Protection from prior infection significantly reduced against Omicron

The big picture: Initial studies have found that coronavirus vaccines — particularly without a booster shot — are much less effective against Omicron infections than other variants, although they appear to hold up well against severe disease.

• This study suggests the same is true for people with prior infections, emphasizing the benefit of vaccines for this population.

By the numbers: The study found that prior infections were 90% effective against symptomatic reinfections for the Alpha variant, 92% effective for the Delta variant and only 56% effective for Omicron.

• Effectiveness against hospitalization and death was 69% for Alpha, 100% for Delta and 88% for Omicron.

The bottom line: Omicron's remarkable ability to evade immune protection — either from vaccines or infection — goes a long way toward explaining its explosive case growth.