Scenario Modeling Hub

In a new round of projections, the Scenario Modeling Hub evaluated the trajectory of COVID-19 during October 30, 2022 to April 29, 2023 (26-weeks), under different assumptions about the uptake of bivalent boosters and extent of immune escape of circulating variants compared to the previously dominant BA.5 variant. Six teams contributed both national and state-specific projections.

Our main findings include:

• In scenarios considering moderate immune escape variants (scenarios A and C, corresponding to a 25% immune escape to BA.5) hospitalizations are projected to remain constant or rise moderately this winter, though the possibility of moderate resurgences still exists.
• The rise in hospitalizations is more pronounced in high immune escape variant scenarios (scenarios B and D, corresponding to a 50% immune escape). In these scenarios, hospitalizations would peak in December 2022 and could reach or exceed the level of the peak of the Delta wave from summer 2021.
• High booster uptake (similar to the uptake of the flu vaccine) would reduce cumulative hospitalizations by 26-31% and deaths by 32-34% on average over the projection period. This is compared to a lower booster coverage, which would follow the first booster campaign in winter 2021-22. Current uptake of boosters appears to be closer to our more pessimistic scenario.
• Weekly incident deaths are projected to follow the same general patterns as hospitalizations. In the worst case scenario D (low booster coverage and high immune escape), weekly deaths are likely to remain substantially below the Omicron peak (Dec 2021 - Jan 2022).
• In the most optimistic scenario (high booster coverage, moderate immune escape), we project 555,000 cumulative hospitalizations (95% PI, 97,000-1,195,000) over the 26-week projection period, and 47,000 cumulative deaths (95% PI, 0-221,000). In the most pessimistic scenario (low booster coverage, high immune escape), we project 959,000 cumulative hospitalizations (95% PI, 179,000-3,382,000) and 128,000 deaths (95% PI, 26,000-241,000).
• Based on recent bivalent vaccine uptake and effectiveness information, the more pessimistic vaccination scenarios (C and D) appear to be the most plausible.
• Immune escape scenarios may be best interpreted as bounding, with high escape scenarios (B and D) currently appearing less likely based on recent trajectories.
• A few caveats are worth noting:
  • We assumed the VE of reformulated boosters would be 80% against symptomatic disease with BA.4/5, immediately after vaccination. The effectiveness of reformulated boosters against existing and new variants remains unclear, as does the pace of waning after multiple booster shots and repeat infections.
  • Multiple variants co-circulate, which makes it difficult to project the epidemiological impact of future dominant variants.
  • There is considerable heterogeneity in the trajectories of individual models.
  • Issues with case ascertainment and delays in death reporting make model calibration difficult for these outcomes. Hospitalizations continue to be a stable outcome.

Table 1. COVID-19 Scenario Modeling Hub round 16 scenarios. More detailed scenario definitions and model characteristics can be found at https://github.com/midas-network/covid19-scenario-modeling-hub.

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.