One chart shows how well vaccines and boosters protect against severe Omicron compared to Delta

• Booster shots appear to be at least 90% effective against hospitalization for Delta and Omicron.
• But Omicron poses a greater challenge to vaccine protection, according to three new CDC studies.

COVID-19 booster shots were effective at preventing hospitalization and death before Omicron started spreading in the US. But protection from a third dose is essential now that Omicron cases are predominate, according to three new studies from the Centers for Disease Control and Prevention.

The first CDC study looked at severe cases of COVID-19 among adults in 10 states from August to mid-December 2021, when the Delta variant was dominant, and from mid-December 2021 to January 2022, when Omicron started to take over. During that time, the CDC's Vision Network recorded around 88,000 hospitalizations for COVID-19.

Adults who received an mRNA booster saw the highest protection during both the Delta and Omicron periods: Boosters reduced the risk of hospitalization by at least 90%. But two mRNA doses were far less effective at preventing severe Omicron cases than severe Delta ones, the study found.

The chart below shows how well vaccines protected against hospitalization during the Omicron and Delta periods.

During the Delta period, the effectiveness of a second dose waned slightly after about 26 weeks — from 90% to 81%. A third dose brought that protection back up to 94%.

But protection significantly declined during the Omicron period: A second dose was 81% effective against hospitalization after 2-26 weeks, but just 57% effective after 26 weeks. Booster shots increased that protection to 90%.

Separately, the study also examined nearly 223,000 COVID-19-related visits to emergency departments or urgent care clinics from August 2021 to January 2022. Overall, booster shots reduced the risk of someone winding up in these settings, but protection was higher before mid-December.

During the Delta period, booster shots reduced the risk of a COVID-19-related visit to the emergency department or urgent care clinic by 94% two weeks after they were administered. During the Omicron period, boosters reduced the risk of those visits by 82%.

The findings nevertheless "underscore the importance of receiving a third dose" to prevent moderate and severe COVID-19, the researchers wrote.

Boosters protect against Omicron infections and symptoms, two more studies found

New research also suggests that boosters protect against Omicron infections and symptoms.

A second CDC study found that vaccinated people who received an mRNA booster had more protection against a coronavirus infection in December 2021, when Omicron was spreading in the US, than people who had received two doses or none at all.

However, Omicron's arrival increased the odds that a vaccinated or boosted person would get infected relative to when Delta was dominant.

In October and November, unvaccinated people had around 14 times the risk of getting infected compared with people who had received booster doses. By the time Omicron emerged in December, unvaccinated people had just five times the risk of infection relative to people who had been boosted.

Like other vaccine doses, boosters didn't offer the same protection for every age group: The shots were more effective at preventing infections and deaths among people ages 50 and up than younger adults, the researchers found.

A third CDC study, released Friday in the Journal of the American Medical Association, also found that three mRNA shots were better than two doses at protecting against symptomatic Omicron — though boosters did a better job of warding off Delta symptoms.