In a study published in JAMA Network Open today, scientists report on how effective the original vaccine and booster shots are against the COVID-19 Delta and Omicron variants.
Researchers in Ontario analyzed data from more than 134,000 people, including those who tested positive for Delta and Omicron infections during December 2021. They found that people who were fully vaccinated (with two doses of an mRNA vaccine, from either Moderna or Pfizer-BioNTech) experienced a decline in vaccine effectiveness against both Delta and Omicron infections, but the drop was greater against Omicron than against Delta. Among the vaccinated, the shots’ effectiveness declined from 36% up to two months after the second dose of the primary series, to 1% up to four months later (or six months after the second dose).