A Note on Scenario Modeling Hub Round 12 (January 26, 2022)

Round 12 is focused on the impact of the Omicron variant in the United States and updates an earlier “emergency” round. We aim to provide a set of planning scenarios around this variant. With updated severity information that has become available since our emergency round, we now provide 4 plausible scenarios that address different levels of severity and immune escape for Omicron. To reflect residual uncertainty in the characteristics of the Omicron variant, we focus on the projection interval rather than the central estimates from the ensemble.

We note several key takeaways from this round:

  • Most models project that both cases and hospitalizations peak before the end of January 2022 in most states. The peak is expected to occur earlier in the North East than in the rest of the country.
  • At the end of projection period in early April 2022, incidence is projected to drop to low levels, assuming no new immune escape variant.
  • In the scenario with low immune escape and optimistic severity, we expect incident national hospitalizations to peak at 146,000 per week (95% PI 94,000-255,000) in the week of January 15th, 2022. In the scenario with high immune escape and high severity we expect national hospitalizations to peak at 193,000 per week (95% PI 79,000-507,000) in the week of Jan 22nd, 2022. Assumptions about immune escape have a low impact on all projections.
  • Nationally during the projection period (January 15 to Apr 2, 2022), we expect between 250,000 and 2,036,00 cumulative hospitalizations and 13,000-97,000 cumulative deaths resulting from the Omicron wave in the low immune escape and optimistic severity scenario. There are variations in cumulative estimates between scenarios.
  • Substantial uncertainty remains, notably:
    • The intrinsic severity of Omicron and the protection afforded by full vaccine schedules and boosters remain debated.
    • Data are scarce on the serial interval for the Omicron variant. A shorter serial interval would result in a lower transmissibility advantage over Delta; hence a smaller Omicron wave.
    • Case projections should be considered with caution due to potential changes in case ascertainment in the Omicron era. Issues include higher rates of asymptomatic infections, unreported positive home tests, and saturation in testing due to the sheer volume of Omicron infections.
    • There is uncertainty in the duration of protection from reinfection and the possible persistence of the Delta variant, which may affect estimates towards the end of the projection period.

A Note on Scenario Modeling Hub Round 11 (January 4, 2022)

Round 11, focused on the impact of the Omicron variant in the United States, is an “emergency” round aimed at providing a timely set of planning scenarios around this variant. Because of the developing situation, these scenarios were necessarily based on the best evidence at the time of their specification, and new information may deviate from main assumptions. Notably, at this point only the “low-severity” scenarios are considered plausible (scenarios A & B, the scenarios selected by default). Additionally, to reflect the larger uncertainty of this round, we focus on the projection intervals rather than the central estimates from the ensemble.

Despite the inherent uncertainty in this developing situation, their remain several key takeaways from this round:

  • The Omicron wave is projected to be sharp and fast in all scenarios, with most models projecting both cases and hospitalizations to peak before the end of January 2022 in every state (all models project peaks before the end of February 2022).
  • Cases, hospitalizations, and deaths will likely have receded substantially from the peak by the end of the projection period (March 12, 2022), but are projected to remain elevated compared to June 2021 (the lowest levels seen in the pandemic so far).
  • Weekly national hospitalizations could peak at substantially higher levels than reported since the start of the pandemic, with the upper bound of the 95% projection interval reaching 280% of the winter 2020-2021 peak, and 370% of the Delta peak (August- November 2021) in low severity scenarios.
  • Nationally during the projection period (December 19, 2021 to March 12, 2022) we expect to see between 409,000-2,380,000 cumulative hospitalizations and 54,000-304,000 deaths. Expected hospitalizations and deaths vary by scenario, with an ensemble median estimate of 832,000 hospitalizations in scenario A and 1,547,000 in scenario B.
  • Substantial uncertainty remains, notably:
    • Contributing models show a high level of qualitative and quantitative heterogeneity, reflecting high levels of uncertainty (though models agree on key points as noted above).
    • There is still uncertainty around the epidemiology and severity of Omicron, some of which (e.g., a possible shortened generation time and risk of severe outcomes in naive individuals) could substantially impact projections.
    • Case projections, particularly, should be considered with caution due to potential changes in the definition and identification of a case during the Omicron wave.

Statement of the COVID-19 Scenario Modeling Hub on the Coming Omicron Wave of COVID-19 Cases, Hospitalizations, and Deaths in the United States (December 22, 2021)

We are a consortium of scientists who contribute to the COVID-19 Scenario Modeling Hub. Currently, we are working to produce a round of planning scenarios to help policymakers and the public prepare for the potential impact of the Omicron variant of SARS-CoV-2 that is spreading rapidly in the United States and throughout the world. This process entails careful scenario specification, running of complex models by multiple independent teams, comparison and review of projections, and summarizing the results using formal techniques. Although we have not completed this round (Round 11), preliminary projections from multiple models raise deep concerns about the speed and severity of the coming Omicron wave. This urgency is further fueled by the unprecedented speed at which the Omicron variant has become the dominant strain in many areas. We believe it is important to share these preliminary results to help individuals and institutions prepare for potential surges in cases and healthcare demand.

The planning scenario projections produced so far universally show a large wave of COVID-19 cases that will likely exceed numbers seen nationally at the peak of the Delta wave by the first week of January 2022. While it is clear that the wave of COVID-19 infections will be large, it is less clear what Omicron’s impact will be in terms of hospitalizations and deaths, as much remains uncertain regarding the severity of primary, secondary and breakthrough infections with Omicron compared to what was witnessed for Delta and prior variants. The scenarios considered by the COVID-19 Scenario Modeling Hub capture a range of possible severities for the Omicron variant consistent with current evidence. Depending on severity, preliminary projections of weekly hospitalizations range from levels twice as high as any previously observed in the United States during the pandemic, to levels around half of what was seen at the peak of the Delta wave. Regardless of where the relative severity of Omicron falls, the sheer number of cases projected means that even a relatively mild severity of the Omicron variant has the potential to severely stress, if not overwhelm, already strained health care systems across the country. This includes straining our ability to provide COVID-19 testing to all who need it. There is substantial uncertainty underlying these projections, but they remain one of the best tools at our disposal to prepare for the effects of Omicron in the face of significant uncertainty.

The work of the COVID-19 Scenario Modeling Hub teams, multiple other modeling groups world wide, and the epidemic situation in countries where Omicron is already widespread, all indicate that the United States must be prepared to face an unprecedented wave of COVID-19 cases in the coming weeks and months. However, we are not helpless in the face of the coming wave, and have proven tools to blunt its impact. Physical distancing, limiting gatherings and masking have repeatedly proven themselves effective in slowing the spread of the virus. Vaccination, even if somewhat less effective due to Omicron’s immune escape, remains one of the best ways to reduce the risk of severe outcomes if infected and still reduces the chances of passing on the virus to others. Reassuringly, boosters have been shown to reduce the risk of symptomatic Omicron infections, and hence will likely provide high levels of protection against hospitalization and death; yet only 30% of those eligible in the US have received a booster shot. Antigen testing also provides an important tool to help prevent spread at holiday gatherings, though the availability of these tests has become limited in recent weeks. To the extent to which individuals and communities proactively use these tools, many of the worst outcomes projected by our models may not come to pass.

The COVID-19 Scenario Modeling Hub is working towards a rapid release of more formal planning scenarios assessing the threats posed by the Omicron wave. It will be important to understand the potential impact of the Omicron wave in different US jurisdictions that have various levels of population immunity. These COVID-19 Scenario Modeling Hub projections will undoubtedly be augmented by other critical information that will emerge in the coming weeks pertaining to viral characteristics, immune response, severity, and population level impact. However, the best information we have at the moment indicates the threat posed by Omicron is substantial and imminent, and individuals and governments should be prepared to respond accordingly.

Signed
  • Justin Lessler, UNC Gillings School of Public Health
  • Katriona Shea, The Pennsylvania State University
  • Rebecca Borchering, The Pennsylvania State University
  • Cecile Viboud, Fogarty International Center, NIH
  • Shaun Truelove, Johns Hopkins Bloomberg School of Public Health
  • Spencer Fox, The University of Texas at Austin
  • Bryan Lewis, University of Virginia
  • Srini Venkatramanan, University of Virginia
  • Przemyslaw Porebski, University of Virginia
  • Alessandro Vespignani, Northeastern University
  • Matteo Chinazzi, Northeastern University
  • Ajitesh Srivastava, University of Southern California
  • Harry Hochheiser, University of Pittsburgh
  • Emily Howerton, The Pennsylvania State University
  • Claire P. Smith, Johns Hopkins Bloomberg School of Public Health
  • Shi Chen, University of North Carolina at Charlotte
  • Thomas Hladish, University of Florida
  • Alexander N. Pillai, University of Florida
  • Jiangzhuo Chen, University of Virginia
  • Guido Camargo España, Univerasity of Notre Dame