When COVID-19 hit a nursing home in France weeks after most residents got vaccinated, 23% of residents became infected. While there were no deaths among the vaccinated, the extent of the outbreak indicates that more than a vaccine is needed to protect the frail elderly from SARS-CoV-2 illness, investigators contend.

The outbreak occurred early in 2021 starting with an infected visitor. Eight residents developed severe disease, two were hospitalized and one unvaccinated resident died. Twelve of the facility’s 102 staff members — 30% of whom were vaccinated — also became infected, although none with serious symptoms.

All vaccinated residents (97%) and staff in the 77-bed facility received the Pfizer-BioNTech vaccine, and all cases were found to be caused by the alpha, or B117 gene variant, the researchers reported. The mean age of facility residents was 88 years.

The large number of breakthrough infections among facility residents is likely associated with the immune function impairment of aging and complications of comorbidities common to long-term care residents, including diabetes, malnutrition and cancer, reported Joël Belmin, M.D., Ph.D., of the Sorbonne University in Paris. Related studies have found a reduced antibody response in this population to several COVID-19 vaccines, including the Pfizer vaccine, he added.

“This cohort study’s findings suggest that an outbreak of COVID-19 can occur among fully vaccinated NH residents,” Belmin and colleagues wrote.

Vaccination may not be sufficient by itself to protect adults in long-term care against COVID-19, the authors concluded.

“[O]ther prevention measures should not be abandoned yet in these settings,” they wrote. “More research is needed to improve the effectiveness of SARS-CoV-2 vaccines in this population.”

Full findings were published Monday in JAMA Network Open.