Why Pfizer, Moderna Boosters Protect Against Omicron but Not 1st 2 Shots

This story was updated on Dec. 21, 2021, at 2:14 p.m. ET.

The omicron variant of the coronavirus has become the dominant strain the U.S., making up 73% of sequenced cases from the week ending Dec. 18, according to new data from the CDC. The previous week, omicron made up less than 13% of sequenced cases in the U.S. More omicron breakthrough cases are likely because omicron more easily evades the protection provided by vaccines than previous variants of the coronavirus.

The virus is spreading rapidly, likely because it's more infectious than previous versions of the coronavirus, but the severity of the illness it causes is still unknown. Preliminary research indicates it may actually cause less severe disease, but it's a bit early to tell.

To prevent the spread of omicron, you should mask up and get the first two doses of the COVID-19 vaccine and get your booster, experts say.

Which booster is best for omicron?

To protect against omicron, it matters less whether you get the Pfizer or Moderna vaccine booster, and more that you've gotten one at all, according to Chicago Department of Public Health Commissioner Dr. Allison Arwady.

That's because research has shown that both the Pfizer and Moderna vaccine boosters are effective against omicron, especially compared to the first two shots. Earlier this month, Pfizer said preliminary data show its booster protects against omicron at a comparable level to what the first two doses provide against previous variants and the original strain of the coronavirus. Moderna said on Monday that its booster appears to provide protection against omicron, increasing neutralizing antibody levels against omicron by 37 times compared to levels pre-booster. A double dose of the booster provided even greater protection.

That said, the first two doses of the vaccines will likely still protect against severe disease from omicron, even if they don't prevent infection altogether, NBC News reported.

If you received J&J as your first vaccine dose, the best booster for you to get is an mRNA vaccine, the CDC announced last week. Both Moderna and Pfizer are mRNA vaccines and authorized in the U.S.

Why the Pfizer and Moderna boosters are effective against omicron

Dr. Timothy Brewer, an infectious disease physician and professor of epidemiology at University of California, Los Angeles, joined LX News to explain the science behind why boosters offer better protection than the first two shots.

"As RNA viruses replicate, they can have mistakes. We call those mutations or deletions, and those changes to the virus can change whether or not the immune system recognizes the virus," Brewer said. "That's why you have to get an influenza vaccine every year: Because as the influenza virus spreads, those changes mean that the virus is slightly different than the virus before, and last year's vaccine may not work. So it's possible there are enough changes in the omicron variant that now the immune system that's been stimulated by the vaccines aren't recognizing it quite as well as it did the earlier versions of the SARS-CoV-2 virus."

But why does that mean the booster can effectively combat the omicron variant, while the first two shots of the vaccine may not? The third dose of the vaccine doesn't use a different formula, in case you were wondering.

"As you stimulate the immune system, it learns to recognize a wider range of variants," Brewer said. "We've seen that both with natural infection and with vaccination. So as you stimulate the immune system more and more, it essentially gives you a broader response than it did after the first dose or the second dose, for example."

Vaccines and boosters: How to protect against omicron

Early data indicate that omicron is more contagious than the delta variant but causes milder illness. Still, that's not a reason to let your guard down, especially ahead of the holiday season, according to Brewer. That's because the more people who become infected with omicron, the more opportunities there are for the virus to mutate, potentially leading to another variant that could be more severe or more easily evade vaccines.

"That's why it's so important to get vaccines all around the world because we have to stop transmission and replication everywhere, not just in the United States," Brewer said, adding that we should take the same precautions with omicron as we have with previous variants.

"Remember, not everybody responds the same to this disease. If you're older or you're overweight or you have [underlying] conditions or you're immunosuppressed from a transplant or treatment ... you may be at higher risk for serious disease," he explained. "Even if the majority of people have mild or moderate disease, that doesn't mean that everybody is going to have mild or moderate disease, and we don't want to risk them getting infected and getting sick."

So what can you do? Get vaccinated and get your booster, if you're eligible. In the U.S., that applies to everyone 18 and older who got a second Pfizer or Moderna shot more than six months ago or who got a Johnson & Johnson shot more than two months ago. The CDC has expanded its booster recommendations to include 16 and 17-year-olds, who are currently only able to receive the Pfizer shot.