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VERIFY: Here's why you’ll likely need to get two doses of the COVID-19 vaccine

Drug companies are racing to make a COVID-19 vaccine available. But it may not be one-and-done. Health experts say you'll likely need two shots of the vaccine.

WASHINGTON — The VERIFY team weeds through misinformation on social media to bring you the facts about COVID-19.

Drug companies have been working on a vaccine for months. Drug company Pfizer said its vaccine is about 90% effective, giving a glimmer of hope that it could be rolled out soon.

The company also said the vaccine will be administered in two doses -- but why?

Let's Verify.

QUESTION: Why are some vaccines, like those with COVID-19, administered in two doses?

SOURCES:

ANSWER:

Our experts say it's not uncommon to need two doses of a vaccine. 

"Many childhood vaccines are multi-dose multi-step vaccines, for example, hepatitis B, varicella or chickenpox," Dr. Nabha said. "In fact, there are even adult vaccines such as the shingles vaccine... that's a two dose-response."

RELATED: Pfizer: COVID-19 vaccine is looking 90% effective

Dr. Beyrer explained that the vaccine schedule, which helps determine if those taking it will need doses, takes some time to determine. 

"The vaccine schedule is really determined in the early phase one and phase two studies," Beyrer explained. "And it's based on the immune response -- the immunogenicity."

So what is immunogenicity?

Mostly, it's a measurement of the vaccine’s ability to generate an immune response, explains Dr. Beyrer. That measurement helps determines how many doses are needed. 

“For example, hepatitis B vaccine is a three-dose vaccine, because about 60% of people are protected with one dose, then you get up to about 85 with two, and then you get up to about 96 or 98 with three doses," he said.

Dr. Nabha said multiple doses help boost immunity. 

“You get a little bit of that bump of protection, but you really don't get full protection unless you get that second shot," Nabha continued.

Dr. Dowdy agreed. Immune systems that see the same vaccine again "develop a much stronger response the second time around," he said.

RELATED: VERIFY: No, the flu shot won't make you test positive for COVID-19

All but one of the COVID-19 vaccines in Phase 3 of the current CDC trials require more than one dose. For more information on the trials, and their statuses and doses, the CDC directed our VERIFY researchers to this webpage.

So we can VERIFY: Yes, it is very likely the COVID-19 vaccine will require two doses. And as you heard, our experts say that’s not unusual.

RELATED: Pfizer: COVID-19 vaccine is looking 90% effective