In vaccinology, a breakthrough infection means a person develops an infection from a pathogen after vaccination and may indicate the pathogen has become resistant to the vaccine. Almost no vaccine is 100% effective in preventing infections, but breakthrough infections are tracked in clinical trials to determine how well inoculation is working in a population.
Deeper dive
With COVID-19, the CDC defines a vaccine breakthrough case as someone who tests positive for COVID-19 (with or without symptoms) 14 days or more after being up-to-date with their vaccines. Up-to-date means depending upon age and health status, the individual has received one dose of Johnson & Johnson vaccine or two doses of the Pfizer/BioNTech, Moderna or Novavax vaccine (called the primary series), and a booster shot in addition to the primary series, if recommended.
The CDC initially tracked all breakthrough infections of COVID-19 to determine if the vaccines were working, but on May 1, 2021 transitioned to tracking only breakthrough infections among people with COVID-19 who were hospitalized or died.
Data from the end of May 2022 indicate that breakthrough infections are common the longer the period after receiving a COVID-19 vaccine, however severe illness is less common.
Adults with three doses of Pfizer/BioNTech, Moderna vaccines had a 62.8% lower chance of developing a breakthrough infection two to four months after the third dose. Data as of the end of March 2022, indicated that three doses of the Pfizer/BioNTech, Moderna vaccines reduced the risk of hospitalization by 86%. As these numbers will continue to evolve, see up-to-date data on the latest on vaccine effectiveness on this CDC site.