Partial vaccination with the Pfizer-BioNTech vaccine was 63% effective against COVID-19 infection in two skilled nursing facilities in Connecticut that were experiencing COVID-19 outbreaks, according to a report the Centers for Disease Control and Prevention released Monday.

"This confirms what we had all hoped: This vaccine is highly effective in perhaps the most at-risk group and can potentially save many lives," said Sunil Parikh, senior author of the study and an associate professor at the Yale School of Public Health, in a news story from Yale. But the study also notes its findings are subject to multiple limitations.

The report doesn't name the facilities, which the Connecticut Department of Public Health identified in January as having outbreaks after each facility's first vaccination clinic.

The Pfizer vaccine is a two-dose regimen meant to be administered 21 days apart. Other CDC guidance considers people "fully vaccinated" two weeks after the second dose, though this study defines "fully vaccinated" as more than seven days after the second dose. In this study, "partial vaccination" is defined as the period between 14 days after the first dose and seven days after the second dose.

By comparison, Pfizer's phase 3 trial — which didn't include nursing home residents — found the vaccine was 52% effective after the first dose.

This study concludes that while partial vaccination "was associated with a significant reduction in the risk for SARS-CoV-2 infection among (skilled nursing facility) residents," the results — coupled with findings from a study in Israel — "suggest that complete 2-dose vaccination is an important strategy for preventing COVID-19 in this disproportionately affected population."

Using electronic medical records, the researchers retrospectively looked at data from Dec. 29 to Feb. 9 for the first facility and Dec. 21 to Feb. 12 for the second, with a total of 463 residents. Dec. 29 and Dec. 21 are the respective dates each facility had its first vaccination clinic.

The study found that excluding residents with past confirmed cases of COVID-19, which was nearly 25% of the residents, didn't substantially change the estimate of 63% effectiveness from the first dose. Considering residents who were previously infected had a first-dose vaccination rate over 90% and only one reinfection was documented, researchers couldn't determine the impact of past infection.

By the last day of the investigation period, or time of discharge, 65.7% of residents had received two doses, 15.6% received one dose, and 18.8% were unvaccinated. Among the 97 COVID-19 cases documented, 65 were among people who were unvaccinated or had gotten the first dose less than two weeks prior, 25 were among people who were partially vaccinated, and seven were among people fully vaccinated.

So, why couldn't the study estimate the effectiveness of being fully vaccinated?

The authors said once residents became fully vaccinated, there weren't enough unvaccinated residents, or cases among them, to serve as a comparison.

Another study limitation was that small sample sizes didn't allow for analysis of effectiveness against symptomatic illness, hospitalization or death. The researchers also noted that ethnicity data was missing for about one-third of residents, and racial minority groups were underrepresented in the investigation "compared with the general population of older adults."

But the report said that nursing home residents, who are "generally older and with more underlying medical conditions than community-dwelling adults," were not included in vaccine trials, and little was known about vaccine effectiveness in this population before this study.

Among the 463 residents, the report said, the median number of high-risk conditions was three.

e.moser@theday.com