Older, frail residents in skilled nursing facilities saw a 63% drop in risk of COVID-19 infection after at least one dose of the Pfizer vaccine, according to a new report from the Centers for Disease Control and Prevention (CDC).

CDC researchers said the findings revealed a "significant reduction in risk" of contracting infection among this vulnerable group left unaccounted for in clinical trials, therefore raising questions about vaccine efficacy.

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Results stemmed from 463 residents across two skilled nursing facilities in Connecticut. Age and race was said to be similar across the facilities; over 90% of the residents in each facility were white, per available data.

Vaccinations with the jab developed by Pfizer-BioNTech began in the facilities in late December through a federal pharmacy program in long-term care facilities. The state department of health joined the CDC to assess patients' electronic records for vaccinations and infections.

"Partial vaccination, defined as the period from >14 days after the first dose through 7 days after the second dose, had an estimated effectiveness of 63%," CDC researchers wrote, finding the results were similar compared to adults across age groups. For instance, a late-stage clinical trial indicated a 52% vaccine efficacy between the first and second doses. Also, findings from Israel's vaccine rollout revealed the Pfizer vaccine cut documented infections by 46% 14-20 days after an initial dose.

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"To optimize vaccine impact among this population, high coverage with the complete 2-dose series should be recommended for SNF [skilled nursing facility] residents and staff members," the CDC researchers wrote.

The report noted that 24.8% of residents had confirmed infections less than three months before the investigation began. During the study period, 97 infections cropped up, with 89% of cases involving at least one symptom. By the end of the investigations around mid-February, 66% of the residents were fully vaccinated, 16% had received one dose and the rest had yet to receive a vaccine.

"Even during a large disease outbreak in a long-term care setting, the Pfizer-BioNTech vaccine provided protection against SARS-CoV-2 infection, including in older adults aged ≥65 years with a high prevalence of underlying medical conditions," the report continues.

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The study authors noted that separate evidence has suggested immunity from prior infection can combine to boost protection after just a single dose, but the report couldn't assess the impact of prior infection due to high first-dose vaccination rates and a single case of reinfection.

The report had its limitations, including 30% of missing data on ethnicity, and a small sample size which prevented conclusions on outcomes like hospitalization or death.