A single unvaccinated health care worker at a Kentucky nursing home set off a 46-case outbreak of Covid-19, including 22 cases—mostly asymptomatic—among vaccinated residents and employees, according to a new study released by CDC on Wednesday.

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Vaccines provided protection, study finds

According to the study, the outbreak happened at a nursing home that cared for 83 residents and had 116 staff members. The outbreak involved 26 residents, including 18 who had been vaccinated, and 20 health care personnel, including four who had been vaccinated. Residents and employees had been vaccinated with the Pfizer/BioNTech vaccine at clinics held on January 10, January 31, and February 21, with most receiving their doses on January 10 and January 31.

Most of those who had been vaccinated did not experience symptoms or have to go to the hospital for care, CDC said. However, three people passed away, including two unvaccinated residents and one vaccinated resident who had been previously infected with Covid-19 some 300 days day prior.

According to the researchers, the outbreak involved the R.1 variant of the coronavirus, which is not currently listed as one of CDC's variants of concern. However, the variant contained several important mutations, including D614G, which appears to increase transmissibility; E484K, which has been identified in the B.1.351 and P.1 variants first identified in South Africa and Brazil, respectively; and W152L, which may curb the effect of neutralizing antibodies.

Based on the rate of infection and symptomatic cases, the researchers said the vaccine appeared to be 66% effective among residents and 75.9% effective among employees against infection. CDC acknowledged those rates are lower than reported from Israel's national vaccination program, potentially because of the particular variant, the small sample size, or the higher risk of exposure in a congregate setting. That said, the agency noted that vaccinated residents and employees were still 87% less likely than those who were not vaccinated to have symptomatic Covid-19.

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According to CDC, the study was published concurrently with separate research that found of the 627 Covid-like infections across 75 nursing homes in the Chicago area, just 22 occurred among vaccinated people—and no facility-related secondary transmission occurred. In addition, nearly 66% of the infections were among vaccinated people were asymptomatic, although two residents were hospitalized and one passed away.

Key takeaways

According to CDC, the findings of the Kentucky study demonstrate the importance of vaccinating both residents and employees at nursing home facilities. At the time the outbreak was identified, 90.4% of the nursing home's residents had been vaccinated, but only about half of the facility's employees had been.

Citing resistance among nursing staff to get vaccinated—and how a low vaccination rate among staff increases the odds of Covid-19 outbreaks—the authors wrote, "To protect skilled nursing facility residents, it is imperative that health care providers, as well as skilled nursing facility residents, be vaccinated."

Similarly, the authors of the Chicago study said their findings underscore the importance of "maintaining high vaccination coverage among residents and staff members" to "reduce opportunities for transmission within facilities and exposure among persons who might not have achieved protective immunity after vaccination." The findings also demonstrate the need, the researchers said, to continue adhering to recommended infection control practices—such as isolation, quarantine, the routine use of personal protective equipment, and regular testing—regardless of vaccination status (Rabin, New York Times, 4/21; Weixel, The Hill, 4/21).