Asymptomatic and Symptomatic COVID-19 Infections Among Health Care Personnel Before and After Vaccination

This cohort study investigates asymptomatic and symptomatic COVID-19 case rates before and after the initial vaccine rollout among health care personnel in Orange County, California.


Introduction
By mid-April 2021, there were 133 million COVID-19 infections and 2.9 million associated deaths worldwide. 1 Several vaccines offer hope to end the pandemic. COVID-19 mRNA vaccines provide 95% protection from symptomatic disease to date, but concern exists over asymptomatic infection and transmission risks in vaccinated individuals. Health care personnel (HCP) may be routinely tested, enabling assessment of vaccine impact on asymptomatic COVID-19 infection. We evaluated COVID-19 rates before and after HCP vaccination in Orange County, California, the sixth largest US county and one of the hardest hit by COVID-19 during the winter surge from 2020 to 2021. 1,2

Methods
This cohort study of HCP was conducted at University of California Irvine (UCI) Health, the sole academic medical center in Orange County, California, between November 1, 2020, and March 31, 2021 (21 weeks). We evaluated HCP COVID-19 cases, both symptomatic and asymptomatic, before and after initiating COVID-19 mRNA vaccination (Pfizer, Moderna) on December 16, 2020. Daily screening for CDC-defined COVID-19 symptoms (fever, fatigue, chills, myalgia, congestion, cough, loss of smell, shortness of breath, sore throat, nausea, diarrhea) and temperature were required for all HCP, with rapid nasopharyngeal testing if positive. Additionally, invitations for asymptomatic bilateral nares testing were provided weekly to randomly selected HCP (approximately 1000 [10%] per week until the week of December 13, 2020, and approximately 2500 [25%] weekly thereafter).
All samples were processed using polymerase chain reaction (PCR) testing for SARS-CoV-2 at UCI Health's Clinical Laboratory Improvement Amendments-certified laboratory. Depending on availability, COVID-19 reverse transcription-PCR testing was performed using Molecular Simplexa (DiaSorin), m2000 RealTime (Abbott), or Xpert Xpress SARS-CoV-2 (Cepheid). Demographic data (age, race, ethnicity) were obtained from occupational health and human resources records to assess for any differences in vaccine distribution.
Every HCP with COVID-19 was interviewed for symptoms assessment regardless of testing pathway (eg, symptomatic or asymptomatic). Rolling 7-day averages (means) of daily COVID-19 cases were calculated for UCI Health and countywide cases reported by public health. 2 COVID-19 cases identified through asymptomatic testing were stratified by presence or absence of symptoms identified on interview.
This study followed the Strengthening the Reporting of Observational Studies in Epidemiology
Approximately 20% of invited HCP participated in weekly asymptomatic testing (approximately 250 HCP/week) when a random 10% were invited, and approximately 15% (approximately 500 HCP/week) participated when 25% were invited. Although symptomatic HCP were not to be tested using the asymptomatic pathway, most HCP with COVID-19 who were identified by asymptomatic testing (81% [34 of 42] within the 21-week study period) reported having