Risk factors for Omicron reinfections among previously infected frontline workers in the United States

By Neha MathurReviewed by Aimee MolineuxFeb 1 2023

In a recent article published in Emerging Infectious Diseases, researchers examined a prospective cohort of frontline workers in the United States (US) previously infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). They attempted to identify risk factors that made this highly vulnerable population prone to reinfection by SARS-CoV-2 new variant of concern (VOC), Omicron, known for its high immune evading potential.

Background

Omicron emerged in the US in December 2021, when prior infection-induced immunity of the majority of the US population had waned. Among the vaccinated population, the effectiveness of two and three doses of a messenger ribonucleic acid (mRNA) vaccine had diminished by 46% and 60%, respectively. Consequently, Omicron, due to its unprecedented transmissibility, led to a re-surge in new SARS-CoV-2 infections and reinfections in the US.

About the study

In the present study, researchers enrolled frontline workers from eight US sites starting July 2020, who self-collected their nasal specimens for reverse transcription-polymerase chain reaction (RT-PCR) testing. The team performed whole-genome sequencing (WGS) on samples testing RT-PCR-positive with a cycle threshold (C_T) value less than 30 to determine the infection-causing SARS-CoV-2 VOC.

Finally, the researchers included those participants who previously had coronavirus disease 2019 (COVID-19) a minimum of 45 days before Omicron predominance or who enrolled in this study from US sites where Omicron was predominant. They also collected data on their demographic and preexisting health at enrollment. Likewise, they noted each participant's time-varying vaccination status, self-reported mask use, and time since the previous infection.

They averaged self-reported mask use and the number of hours worked per month (week-wise) for the study duration. Furthermore, the researchers verified each participant's self-reports using state vaccine registries or electronic health records (EHRs). Finally, the team used Cox proportional-hazards models to estimate hazard ratios (HRs) for Omicron reinfection, both adjusted and unadjusted.

Study findings

In the present study, while 42.5% and 26.1% were healthcare personnel and first responders, 31.4% of the participants were other frontline workers. Around 60% of the study participants were from Arizona; females and non-Hispanic Whites constituted 60.5% and 66.9% study population, respectively. Of the 4,707 active participants, 1,587, i.e., 33.7%, had a SARS-CoV-2 infection more than 45 days before the Omicron predominance period.

Based on the Omicron predominance period, 1,530 participants with prior COVID-19 added 124,665 person-days at reinfection risk. Among mRNA vaccine recipients, 71.2% and 28.8% had received BNT162b2 and mRNA-1273 COVID-19 vaccines, respectively. Within a year of its predominance, Omicron caused the first infections in around 51% of the participants in that area. Likewise, it caused reinfections in 20.8% of the participants, of which 27% remained asymptomatic. Perhaps, vaccination protected vaccinated workers who acquired Omicron reinfections unknowingly.

The Cox proportional hazards model revealed that the study participants who had received two or three mRNA vaccine doses had a >40% (lowered) risk for reinfection by Omicron than their unvaccinated counterparts. The adjusted HRs for two and three vaccine doses were comparable at 0.57 and 0.54, respectively.

The researchers noted that Utah residents and non-Hispanic Blacks were at markedly higher risk of reinfection, with aHRs of 1.61 and 2.14. Age and gender, however, did not present as substantial risk factors, with significant predictive value for reinfection in this study. Nonadherence to masking in community settings, defined as wearing a mask less than the average reported percentage of the time, increased the likelihood of reinfection.

Finally, the time since infection contributed to an increased risk of reinfection by Omicron. The essential workers for whom more than 12 months had passed since their first infection were at a higher risk of contracting COVID-19, with an aHR of 1.63 compared to those for whom less than a year had passed since their first infection.

Conclusions

The study results confirmed that risk factors for primary infections and reinfections were similar. Vaccinated and previously infected frontline workers were at a reduced risk of reinfection by Omicron, whereas not masking in public and more time since the first infection (>1 year) increased the risk for reinfection. Together, these findings suggested that prior infection-induced immunity depletes over time.

Yet again, the observed increased risk for reinfection among non-Hispanic Black highlighted the need for addressing race-based health inequalities in the US. Note that these racial minorities work in abundance as frontline workers in the US. As the COVID-19 pandemic reaches a phase where reinfections are becoming frequent, there is a heightened need to adopt a multidisciplinary approach to protect frontline workers. It should cover using vaccines, nonpharmaceutical interventions, and reducing racial health inequalities.