This study compared the clinical characteristics and laboratory biomarkers in COVID-19 patients with critical care admissions during the Alpha wave of the COVID-19 pandemic (March 28, 2020, to April 30, 2020) and the Omicron wave (November 1, 2021, to January 31, 2022) at the UHD. Patients in the Omicron wave had differing characteristics and outcomes compared to those in the Alpha wave. The patients admitted to the ICU in the Omicron wave were slightly younger, and there was a higher proportion of females. This finding contrasts with the Alpha wave where men were disproportionately affected and had an increased risk of mortality. In our study of the Omicron wave, these mortality trends were reversed with women constituting the majority of the Omicron related deaths. Recently, a large review of COVID19 outcomes identified a higher prevalence of Omicron infection amongst females [10]. While the mortality of the Omicron variant is significantly lower than the Alpha variant, the increased prevalence of Omicron infection in females could have long-term health consequences as women have an increased risk of developing long haul COVID [12]. It is conceivable that gender related differences in cell mediated immunity or changes in the spike protein in the Omicron variant could skew infection towards females. However, further research is needed to better understand why these variants exhibited varying sex predilections.

Compared to the Alpha wave, the Omicron wave had much fewer ICU admissions (27 vs. 271) and a lower mortality rate (55.6% vs. 67.2%), suggesting a decreased disease severity during the Omicron wave. This finding is consistent with studies from South Africa [13,14,15] and European countries [16, 17]. Also, the mean SOFA score of the Omicron wave was lower than that of the Alpha wave (5.63 ± 4.01 vs. 8.33 ± 0.21) [11]. The lower mortality rate, ICU admissions, and mean SOFA score in the Omicron wave suggest that the Omicron variant, the predominant variant during this period, is associated with less disease severity and mortality compared to the previous SARS-CoV-2 variants as supported by other studies [2,3,4,5]. Interestingly, as seen in the Kaplan Meier survival curves (Fig. 2), although there was no significant difference in the survival probabilities of patients in the two waves (p = 0.34), the patients in the Alpha wave had longer hospital admission times. Another notable point is that treatments for COVID-19-related hospitalized patients have improved with more research and clinical findings and hospitals overall have more knowledge on the SARS-CoV-2 virus [18, 19], so these advances have also contributed to the overall lower mortality of COVID-19 patients. Lastly, the decreased volume of patients in the Omicron wave placed less strain on ICU resources which may also partly explain the improved outcomes in this cohort.

The UHD serves a predominantly Afro-Caribbean patient population with African Americans representing 91.2% of the local population. Accordingly, most of the patients seen were Black/African American in the Omicron wave (n = 23, 85.2%) and the Alpha wave (n = 239, 88.2%). Our findings, which showed that the majority of the non-survivors were Black in both waves with 165 patients (90.7%) in the Alpha wave and all of the patients in the Omicron wave (n = 15, 100%), could therefore be correlated with our hospital census. Though these numbers are consistent with our local demographics, it appears that the Black population continues to be disproportionately affected by COVID-19. In fact, as of April 2022, Kings County, the county where the UHD is located, continues to be the county with the highest number of COVID-related deaths in the state of New York and the fourth-highest number of deaths nationwide [6]. The high mortality rate in the Black population could be due to the “longstanding inequalities rooted in systemic and pervasive problems in the United States,” [20] as articulated by some scholars. Some examples include an overrepresentation of racial and ethnic minority populations in essential jobs, leading to greater chances of being exposed to COVID-19, or these groups facing multiple barriers to accessing health care [21]. Also, comorbidities, such as hypertension, or obesity, are prevalent in the Black population, thus contributing to worsened disease outcomes [7,8,9, 21,22,23,24]. Addressing the underlying health issues of obesity, diabetes and hypertension in the African American community could be an effective means of reducing the susceptibility of this community to future pandemics.

Mechanical ventilation is an indicative measure of poor disease outcome [25]. All the patients who were admitted to the ICU in the Alpha wave had severe respiratory illness requiring high-level oxygen supplementation or some form of mechanical ventilation [11]; 38.7% (n = 105) of the patients required invasive mechanical ventilation at the time of their initial critical care consult. In the Omicron wave, 29.6% (n = 8) of the patients required invasive mechanical ventilation. Our finding that there was less invasive mechanical ventilator use in the Omicron wave compared to the Alpha wave is consistent with findings in a study by Modes et al. [26] We also found that while most of the initial consult diagnoses in the Alpha wave were related to a severe respiratory illness, the initial admitting diagnoses in the Omicron wave had many other diagnoses that were not related to a respiratory-related illness. These included diabetic ketoacidosis, non-infectious diarrhea, facial weakness, hypokalemia, and altered mental status. This difference may be because the Omicron variant results in a decreased disease severity possibly due to its lower efficiency of replication in the lung parenchyma [27], which reduces severe respiratory illnesses and the need for invasive mechanical ventilators as observed in the Omicron wave. Indeed, almost fifty percent of the Omicron related deaths occurred in individuals who did not require mechanical ventilatory support. This suggests that infection with the Omicron variant was not the primary factor causing death. Nevertheless, we found that the Omicron variant infection can still cause severe lower respiratory illness as seen in other studies [26].

A bivariate comparison of the laboratory findings of the two waves found that the patients in the Omicron wave had overall significantly lower ALT levels compared to the patients in the Alpha wave. Also, multivariable analyses using both waves found that Neu % was significantly associated with a 5% increased odds of mortality and Lym % was significantly associated with 5% reduced likelihood of mortality. Several studies found that an increased neutrophil ratio, a lower Lym %, and an elevated ALT are independently correlated with disease mortality [11, 28,29,30,31]. Accordingly, these parameters support our observation that patients in the Omicron wave experienced a less severe disease, possibly due to less systemic inflammation, better immunological response to viral infection [32], and less liver injury [29]. However, another finding in our study is that the patients in the Omicron wave had a significantly lower PLT count than the patients in the Alpha wave, which conflicts with what would be expected as studies show that thrombocytopenia is associated with a higher risk of poor outcomes [28,29,30]. This conflicting result may be because a PLT count is reported to have a worse prognostic value than other biomarkers, such as CRP [33]. Also, we found that the Omicron wave patients had a significantly higher Mono % than the Alpha wave patients and higher Mono % in the Omicron wave survivors than Alpha wave survivors. Several studies describe an increased monocyte count in severe COVID-19 disease [34,35,36], but some studies have shown decreased monocytes in patients with severe disease [37,38,39]. The contrasting findings in these studies suggest the need for further studies or may suggest that Mono % is not a reliable marker of disease severity due to its dynamic effects.

One factor in the Omicron wave that differs from the Alpha wave was the addition of the COVID-19 vaccine, as the COVID-19 vaccine was developed after the Alpha wave [40]. Many studies show that the vaccine reduces COVID-19-related hospitalization and death [41,42,43]. Specifically, in regards to critical care, Thompson et al. found that the effectiveness of mRNA-based vaccines was 90% against infection leading to ICU admission [44]. In addition, a recent study conducted by the Centers for Disease Control and Prevention found that there were more hospitalizations during the Omicron-dominant period compared to the Delta-dominant period, but vaccination, including a booster dose, was associated with a lower likelihood of ICU admission [26]. In our study, interestingly, more non-survivors were vaccinated compared to survivors (53.3% vs. 33.3%) during the Omicron wave. The high rate of breakthrough cases and mortality among the vaccinated patients could have a manifold explanation. Firstly, the non-survivors were older than the survivors (mean age = 68.7 vs. 61.2), so the higher age could have been a cofactor in mortality. Secondly, only two patients in the Omicron wave were fully vaccinated with a booster dose, both of whom were non-survivors. Lastly, many of the patients admitted to the ICU have comorbidities, which are particularly more likely in the Black community [7,8,9, 21,22,23,24], and could have also contributed to the higher mortality seen in the vaccinated patients.

On a broader level, our findings reflect a lessening of the overall disease burden of COVID-19 in the Omicron wave, which is indicative of a less-lethal variant, and possibly a higher prevalence of natural immunity amongst the local community. The progress thus far on COVID-19 has been promising, and continued research in this domain is needed, which will be instrumental in addressing COVID-related health disparities and improving clinical outcomes.

The findings in this study should be interpreted in light of several limitations. Firstly, the sample size of the Omicron wave was smaller than the Alpha wave, limiting some statistical analyses and possibly affecting our conclusions. Also, we did not have the sequencing data for each patient to identify the SARS-CoV-2 variant. Nevertheless, the New York City Department of Health reported that nearly all COVID-19 cases were the Omicron variant during the time period studied. In addition, our study population consisted largely of Afro-Caribbeans, so future studies on other patient populations are needed to extrapolate our findings to other racial and ethnic groups. Furthermore, the two waves had different durations, which could have also potentially affected our data. Moreover, patients admitted to ICU require critical care and generally have more severe manifestations compared to other hospitalized patients, limiting the ability to generalize our findings to less severe patient populations. Despite these limitations, to our knowledge, this is the first study to compare the clinical outcomes of COVID-19 critical care patients (of a predominantly Afro-Caribbean community) in the specific waves studied, in Brooklyn, New York. These findings contribute to the growing literature on COVID-19, as different variants emerge, and highlight the outcomes of a generally underserved population, which is vital to their clinical care, and more broadly, informative for developing effective public health interventions.