April 21 (UPI) -- Twenty-two residents and staff members at 15 Chicago-area skilled nursing facilities developed COVID-19 infections more than 14 days after receiving a second dose of the coronavirus vaccine, according to data released Wednesday by the U.S. Centers for Disease Control and Prevention.
These cases were reported among the nearly 15,000 residents and staff members across 75 facilities considered "fully vaccinated," meaning they had received both doses of either the Pfizer-BioNTech or Moderna inoculations, the agency said.
Nearly two-thirds of these cases were asymptomatic, or mild illness with no outward symptoms, the data showed.
"The results in this report highlight the importance of COVID-19 vaccination in high-risk congregate settings such as skilled nursing facilities," the CDC researchers wrote.
"Most fully vaccinated persons were not infected, did not have COVID-19-like symptoms, and did not have severe illness," they said.
Fewer than 6,000 cases of COVID-19 have been reported among the more than 84 million people in the United States who were fully vaccinated as of earlier this week, CDC Director Dr. Rochelle Walensky said Monday.
That figure amounts to 0.007% of the fully vaccinated population, she said.
In clinical trials, both the Pfizer-BioNTech and Moderna vaccines prevented COVID-19 infection in most people, and also offered 95% protection against developing severe COVID-19 in the event a vaccinated person still becomes infected.
This analysis covers the period from late December through the end of March, when 78 skilled nursing facilities that provide 24-hour residential care for the elderly and disabled participated in a COVID-19 vaccination drive.
During that time, nearly 8,000 residents and almost 7,000 staff members at this facilities received both doses of the two-shot vaccines, according to the CDC.
After the start of the vaccination program 627 people -- 353 residents and 274 staff members -- were infected with the coronavirus, the agency said.
Just over 70%, or 447, of these cases occurred in unvaccinated people and 23%, or 145, were reported in those who had received only one dose of the vaccine, the data showed.
In addition to the 22 cases in fully vaccinated residents and staff, 13 occurred in people who received both doses, but were not yet immune because they became infected fewer than 14 days after the second shot, the CDC said.
Of the 22 cases in fully vaccinated people, 12 involved facility residents, and the rest involved staff members, according to the agency.
Although some of the virus samples in these cases were submitted for genetic analysis, the results are not yet available, so it is unclear if these post-vaccination infections involved a new variant of the coronavirus, the agency researchers said.
A separate analysis also released Wednesday revealed that 18 residents and four staff members at a skilled nursing facility in Kentucky who were fully vaccinated were later infected with the R.1 variant of the virus, a strain first identified in the southeastern United States.
Still, these cases of the R.1 variant in fully vaccinated people were nearly 90% less likely to develop symptoms of COVID-19 compared to unvaccinated people, the agency said.
"Early studies suggest that COVID-19 vaccines protect against severe illness," the CDC researchers wrote.
"However, post-vaccination ... infections can occur because COVID-19 vaccines do not offer 100% protection," they said.