A study released April 22 in The Lancet Respiratory Medicine shows that a booster dose of the Pfizer COVID-19 vaccine provides strong protection, roughly 80% to 90%, in the first few months against hospital admissions and emergency department visits caused by the delta and omicron variants. However, against omicron, this protection wanes over time—even after a third dose.

For this study, the researchers analyzed 11,123 hospital admissions and emergency department visits that did not result in hospital admission for acute respiratory infection. The study focused on Kaiser Permanente patient records in Southern California from December 1, 2021, through February 6, 2022, when both the delta and omicron variants were circulating.

• After 2 doses of the Pfizer COVID-19 vaccine effectiveness against omicron was 41% against hospital admission and 31% against emergency department visits at 9 months.
• After 3 doses, effectiveness against omicron-related hospitalization was 85% at less than 3 months but fell to 55% at 3 months or longer.
• Against emergency department visits that did not result in hospitalization, vaccine effectiveness of 3 doses against omicron was 77% at less than 3 months but fell to 53% at 3 months or longer.

“Although the Pfizer COVID-19 protection levels against omicron after 3 doses are substantially higher than those seen after 2 doses, they are less than those observed for delta or other COVID-19 strains,” Tartof said. “Additional doses of current, adapted, or novel COVID-19 vaccines may be needed to maintain high levels of protection against subsequent waves of COVID-19 caused by omicron or future variants with similar potential to escape protection.”