Alpha Variant Coronavirus Outbreak in a Nursing Home Despite High Vaccination Coverage: Molecular, Epidemiological, and Immunological Studies

Abstract

Background: Vaccination may control the coronavirus disease 2019 (COVID-19) pandemic, including in nursing homes where many high-risk people live. We conducted extensive outbreak investigations.

Methods: We studied an outbreak at a nursing home in Switzerland, where the uptake of messenger RNA vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was 82% among residents as of 21 January 2021. After diagnosis of COVID-19 in a vaccinated symptomatic healthcare worker (HCW) on 22 February, we performed outbreak investigations in house A (47 residents; 37 HCWs), using SARS-CoV-2-specific polymerase chain reaction testing of nasopharyngeal swab samples. We performed whole-genome sequencing of SARS-CoV-2 and serological analyses.

Results: We identified 17 individuals with positive polymerase chain reaction results, 10 residents (5 vaccinated) and 7 HCWs (3 vaccinated). The median age (interquartile range) was 86 (70-90) years among residents and 49 (29-59) years among HCWs. Of the 5 vaccinated residents, 3 had mild disease and 2 had no symptoms, whereas all 5 unvaccinated residents had mild to severe disease, and 2 died. Vaccine effectiveness for the prevention of infection among residents was 73.0% (95% confidence interval, 24.7%-90.1%). The 12 available genomes were all alpha variants. Neutralizing titers were significantly higher in vaccinated individuals on reexposure (>1 week after diagnosis) than in vaccinated, unexposed HCWs (P = .01). Transmission networks indicated 4 likely or possible transmissions from vaccinated to other individuals and 12 transmission events from unvaccinated individuals.

Conclusions: COVID-19 outbreaks can occur in nursing homes, including transmission from vaccinated persons to others. Outbreaks might occur silently, underlining the need for continued testing and basic infection control measures in these high-risk settings.

Keywords: B.1.1.7; COVID-19; Outbreak; UK variant; Vaccine; nursing home.

Figure 1.

Epidemic curves based on dates of the first positive severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) polymerase chain reaction (PCR) test result (A) and dates of symptom onset (or first positive PCR result in asymptomatic persons) (B). Numbers inside squares represent numbers assigned to the 17 infected persons (residents and healthcare workers (HCWs).

Figure 2.

Serological profiling of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–specific humoral responses in the infected persons at the nursing home, from multiplex SARS-CoV-2 ABCORA assessment of unvaccinated (light and dark red circles) and vaccinated (gray and black circles) residents and healthcare workers (HCWs). Light red and gray circles represent the first sampling time point (T1; 1–18 days after diagnosis); dark red and black circles, the second time point (T2; at 16–30 days). Signal-over-cutoff (SOC) values are depicted; all values >1 are positive. Abbreviations: Ig, immunoglobulin; N, nucleocapsid protein; RBD, receptor-binding domain; S1, spike 1; S2, spike 2 (RBD, S1, and S2 are subunits of the SARS-CoV-2 spike protein).

Figure 3.

Neutralization titers assessed in unvaccinated and vaccinated infected individuals during outbreak, as well as uninfected health care workers (HCWs). Values represent 50% neutralization titers (NT50) against Wuhan-Hu-1 pseudotype after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in unvaccinated (light and dark red circles) and vaccinated (grey and black circles) health care workers (open circles) and residents (closed circles at T1 [1–18 days] and T2 [16–30 days] after positive polymerase chain reaction (PCR) results. Black line depicts the median. Vaccinated health care workers (HCW) not involved in the outbreak are shown in light blue.

Figure 4.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) phylogeny of the infected persons in the outbreak compared with selected alpha (B.1.1.7) strains from Switzerland and neighboring countries. A, Phylogeny of B.1.1.7 genomes; including European genomes; vertical blue line represents the outbreak. B, Sequences of the persons involved in the outbreak (person numbers in the hexagons), with 4 subclusters identified, subclusters a–d.

Figure 5.

Transmission network based on symptom onset, test date, epidemiological data, and observations of the nursing home management and nurses. The colors of the shaded ovals identify genomic subclusters a (blue), b (green), c (red), and d (yellow); absence of shading indicates missing data, and black outlines around ovals indicate symptomatic cases (any symptoms). Abbreviation: HCW, healthcare worker.