Why is Omicron Causing a Rise in Breakthrough Infections?

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The omicron variant is causing an increase in breakthrough infections among the fully vaccinated and, in some cases, the boosted, but what is it about this variant that is leading to a rise in infections?

"Omicron is really different in the same way that, you know, we've seen a lot more breakthrough infections, we also have seen people who previously had COVID, especially if it was like last year... with the variants before, we've seen all this transmission in multiple studies," Chicago Department of Public Health Commissioner Dr. Allison Arwady said.

People might mistakenly think the COVID-19 vaccines will completely block infection, but the shots are mainly designed to prevent severe illness, says Louis Mansky, a virus researcher at the University of Minnesota.

And the vaccines are still doing their job on that front, particularly for people who've gotten boosters.

Omicron appears to replicate much more efficiently than previous variants. And if infected people have high virus loads, there's a greater likelihood they'll pass it on to others, especially the unvaccinated. Vaccinated people who get the virus are more likely to have mild symptoms, if any, since the shots trigger multiple defenses in your immune system, making it much more difficult for omicron to slip past them all.

Chicago's top doctor explained what exactly happens in the body when an omicron infection begins, which could explain in part what is believed to be behind the increased transmission risk, but also why many of the breakthrough cases are reporting milder symptoms.

It all starts with antibodies, Arwady said.

"Antibodies are your immediate immune response... When you get either COVID infection or you get vaccinated with COVID you are teaching your body how to fight off COVID the next time and you build up what are called antibodies. Those antibodies are the thing that is sort of the first line of defense for your immune system, the fastest part," Arwady said. "And so what we think we are seeing - and again, we're still learning the details of the science here with omicron - is that we know that antibody levels fall over time. We know that that happens after vaccination, we know that that happens after natural infection. One of the reasons why boosters seem to help quite a bit here is that they quickly get your antibody level back up and so the antibody is really trying to protect you from infection. But omicron moves so quickly that we think that there are more cases of people getting infected sort of like in their nose... to a point where they're infected, you can pick up their test. But antibodies are not the whole story of your immune system, thankfully."

She went on to say that behind antibodies are "slower parts of your immune system" known as B cells and T cells.

"They don't stop you from getting infected, but they come along after the antibodies and kill off cells that were infected, stop them from being able to replicate and build and get into your lungs and make you seriously ill," she said. "And so we're still learning about it, but I think this idea why we continue to see the vaccines protect so well against severe infection is because it's not all about the antibodies, but when the antibodies have fallen, people may still get infected."

Arwady noted she is "hypothesizing based on what I know about the science of the immune system and what we're seeing."

Despite many cases remaining mild, there are still concerns that even a mild infection can lead to what are known as "long COVID" symptoms.

"What I'm particularly interested in is what we see after these breakthrough cases where we've got a lot of people getting very mildly ill," Arwady said. "I'm not very concerned just from what I've seen about long COVID for them, but it's been...we have to study it. We've not had this before."

U.S. infectious disease expert Dr. Anthony Fauci told the Center for Strategic and International Studies Commission Tuesday that the omicron variant "will ultimately find just about everybody" While there are some exceptions, he stressed the added pressure any additional hospitalizations will put on hospitals across the country.

Although some cases are appearing with mild illness, Arwady said people should continue trying to avoid getting COVID, if possible.

"Yes, the whole world has COVID right now, but do not think that you should just go and get COVID," she said.

The changing response to COVID variants is largely why Arwady said herd immunity is likely unachievable.

"[COVID] is not behaving like the measles. If you get the measles once, you get your measles vaccines as a child, I'm not really worried about measles in you," Arwady said. "This is, in terms of the way it continues to change and the way it continues to evolve, it is more like the flu. And I'm always careful when I make that comparison, because I know there are people who say, 'Oh, you're just saying COVID is like the flu. We don't care about the flu.' I am not saying that. In vaccinated and boosted adults in children, the way they are being hospitalized is more similar to the flu right now. But that is not true for people who are unvaccinated and unboosted."

Arwady said COVID and flu could be similar in that just like there are yearly flu strains, there could be yearly COVID variants.

"I'm not sure there is a mythical quote herd immunity just because the virus continues to change so much," she said.

Advice for staying safe hasn't changed. Doctors say to wear masks indoors, avoid crowds and get vaccinated and boosted. Even though the shots won’t always keep you from catching the virus, they'll make it much more likely you stay alive and out of the hospital.

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