As COVID-19 cases increase along with spread of the more transmissible Delta variant, the effectiveness of COVID-19 vaccines continues to be an important topic. A new KFF analysis looks at COVID-19 vaccine “breakthrough cases,” when fully vaccinated individuals become infected, as well as hospitalizations and deaths, to see which states are providing data on breakthrough events, how regularly, and what those data reveal.

After a review of the websites of all states and D.C. and other official sources, the new analysis found that half of states (25) report some data on COVID-19 breakthrough events. Within that, 15 states report these data on a weekly basis and one state reports on a daily basis, while the other nine report more infrequently. Overall, the data found that breakthrough events are extremely rare among those who are fully vaccinated, and that the vast majority of reported COVID-19 cases, hospitalizations, and deaths in U.S. are among those who are unvaccinated or not fully vaccinated. Some key highlights include:

  • The rate of breakthrough cases reported among those fully vaccinated is well below 1% in all reporting states, with Connecticut the lowest (0.01%) and Oklahoma the highest (0.9%). Hospitalization rates among those unvaccinated ranged from effectively zero (0.00%) in California, Delaware, D.C., Indiana, New Jersey, New Mexico, Vermont, and Virginia to 0.06% in Arkansas. Death rates were even lower at 0.00% for all but two states, Arkansas and Michigan, where they were 0.01%.
  • More than 9 in 10 COVID-19 cases, hospitalizations, and deaths occurred among people who are unvaccinated or not yet fully vaccinated. The share of COVID-19 cases among those not fully vaccinated ranged from 92.02% in Oklahoma to 99.85% in Connecticut.

This analysis highlights how rare breakthrough events are but also how data remain limited, and only available for a subset of states.



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