one big thing: You’ll need to get two doses in order for the vaccine to work.

With every day seeming to break new records for hospitalizations and deaths, COVID-19 is now the No. 1 killer in the U.S, according to the Institute for Health Metrics and Evaluation.

“These are really very, very disturbing numbers. That’s not the time to throw our hands up and say, well, you know, we’re helpless. We’re not.” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

Fauci says help is just around the corner.

“We’re going to start distributing vaccine doses to health care providers and people in nursing homes and other facilities literally within a week or two. It will be in the middle or end of December and then as we get into January, there will be more doses for more people.”

While highly effective, both vaccines – from Pfizer and Moderna – up for emergency authorization require two doses given weeks apart.

“The only way that we have been able to demonstrate that these are effective is if two shots are taken,” said Dr. Marissa Levine.

Levine, director of the Center for Leadership in Public Health Practice at USF, says that’s because of how our bodies react to foreign substances, like vaccines.

“When we first see something that’s foreign to our body, we kind of prime our system, if you will. The second time we see it is when we actually get that boost of immunity.”

But if you skip – or forget – to get your second dose, are you worse off than if you hadn’t gotten vaccinated at all?

“The worst thing is you might think you’re protected when you’re not, and you might let your guard down,” which means you could let the virus in.

Levine says one reason people sometimes skip out on their second dose is that they have a reaction after getting the first shot, but she says that’s proof your body is reacting to the vaccine, which is a good thing.

Experts hope that lockdowns and closures will motivate people to take the vaccine when it’s ready.

“The very careful rigorous review by the FDA will lead us to believe that these vaccines are both safe, certainly in the short term, and highly efficacious, and that we’ll be moving forward with an Emergency Use Authorization,” said Dr. William Moss, executive director of the International Vaccine Access Center.

Dr. Kartik Cherabuddi with UF Health says that one thing health care leaders can do is offer transportation to and from vaccination sites. They can also offer clear information about side effects so that people are comfortable coming back for their second dose.

“If you go too long, we might have to start all over again and you’re now going to get dose number two and three. And we don’t have as many doses of the vaccine, so you’re going to use up someone else’s dose,” Cherabuddi said.

Something else to consider: Health experts say we need to get people vaccinated as soon as possible so we can knock the virus out before it gets a chance to mutate. If it doesn’t happen fast enough, we may end up dealing with a different coronavirus that needs a different vaccine.


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