key: cord-0912090-7q1i9jcy
authors: Novazzi, Federica; Taborelli, Stefano; Baj, Andreina; Focosi, Daniele; Maggi, Fabrizio
title: Asymptomatic SARS-CoV-2 Vaccine Breakthrough Infections in Health Care Workers Identified Through Routine Universal Surveillance Testing
date: 2021-10-19
journal: Ann Intern Med
DOI: 10.7326/m21-3486
sha: 7ce194a22d2951dc02aa0b79ce84537c21d65403
doc_id: 912090
cord_uid: 7q1i9jcy

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PCR Cts for SARS-CoV-2 cases among HCWs and community COVID-19 cases during the study period.

Positive Circles indicate asymptomatic infection, and squares indicate symptomatic infection. The color indicates Alpha variant (green), Delta variant (blue), or undefined strain (black). Arrows indicate attribution based on spike gene sequencing. Ct = cycle threshold; HCW = health care worker; PCR = polymerase chain reaction. A. All asymptomatic and symptomatic cases. B. Cases among HCWs screened every 2 wk working in wards deemed to have high exposure risk (hematology, pediatric oncohematology, general oncology, gynecologic oncology, dialysis and nephrology, general and transplant surgery, infectious diseases, and neonatal intensive care unit). C. Cases among HCWs screened every 4 wk working in wards deemed to have moderate exposure risk (radiotherapy, thoracic surgery, heart surgery, rheumatology, pneumology, COVID hub 1, COVID hub 2, and emergency department).D. Weekly positive SARS-CoV-2 PCR incidence rate in the area of Insubria, Italy (where HCWs resided), during the study period.

This article was published at Annals.org on 19 October 2021.

Annals of Internal Medicine © 2021 American College of Physicians 1

LETTERS was twice as high in the asymptomatic than the symptomatic group.

Discussion: Our results indicate that systematic surveillance of asymptomatic vaccinated HCWs uncovers many times more cases of VBT infection than symptom-based testing. The incidence of asymptomatic VBT infections seemed to depend on the frequency of testing and not occupational risk or community prevalence; once corrected for frequency of testing, incidence was similar in the highand moderate-risk groups. Asymptomatic cases seemed to clear much more quickly and had higher mean antibody levels than symptomatic cases. Because of the rapid viral clearance in asymptomatic VBT infections, our surveillance program likely missed many other such infections.

Benenson and colleagues (3) reported that 689 of 6680 HCWs (10.3%) had VBT infections across 7 weeks of study, but this estimate included HCWs who had received a single dose of BNT162b2. Keehner and colleagues (4) reported 7 cases out of 5455 HCWs at the University of California, San Diego, and 9535s HCWs at the University of California, Los Angeles (0.05%), who were fully vaccinated with BNT162b2 or mRNA-1273. Another study reported 33 cases (16 mildly symptomatic and 17 asymptomatic) among 3694 HCWs fully vaccinated with BNT162b2, without details on time to viral clearance (5)-consistent with our estimates of symptomatic cases. All of these studies were done at times when Alpha was the dominant SARS-COV-2 variant, whereas our study was mostly run during the Delta predominance.

Study limitations include the inability to perform genetic sequencing for more accurate VOC imputation in patients with cycle thresholds above 25 and the lack of viral neutralization assays to assess neutralizing antibody levels. We likely missed a large proportion of asymptomatic VBT infections cleared during intervals between NPSs.

Our findings suggest that regular testing of asymptomatic vaccinated HCWs who tested positive for SARS-CoV-2 may preclude the need for long quarantines, but this requires confirmation in larger studies. Surveillance programs aimed at capturing all VBT infections would need to include frequent testing. In the absence of such frequent surveillance, maintaining vigilant use of personal protective equipment seems to be warranted. 

Delta variant reveals multiple vaccine breakthrough infections. med. Rxiv. 2021

Vaccinated and unvaccinated individuals have similar viral loads in communities with a high prevalence of the SARS-CoV-2 delta variant. medRxiv. Preprint posted online 31

BNT162b2 mRNA Covid-19 vaccine effectiveness among health care workers

SARS-CoV-2 infection after vaccination in health care workers in California

SARS-CoV-2 vaccine breakthrough infections are asymptomatic or mildly symptomatic and are infrequently transmitted. medRxiv. Preprint posted online 3