While the CDC has stopped reporting breakthrough infections that do not end up in hospitalization, the Wisconsin Department of Health Services has recently started to publish the breakdown of new COVID-19 infections among vaccinated and unvaccinated people. While the numbers are just a snapshot of one state and one month, in this case July, they do give an impression of how infections, hospitalization and death rates differ between the vaccinated and the unvaccinated.
In July 2021, around 125 breakthrough infections happened per 100,000 fully vaccinated Wisconsinites, compared with around 369 cases per 100,000 inhabitants of the state who had not been fully vaccinated. At a vaccination rate of around 50 percent in the state in July, this means that around 3 in 4 new cases occurred in unvaccinated people. That would have made the COVID-19 vaccines in the state 66 percent effective in preventing infection in real world conditions opposite unvaccinated people, as for every three unvaccinated Wisconsinites who were infected with COVID-19, two vaccinated people were spared an infection, assuming that both groups had the same exposure to the virus on average.
When it comes to hospitalizations and deaths, the differences in outcomes for vaccinated and unvaccinated people were even starker. Around four in five hospitalizations for COVID-19 occurred in unvaccinated people in Wisconsin, translating to a vaccine effectiveness of 73 percent in preventing hospitalizations. For preventing death, vaccines proved 91 percent effective, as only one in twelve Wisconsinites who succumbed to COVID-19 was vaccinated.
The efficacy of COVID-19 vaccines varies from study to study, as these are not carried out in a lab but in the real world where conditions vary. Efficacy also differs depending on the population that the vaccine is given to, in addition to the fact that the effectiveness of COVID-19 vaccines is expected to decline with time and when up against a high virus load coronavirus mutation like the Delta variant. A rundown of different efficacy studies, most of them carried out shortly after full vaccination, can be found on the CDC website.