COVID-19

Scenario Modeling Hub

A Note on Scenario Modeling Hub Round 12 (January 20, 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 93,000-240,000) 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 101,000-319,000) 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 252,000 and 2,360,00 cumulative hospitalizations and 16,000-98,000 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.

Rationale

Even the best models of emerging infections struggle to give accurate forecasts at time scales greater than 3-4 weeks due to unpredictable drivers such as a changing policy environment, behavior change, the development of new control measures, and stochastic events. However, policy decisions around the course of emerging infections often require projections in the time frame of months. The goal of long-term projections is to compare outbreak trajectories under different scenarios, as opposed to offering a specific, unconditional estimate of what “will” happen.

As such, long-term projections can guide longer-term decision-making while short-term forecasts are more useful for situational awareness and guiding immediate response. The need for long-term epidemic projections is particularly acute in a severe pandemic, such as COVID-19, that has a large impact on the economy; for instance, economic and budget projections require estimates of outbreak trajectories in the 3-6 month time scale.