Decline in SARS-CoV-2 Infections Among Health Care Workers at 2 Hospitals Following Rollout and Administration of mRNA Vaccines

Dear Editor,

Two severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mRNA vaccines received Food and Drug Administration Emergency Use Authorization in December 2020 after showing 94%–95% efficacy in phase 3 clinical trials [1,2]. We evaluated trends in health care worker (HCW) SARS-CoV-2 infections before and after vaccine deployment at Brigham Health, an integrated health care system in Massachusetts that includes Brigham and Women’s Hospital (an 803-bed academic hospital) and Faulkner Hospital (a 162-bed community teaching hospital) and employs over 23 000 HCWs.

METHODS

Brigham Health began administering the BNT162b2 (Pfizer/BioNTech) vaccine on December 17 and the mRNA-1273 (Moderna) vaccine on December 23. Vaccines were initially prioritized for emergency, inpatient, and ambulatory staff caring for suspected or confirmed coronavirus disease 2019 (COVID-19) patients. Access was expanded to include all staff working on-site within 3 weeks.

We compared weekly trends in HCW SARS-CoV-2 infections from October 5 to January 31 with the 7-day average of new confirmed cases in Massachusetts [5] and the hospitals’ 7-day average inpatient COVID-19 census. HCWs with any symptoms consistent with COVID-19 or unprotected exposures were required to undergo polymerase chain reaction (PCR) testing; elective asymptomatic testing was also available at no cost. The hospitals’ Occupational Health Service interviewed all test-positive employees to assess symptoms and exposures. We compared trends in the prevaccine (October 5–January 3) vs postvaccine period (January 4–31) by applying an interrupted time series model to weekly case counts (log-transformed) and comparing slopes between HCWs, Massachusetts cases, and inpatient COVID-19 census. We used January 4 as the inflection point to allow adequate time for a meaningful fraction of HCWs to be vaccinated and because the trials suggested vaccine efficacy after about 2 weeks [1,2]. Analyses were done using R, version 4.0.3 (R Foundation).

Patient Consent Statement

This study was approved by the Mass General Brigham Institutional Review Board. The need for written consent was waived as the data were collected for the purpose of hospital operations.

RESULTS

Between October 5 and January 31, 1004 HCWs tested positive for SARS-CoV-2. Weekly HCW infections increased from October through early January, closely mirroring trends in community and hospital cases (Figure 1). Of 23 329 HCWs, 7542 (32%) received at least 1 vaccine dose by January 4, 12 438 (53%) by January 18, and 14 901 (64%) by January 31. After January 4, there was a 33% weekly reduction in HCW infections from a peak of 116 to a low of 33 from January 25 to 31, while Massachusetts community cases and inpatient census declined more modestly (weekly reduction rate, 18% and 9%, respectively; P < .001 for difference in both trends compared with HCW trends). HCWs underwent a mean of 2226 tests per week during the study period, including 2852 per week from January 4 to 31. Overall, 824/1004 HCWs (82%) were symptomatic when tested, including 610/760 (80.3%) in the October 5–January 3 period and 214/244 (87.7%) in the January 4–31 period.

Figure 1.

Trends in weekly health care worker SARS-CoV-2 infections vs community infections and inpatient COVID-19 census. Weekly HCW infections refer to the total number of PCR-confirmed infections during the listed date and the preceding 6 days (primary y-axis). The 7-day average inpatient COVID-19 census denotes the average of the daily number of inpatients at both hospitals who have PCR-confirmed infection and have not yet met criteria for discontinuing transmission-based precautions on the listed date and the prior 6 days (primary y-axis). Massachusetts 7-day average of new cases denotes the average of molecular test–confirmed cases during the listed date and the prior 6 days (secondary y-axis). The infection point (January 4) was chosen to allow adequate time for a meaningful fraction of HCWs to be vaccinated after the rollout on December 17 and because the trials suggested vaccine efficacy after about 2 weeks. The proportion of HCWs vaccinated at various time points was as follows: 32% received 1 dose by January 4, 53% received 1 dose (10% received 2 doses) by January 18, and 64% received 1 dose (30% received 2 doses) by January 31. Abbreviations: COVID-19, coronavirus disease 2019; HCWs, health care workers; PCR, polymerase chain reaction; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.

Between January 4 and 31, 98/244 HCW infections (40%) were in vaccinated individuals; 74 (76%) occurred within 14 days of the first dose. Of the 24 vaccinated HCWs who tested positive >14 days after their first vaccine dose, 18 had symptoms at diagnosis while 6 were asymptomatic. Six of the vaccinated HCWs who tested positive did so ≥14 days after receiving the second vaccine dose; 5/6 had symptoms at the time of diagnosis. None of the vaccinated HCWs who tested positive were hospitalized at the time of interview by Occupational Health. Of the 14 samples with available PCR cycle threshold (Ct) values from the 24 HCWs who tested positive >14 days after the first vaccine dose, the median Ct value (interquartile range) was 24.5 (20.9–26.8), compared with 21.8 (18.6–27.3) from the 109 samples with available Ct values from the other HCWs who tested positive after January 4.

DISCUSSION

Our study mirrors other data on the real-world impact of the mRNA vaccines, including in the general population in Israel and in HCWs in the United Kingdom and the United States [3–8]. Our analysis adds insight into the timeline after vaccine rollout before which a substantial decline on infection rates is likely to be seen, and also underscores the fact that infections can still commonly happen within the first few weeks of vaccination, and occasionally thereafter in fully vaccinated individuals as well.

Limitations of our study include the focus on 2 hospitals alone, incomplete vaccine uptake by staff, and insufficient time for all HCWs to receive 2 doses. The observation that a higher proportion of HCWs tested positive while symptomatic in the post vs prevaccination period also suggests that there may have been changes in HCWs’ propensity to get tested following vaccination. The reasons underlying the decline in community infections in the postvaccination period are unclear, but similar trends were seen nationally and are unlikely to be due to vaccination given that <10% of the population had been vaccinated by the end of January.

In summary, we observed a substantial decline in SARS-CoV-2 infections among HCWs ~3–4 weeks after rollout of the mRNA vaccines. HCW infection counts decreased at a significantly greater rate than concurrent declines in community and hospital infections. After vaccinations began, most infections occurred in unvaccinated individuals; the vaccinated HCWs who did test positive tended to do so within 2 weeks of their first vaccine dose. At the same time, a relatively small number of HCWs tested positive after 1 and even 2 doses of the vaccine. While these results suggest that widespread vaccinations will have a substantial impact on SARS-CoV-2 infection rates among HCWs and in the community, they also indicate that breakthrough infections do occur and thus underscore the need for ongoing vigilance and adherence to safe practices.

Acknowledgments

Financial support. This work was funded by the Centers for Disease Control and Prevention (6U54CK000484-04-02).

Potential conflicts of interest. Drs. Rhee and Klompas have received royalties from UpToDate, Inc., for writings on unrelated topics. None of the authors have any conflicts of interest to disclose. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.

Role of the funder.  The Centers for Disease Control and Prevention and Agency for Healthcare Research and Quality had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.