“Breakthrough” hospitalizations involving COVID-19 among people who are fully vaccinated against the disease most often affected older adults and people with other chronic health conditions, finds a new analysis of hospital data from June through September by KFF and Epic Research.

More than two-thirds (69%) of breakthrough COVID-19 hospitalizations occurred among people ages 65 and older, who are more likely than younger age groups to have gotten vaccinated. A fifth (21%) of breakthrough hospitalizations occurred among people ages 50-64, while 10% occurred among younger adults.

COVID-19 hospitalizations among people who were not fully vaccinated skew much younger, with about 3 in 10 (30%) involving patients ages 50-64 and 4 in 10 (41%) involving patients under age 50.

The analysis examines data from June to September from Epic’s Cosmos research platform, which includes data for more than 120,000 hospitalizations with a COVID diagnosis during the four-month period. Patients are considered “fully vaccinated” if they received a dose of Johnson & Johnson vaccine or two doses of the Pfizer or Moderna vaccine at least two weeks before they were hospitalized, regardless of whether they were eligible for or received a booster shot.

Compared to those who are unvaccinated, a small share (15%) of hospital admissions for COVID-19 during the four-month period involve people who were fully vaccinated against the disease.

Other findings include:

• Larger shares of fully vaccinated adults hospitalized with breakthrough COVID-19 had selected chronic conditions including, hypertension, diabetes, heart failure, or chronic obstructive pulmonary disease) compared to those hospitalized with COVID-19 who were not fully vaccinated.
• Fully vaccinated people who are hospitalized with breakthrough COVID-19 are less likely than those who are not fully vaccinated to have COVID-related complications such as viral pneumonia or respiratory failure, or to receive a ventilator or dexamethasone treatment. This suggests that fully vaccinated patients with COVID-19 diagnoses may be somewhat more likely to be in the hospital primarily for reasons other than COVID-19.
• Fully vaccinated people with breakthrough infections had shorter hospital stays compared to others in their age group who were not fully vaccinated. For example, among those at least 65 years old with COVID-19, the median stay was 5.6 days for those who were fully vaccinated compared to 6.7 days for those who were unvaccinated or partly vaccinated.

The analysis is available on the Peterson-KFF Health System Tracker, an online information hub dedicated to monitoring and assessing the performance of the U.S. health system