What you need to know about the second booster

Last week, the Food and Drug Administration and the Centers for Disease Control and Prevention authorized a fourth dose of the COVID-19 vaccine for people 50 years and older, as well as some immunocompromised people.

Yahoo News spoke to Dr. Katelyn Jetelina, an epidemiologist at the University of Texas Health Science Center, to break down what Americans need to know about the second booster.

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JOE BIDEN: The Food and Drug Administration-- the FDA-- and the Center for Disease Control and Prevention-- the CDC-- authorized a fourth shot. Wonderful.

KATELYN JETELINA: The fourth dose has already been rolled out in Israel for quite a few months now, and it's already been rolled out again in Germany as well as the UK. So we're already starting to see the evidence roll through. And what we're seeing is that a fourth dose is beneficial to some degree, especially for older adults.

The other thing that's happening right now is that the BA.2 footprint is starting to take hold in the United States. We saw a big wave of BA.2 in Europe, and the US typically mirrors what happens in Europe.

And so we assume that we are going to see case increases in the United States. We don't know how big that wave is going to be, but there will be increased transmission in a number of areas. | also the timing of wanting to get a fourth booster before too many people have waning antibodies and there's still benefit to protecting against that new wave that may or may not come.

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There's really two debates going on right now with the booster. Are vaccines meant to protect against infection, or are they meant to protect against severe disease and hospitalization?

And what we're seeing with the fourth dose is that there's benefit to both of those. For example, there was one study that looked at Israelis 60 years and older, and found that a fourth dose reduced the rate of infection by 2 times and reduce the rate of severe disease by 4 times, making it 4 times lower. So a fourth dose has a meaningful impact on older adults.

I haven't seen the evidence to give that to younger people, like under 50 years old in the United States. That may change with variants of concern coming. That may change with the fall season in a couple of months, but today, I would be surprised if it was approved.

A third dose is incredibly important against omicron, and that's because there's really some serious immune invasion with omicron, as well as waning protection over time. And so people need to get that booster as soon as possible for ultimate protection.

But if you had the three doses of your vaccine, and you had a confirmed omicron case, meaning you had an antigen test that was positive or a PCR test that was positive, there's actually really no need or very little need to get a fourth dose. And that's because of this thing called hybrid immunity.

Many studies have shown very fantastic protection, and that's because the vaccine immunity is directly towards the spike protein, whereas infection-induced immunity is directed towards the virus as a whole. So this doesn't mean that you should purposely go get COVID, but it is a viable path to protection that I think that we need to start recognizing.