SARS-CoV-2 infection after vaccination among healthcare workers in California

For the editor:

Data from phase 3 clinical trials of messenger RNA (mRNA) vaccines through November 2020 showed 94.1% efficacy in preventing symptomatic infection with severe acute respiratory syndrome (SARS) coronavirus 2 -CoV-2) 14 days after second dose of mRNA-1273 vaccine (Moderna)¹ and 95% efficacy at 7 days after the second dose of the BNT162b2 vaccine (Pfizer).² Since the publication of the results of these trials, a national increase in coronavirus disease 2019 (Covid-19) has been noted, variants of SARS-CoV-2 with increased infectivity have appeared, the Food and Drug Administration has granted authorization of emergency use for these two mRNA vaccines, and vaccination has been initiated across the United States.

Since the start of the vaccination campaign, the development of Covid-19 has been reported in people who received one or both doses of the vaccine.³ The University of California, San Diego (UCSD) and University of California, Los Angeles (UCLA) health systems began vaccinating healthcare workers on December 16, 2020. On December 2, in addition to To set a low threshold for testing symptomatic people, UCSD required that asymptomatic healthcare workers undergo weekly testing by polymerase chain reaction (PCR) testing of nasal swabs. On December 26, UCLA instituted an optional testing program for asymptomatic healthcare workers with a PCR test of nasal swabs. This program allowed for increased detection of asymptomatic SARS-CoV-2 infections after vaccination.

Aggregated data was obtained in an anonymized format from an employee electronic health record system at UCSD and UCLA.⁴ Exemption from institutional review board approval has been obtained.

Table 1. Table 1. New SARS-CoV-2 infections among vaccinated healthcare workers from December 16, 2020 to February 9, 2021.

From December 16, 2020 to February 9, 2021, a total of 36,659 healthcare workers received the first dose of vaccine, and 28,184 of these people (77%) received the second dose. Among vaccinated healthcare workers, 379 unique people tested positive for SARS-CoV-2 at least 1 day after vaccination, and the majority (71%) of these people tested positive within the first 2 weeks after vaccination. the first dose. After receiving both vaccines, 37 health workers tested positive; of these workers, 22 had positive test results 1 to 7 days after the second dose. Only 8 health workers tested positive 8–14 days after the second vaccination, and 7 tested positive 15 or more days after the second vaccination (Table 1). As of February 9, a total of 5,455 healthcare workers at UCSD and 9,535 at UCLA had received the second dose 2 or more weeks previously; these results correspond to a positivity rate of 0.05%.

In our cohort, the absolute risk of testing positive for SARS-CoV-2 after vaccination was 1.19% among health care workers at UCSD and 0.97% among those at UCLA; these rates are higher than the risks reported in trials of the mRNA-1273 vaccine¹ and the BNT162b2 vaccine.² Possible explanations for this elevated risk include the availability of regular testing for asymptomatic and symptomatic individuals at our facilities, a regional increase in infections in Southern California during our vaccination campaigns,⁵ and differences in demographic characteristics between trial participants and healthcare workers in our cohort. Healthcare workers were younger and had a higher overall risk of SARS-CoV-2 exposure than clinical trial participants. Additionally, reporting deadlines in the first two vaccine trials were well before this surge, no testing of asymptomatic people was included in the BNT162b2 vaccine trial,² and only one screening of asymptomatic people was done in the mRNA-1273 vaccine trial before the second dose was given.¹

The paucity of positive test results 14 days after administration of the second dose of vaccine is encouraging and suggests that the efficacy of these vaccines is maintained outside of the trial setting. These data underscore the critical importance of continuing public health mitigation measures (masking, physical distancing, daily symptom screening and regular testing), even in high vaccination incidence settings, until the herd immunity is achieved as a whole.

Jocelyne Keehner, MD
Lucy E. Horton, MD, MPH
UC San Diego Health, San Diego, CA

Michael A. Pfeffer, MD
University of California, Los Angeles (UCLA) David Geffen School of Medicine, Los Angeles, CA

Christopher A. Longhurst, MD
Robert T. Schooley, MD
UC San Diego Health, San Diego, CA

Judith S. Currier, MD
David Geffen School of Medicine at UCLA, Los Angeles, California

Shira R. Abeles, MD
Francesca J. Torriani, MD
UC San Diego Health, San Diego, CA
[email protected]

This letter was published on March 23, 2021 on NEJM.org.