Cryptic Transmission of the Delta Variant AY.3 Sublineage of SARS-CoV-2 among Fully Vaccinated Patients on an Inpatient Ward

Abstract

Background: Recent reports indicate that vaccination is effective in reducing symptomatic infection with the Delta variant of SARS-CoV-2 (DV) but is less protective against asymptomatic transmission of DV in outpatients than for earlier variants. Here we report cryptic transmission associated with high DV viral load among vaccinated patients on an inpatient medical-surgical ward. Methods: This observational study included all persons diagnosed with breakthrough SARS-CoV-2 infections at the VA Boston Healthcare System (VABHS) from March 11, 2021 to July 31, 2021, including those tested for surveillance, admission, symptoms, and as part of an outbreak investigation in July 2021. SARS-CoV-2 infection was diagnosed by reverse-transcription polymerase chain reaction (PCR) (Cepheid). Variants were identified by MassARRAY SARS-CoV-2 Variant Panel (36-plex PCR, Agena BioScience) for most breakthrough cases after June 2021. Results: An inpatient was diagnosed with asymptomatic DV infection on routine pre-discharge testing. Contact tracing detected infection in 6 of 38 patients (15.8%), 1 of 168 staff (0.6%), and 1 of 6 visitors (16.7%). Infection at the time of diagnosis was asymptomatic in 4 proximate, vaccinated patients, 1 vaccinated visitor, and 1 vaccinated employee caring for 1 undiagnosed, infected, vaccinated patient. Patients were unmasked, whereas staff wore surgical masks. PCR cycle threshold (Ct) for breakthrough infections indicated more than 1000-fold higher viral load for DV (Ct:21.7 +/- 4.3; n=15) than for earlier variants (Ct: 31.8 +/-10.9, n=12; p=.003 (t-test)). Conclusion: These findings demonstrate transmission of DV with high viral load between vaccinated inpatients, the continued efficacy of masking and vaccination for protecting healthcare personnel, and the potential need for post-admission surveillance to prevent cryptic DV transmission.