Jan. 10, 2022 -- Some Americans with a weakened immune system who face high risks for severe COVID-19 become eligible this week to receive a fourth dose of a coronavirus vaccine.

The CDC endorsed a third dose of the Pfizer or Moderna vaccines for moderately and severely immunocompromised people on Aug. 13, which is considered part of their first immunization series rather than a booster shot.

In October, the CDC said moderately and severely immunocompromised people could receive a booster shot, or a fourth dose of the vaccine, 6 months after their third dose.

How Do COVID-19 mRNA Vaccines Work?Some of the COVID-19 vaccines are known as mRNA shots. How are they different from traditional vaccines? And do they contain the real virus?189


SPEAKER: How does a COVID-19

mRNA vaccine work?

COVID vaccines are now


Some of the COVID-19 vaccines

are mRNA vaccines, but what does

this mean?

mRNA vaccines are

different from traditional


mRNA vaccines don't expose you

to any real virus instead,

they're made with messenger

Ribonucleic Acid or mRNA.

This is a type of molecule that

gives instructions to the cell

for how to make different kinds

of proteins.

mRNA molecules are

a natural part of our cells

and how our bodies work.

Researchers have been working

with mRNA vaccines

for many years.

They are made more easily

and safely in a lab

than a vaccine that uses

a virus.

Because of this they can also

be made faster.

The COVID-19 mRNA vaccines

have passed many tests in labs

and in thousands of people,

and meet strict standards

from the FDA.

So how do these vaccines work?

First, a COVID-19 mRNA vaccine

is injected into a muscle

in your upper arm.

Some muscle cells take the mRNA

instructions in the vaccine

and make a harmless piece

of a protein called

a spike protein.

This protein is found

on the outside of the SARS-CoV-2

virus that causes COVID-19.

The muscle cells then destroy

the instructions for how to make

the spike protein.

The mRNA never goes

into the nucleus of your cells

where your DNA is stored.

The newly made spike protein now

sits on the surface

of the muscle cells.

Your immune system senses

the spike protein

as a foreign threat to destroy,

it starts making antibodies

to fight anything

with that spike protein on it.

This will help your body's

immune system recognize

and fight the real virus if it

ever shows up.

It's like recognizing someone

by the hat they wear.

Your body is then

prepared to spot COVID-19

and fight it off before it grows

in your body's cells.

Fast facts to remember

about COVID-19 mRNA vaccines.

They help get your body

ready to fight off the COVID-19

virus before it makes you sick,

they don't use

any live, dead, or weak virus,

they can't give you COVID-19,

they don't affect your DNA.

Want to learn more,

go to cdc.gov to find more

information about mRNA vaccines.

You can also learn more about

how the vaccines were approved

at fda.gov.



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But the CDC last week shortened the timeline to 5 months for a booster shot of the Pfizer or Moderna vaccines. That means immunocompromised people could begin signing up for a fourth shot later this week, The New York Times reported.

About 2.7% of U.S. adults, or about 7 million adults, are considered immunocompromised, according to the CDC. They’re more likely to contract severe COVID-19, have a higher risk for long COVID, have lower antibody levels after vaccination, and develop serious breakthrough infections. About 40% of hospitalized breakthrough cases are among immunocompromised people.

According to CDC guidance, people are considered to be “moderately or severely immunocompromised” if they have:

  • Active cancer treatment for tumors or cancers of the blood
  • Had an organ transplant and are taking medicine to suppress the immune system
  • Had a stem cell transplant in the last 2 years and are taking medicine to suppress the immune system
  • Advanced or untreated HIV infection
  • Moderate or severe primary immunodeficiency, such as DiGeorge syndrome or Wiskott-Aldrich syndrome
  • Active treatment with high-dose corticosteroids or other drugs that suppress the immune response

So far, only moderately and severely immunocompromised Americans are eligible for a fourth shot. Israel has begun offering fourth doses to high-risk groups, including older adults, but the Biden administration hasn’t yet said whether the U.S. will follow, the Times reported.

Overall, the focus remains on getting third shots to Americans who are eligible for boosters, Rochelle Walensky, MD, the CDC director, told reporters on Friday. U.S. officials will remain in touch with Israel to follow their data on fourth shots.

“We will be following our own data carefully as well, to see how these boosters are working in terms of waning effectiveness, not just for infection but, importantly, for severe disease,” she said.

Show Sources

The New York Times: “Starting later this week, some at-risk Americans become eligible for a 4th shot.”

CDC: “ACIP Evidence to Recommendations for Use of an Additional COVID-19 Vaccine Dose in Immunocompromised People.”

CDC: “COVID-19 Vaccines for Moderately or Severely Immunocompromised People.”

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