COVID-19 Vaccine Effectiveness Wanes Rapidly Against Omicron Variant

COVID-19 vaccines were less protective against symptomatic infection from the Omicron COVID-19 variant than the Delta variant.


Primary vaccination with the AstraZeneca, Pfizer-BioNTech or Moderna COVID-19 vaccines provided less protection against symptomatic infection with the Omicron variant compared with the Delta variant, a new study found.

The study, published in the New England Journal of Medicine, included 886,774 who tested positive for the Omicron variant, 204,154 infected with the Delta variant and 1,572,621 who were confirmed to be negative for infection between Nov. 27 and Jan 12.

“After two doses, vaccine effectiveness waned rapidly, with very limited vaccine effects seen from 20 weeks after the second dose of any vaccine,” the authors, led by Nick Andrews, PhD, and Julia Stowe, PhD, of the UK Health Security Agency, wrote. “Booster doses resulted in a substantial increase in protection against mild infection; however, waning of protection against symptomatic disease was also seen after booster doses.”

Against the Delta variant, vaccine effectiveness against symptomatic disease was 82.8% 2 to 4 weeks after 2 doses of AstraZeneca vaccine and waned to 43.5% after 25 weeks or more. Effectiveness was higher for mRNA vaccines: 90.9% for the Pfizer vaccine and 94.5% for Moderna at 2 to 4 weeks, falling to 62.7% and 80.4% respectively.

Against the Omicron variant, vaccine effectiveness against symptomatic disease was 48.9% 2 to 4 weeks after two doses of AstraZeneca vaccine, which had no effect after 25 weeks or more. For mRNA vaccines, effectiveness was 65.5% for the Pfizer vaccine and 75.1% for Moderna at two to four weeks, falling to 8.8% and 14.9% respectively.

Booster doses increased effectiveness for all vaccines against the Omicron variant. For those who received the AstraZeneca vaccine as their primary course, a Pfizer-BioNTech booster increased effectiveness to 62.4% at two to four weeks before decreasing to 39.6% at 10 weeks or more. A Moderna booster increased effectiveness to 70.1%, falling to 60.9% at five to nine weeks.

Those who received the Pfizer-BioNTech vaccine as their primary course saw vaccine effectiveness after a Pfizer-BioNTech booster increase to 67.2% at two to four weeks before falling to 45.7% at 10 or more weeks. A Moderna booster increased effectiveness to 73.9%, falling to 64.4% at five to nine weeks.

Those who received the Moderna vaccine as their primary course saw vaccine effectiveness increase to 64.9% after a Pfizer-BioNTech booster and 66.3% after a Moderna booster at two to four weeks.

The study didn’t assess the ability of vaccines to protect against severe outcomes of the disease, but the authors noted that studies of previous variants have shown that vaccine effectiveness against severe disease, hospitalization and death has been higher and lasted longer than protection against mild disease.

“Our findings support maximizing coverage with third doses of vaccine in highly vaccinated populations such as in the United Kingdom,” the authors wrote. “Further follow-up will be needed to assess protection against severe disease and the duration of protection after booster vaccination.”