Vaccination greatly reduces the risk for severe COVID-19, but fully vaccinated people who get breakthrough cases serious enough to send them to the hospital are notably older and less healthy than unvaccinated COVID-19 patients, according to a report released Wednesday by the Peterson-KFF Health System Tracker.

The report analyzed data on 120,000 hospitalizations from June through September.

It found that the majority of breakthrough hospitalizations were in people older than 64, while most hospitalized, unvaccinated patients were younger than that. In addition, more of the people with a breakthrough case had another serious health problem like high blood pressure or obesity and they were less likely to have classic COVID-19 respiratory complications or require steroid or ventilator treatment. That could mean that the primary reason for their hospital stay was something other than COVID-19.

“If this is the case, it would mean that the gap in risk of COVID-19 hospitalization between vaccinated and unvaccinated people is even greater than previously known,” the researchers wrote.

Other key findings:

  • While 56% of the population was fully vaccinated in September, only 15% of hospitalized COVID-19 patients were.

  • Thirty-one percent of fully vaccinated people in the hospital were 18 to 64, compared with 66% of unvaccinated people.

  • Fully vaccinated people of all ages had shorter hospital stays.