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 percent 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 Centers for Disease Control and Prevention 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 at least 1 dose of Johnson & Johnson vaccine or 2 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 that were hospitalized or died.
Data from the end of May 2022 indicate that breakthrough infections are common the longer the time after receiving a COVID-19 vaccine, however severe illness is less common.
Adults with 3 doses of Pfizer/BioNTech, Moderna vaccines had a 62.8 percent 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 3 doses of the Pfizer/BioNTech, Moderna vaccines reduced the risk of hospitalization by 86 percent. As these numbers will continue to evolve, see up-to-date data on the latest on vaccine effectiveness, on this CDC site.