Scientists now know that the potency of the booster shot wanes quickly after about three months. We look at the latest research on a fourth shot.

Transcript

AYESHA RASCOE, HOST:

Scientists now know that the potency of the COVID booster shot wanes quickly. After about three months, antibody levels begin to decline, and people become vulnerable to an infection. Many Americans received a booster shot more than three months ago. So with omicron still circulating, does that mean it's time for the fourth shot?

Here to explain what the science says is NPR's global health correspondent Michaeleen Doucleff. Hi, Michaeleen.

MICHAELEEN DOUCLEFF, BYLINE: Hi, Ayesha.

RASCOE: So I understand that several countries are already starting to roll out fourth doses, including Sweden and Chile. Do people here in the U.S. need a fourth dose?

DOUCLEFF: Yeah. So right now, the data suggests that for the general population, people don't need a fourth dose. And here's why. Last week, a study came out of Israel looking specifically at what a fourth dose does. How much does it help? It's a preliminary study, and it's pretty small. It included only about 700 people. But it found that quite clearly that a fourth shot of either Pfizer or Moderna doesn't add much protection against infection beyond the third dose. Specifically, it increased protection by only about 10- to 30%, and that protection will likely wane over time.

RASCOE: So a 10- to 30% increase - why would the fourth shot not boost protection like the third shot did? Why isn't it the more the better?

DOUCLEFF: Yeah. I was talking to Jenna Guthmiller about this. She's an immunologist at the University of Chicago. She said, you know, this vaccine really isn't designed to stop omicron, stop omicron infections. It's designed to stop the original variants of the virus that circulated two years ago and are very different than omicron.

JENNA GUTHMILLER: We know that this virus is way more likely to cause an infection just because it's more infectious, right? And so what used to work for something like, you know, the alpha variant and even to a certain degree with the delta variant is perhaps not the same thing that's going to be necessary for omicron.

DOUCLEFF: In other words, no matter how many shots you throw at it - four, five, six - it's just not going to stop infections.

RASCOE: OK. Well, for people who may be looking for a little hope, the fourth dose isn't going to stop infections, but what about preventing severe disease and hospitalizations? Like, would a fourth shot at least help with that?

DOUCLEFF: Yeah. So first off, three doses for healthy people under age 65 or so are still doing a good job of preventing what you said, severe disease and death. But a recent study from the Centers for Disease Control and Prevention shows that this protection may wane just a little bit over time. We're talking about a decline after four months from about 90% to 80%.

RASCOE: OK, so a drop in protection of about 10% - that doesn't sound like a huge amount, but for some people, could that be a problem?

DOUCLEFF: Yeah, you know, that right there is really the key. So for people who are at high risk of severe disease to start off with, losing even a small fraction of protection could be a problem. These are people who are over age 65 or who have health problems like heart disease or obesity, which put them at higher risk, or people with compromised immune systems.

Akiko Iwasaki is an immunologist at Yale University. She says these people will likely benefit from an additional dose.

AKIKO IWASAKI: Their immune response is not as robust as a healthy young person, and then that is probably the first target group that would require such a booster.

DOUCLEFF: But she says when that booster will be needed isn't it known right now and depends on just how quickly this immunity is waning in this group. Hopefully we'll have data on that in the next month or so.

RASCOE: OK, so I hear you, but some people may be wondering, is there any harm in getting a fourth shot?

DOUCLEFF: You know, that's a really good question, and probably not for the fourth shot. But some immunologists say there could be harm in getting many shots in a short period of time. This is all speculative, but in the lab, sometimes researchers do see negative effects with other vaccines. Parts of the immune system can kind of start slowing down a bit. And one immunologist told me that you could actually make your immune response worse with too many shots.

RASCOE: Well, that's NPR's Michaeleen Doucleff. Thank you.

DOUCLEFF: Thank you. Transcript provided by NPR, Copyright NPR.