SARS-CoV-2 infection after vaccination in healthcare professionals in California

For the publisher:

Data from phase 3 clinical trials of messenger RNA (mRNA) vaccines until November 2020 showed 94.1% efficacy for the prevention of symptomatic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) 14 days after the second dose of mRNA- Vaccine 1273 (Modern)¹ and 95% efficacy at 7 days after the second dose of the BNT162b2 vaccine (Pfizer).^two Since the results of these tests were published, a national increase in coronavirus disease 2019 (Covid-19) has been observed, variants of SARS-CoV-2 with increased infectivity have emerged, the Food and Drug Administration has granted emergency use authorization for these two mRNA vaccines, and vaccination has started in 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 health systems at the University of California, San Diego (UCSD) and the University of California, Los Angeles (UCLA) started vaccinating health professionals on December 16, 2020. On December 2, in addition to setting a low threshold for testing of symptomatic people, UCSD determined that asymptomatic health professionals should undergo weekly tests for polymerase chain reaction (PCR) of nasal swabs. On December 26, UCLA instituted an optional testing program for asymptomatic healthcare professionals with a PCR test on nasal swabs. This program allowed an increase in the detection of asymptomatic SARS-CoV-2 infections after vaccination.

The grouped data were obtained in an unidentified format from an employee electronic health record system at UCSD and UCLA.⁴ Exemption from the approval of the institutional review board was obtained.

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

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

In our cohort, the absolute risk of positive testing for SARS-CoV-2 after vaccination was 1.19% among health professionals at UCSD and 0.97% among those at UCLA; these rates are higher than the risks reported in mRNA-1273 vaccine trials¹ and BNT162b2 vaccine.^two Possible explanations for this high risk include the availability of regular tests for asymptomatic and symptomatic people in our institutions, a regional increase in infections in Southern California during our vaccination campaigns,⁵ and differences in demographic characteristics between trial participants and health professionals in our cohort. Health workers were younger and had a higher overall risk of exposure to SARS-CoV-2 than participants in clinical trials. In addition, the cut-off dates to report in both initial vaccine trials were well before this increase, no asymptomatic test was included in the BNT162b2 vaccine trial,^two and only a single screening of asymptomatic people was performed in the mRNA-1273 vaccine trial before the second dose was administered.¹

The rarity of positive test results 14 days after administration of the second dose of the vaccine is encouraging and suggests that the efficacy of these vaccines is maintained outside the assay environment. These data highlight the critical importance of continuous public health mitigation measures (masking, physical distance, daily symptom tracking and regular testing), even in environments with a high incidence of vaccination, until herd immunity is widely achieved.

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

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

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, at NEJM.org.