Modeling of Future COVID-19 Cases, Hospitalizations, and Deaths, by Vaccination Rates and Nonpharmaceutical Intervention Scenarios

United States, April-September 2021

Rebecca K. Borchering, PhD; Cécile Viboud, PhD; Emily Howerton; Claire P. Smith; Shaun Truelove, PhD; Michael C. Runge, PhD; Nicholas G. Reich, PhD; Lucie Contamin, MS; John Levander; Jessica Salerno, MPH; Wilbert van Panhuis, PhD; Matt Kinsey, PhD; Kate Tallaksen, MS; R. Freddy Obrecht, PhD; Laura Asher, MPS; Cash Costello, MS; Michael Kelbaugh; Shelby Wilson, PhD; Lauren Shin; Molly E. Gallagher, PhD; Luke C. Mullany, PhD; Kaitlin Rainwater-Lovett, PhD; Joseph C. Lemaitre, MS; Juan Dent, ScM; Kyra H. Grantz; Joshua Kaminsky, MS; Stephen A. Lauer, PhD; Elizabeth C. Lee, PhD; Hannah R. Meredith, PhD; Javier Perez-Saez, PhD; Lindsay T. Keegan, PhD; Dean Karlen, PhD; Matteo Chinazzi, PhD; Jessica T. Davis; Kunpeng Mu; Xinyue Xiong, MSc; Ana Pastore y Piontti, PhD; Alessandro Vespignani, PhD; Ajitesh Srivastava, PhD; Przemyslaw Porebski, PhD; Srinivasan Venkatramanan, PhD; Aniruddha Adiga, PhD; Bryan Lewis, PhD; Brian Klahn, MS; Joseph Outten; James Schlitt, PhD; Patrick Corbett; Pyrros Alexander Telionis, PhD; Lijing Wang, MS; Akhil Sai Peddireddy; Benjamin Hurt, MS; Jiangzhuo Chen, PhD; Anil Vullikanti, PhD; Madhav Marathe, PhD; Jessica M. Healy, PhD; Rachel B. Slayton, PhD; Matthew Biggerstaff, ScD; Michael A. Johansson, PhD; Katriona Shea, PhD; Justin Lessler, PhD

Disclosures

Morbidity and Mortality Weekly Report. 2021;70(19):719-724. 

In This Article

Abstract and Introduction

Introduction

After a period of rapidly declining U.S. COVID-19 incidence during January–March 2021, increases occurred in several jurisdictions[1,2] despite the rapid rollout of a large-scale vaccination program. This increase coincided with the spread of more transmissible variants of SARS-CoV-2, the virus that causes COVID-19, including B.1.1.7[1,3] and relaxation of COVID-19 prevention strategies such as those for businesses, large-scale gatherings, and educational activities. To provide long-term projections of potential trends in COVID-19 cases, hospitalizations, and deaths, COVID-19 Scenario Modeling Hub teams used a multiple-model approach comprising six models to assess the potential course of COVID-19 in the United States across four scenarios with different vaccination coverage rates and effectiveness estimates and strength and implementation of nonpharmaceutical interventions (NPIs) (public health policies, such as physical distancing and masking) over a 6-month period (April–September 2021) using data available through March 27, 2021.[4] Among the four scenarios, an accelerated decline in NPI adherence (which encapsulates NPI mandates and population behavior) was shown to undermine vaccination-related gains over the subsequent 2–3 months and, in combination with increased transmissibility of new variants, could lead to surges in cases, hospitalizations, and deaths. A sharp decline in cases was projected by July 2021, with a faster decline in the high-vaccination scenarios. High vaccination rates and compliance with public health prevention measures are essential to control the COVID-19 pandemic and to prevent surges in hospitalizations and deaths in the coming months.

Following previous short-term disease forecasting efforts, the COVID-19 Scenario Modeling Hub[4] convened six modeling teams in an open call to provide long-term, 6-month (April–September 2021) COVID-19 projections in the United States using data available through March 27, 2021.[2,5] Teams each developed a model to project weekly reported cases, hospitalizations, and deaths, both nationally and by jurisdiction (50 states and the District of Columbia), for each scenario, using data from the Johns Hopkins Center for Systems Science and Engineering Coronavirus Resource Center and federal databases.[2,5] Four scenarios were considered in each model: high vaccination with moderate NPI use, high vaccination with low NPI use, low vaccination with moderate NPI use, and low vaccination with low NPI use[4] (Table) Vaccination scenarios took into account vaccine effectiveness (VE), weekly state-specific data on COVID-19 vaccination rates, and age- and risk-specific vaccine prioritization (e.g., older adults and health care workers); VE estimates were based on protection against clinical disease in randomized clinical trials§; parameters for effectiveness against infection and transmission were determined by each modeling team.[4] For each NPI scenario, teams estimated a level of NPI adherence in March 2021 and then implemented a linear decrease of that level beginning in April to be 50% or 80% lower in September 2021. All scenarios included the spread of the B.1.1.7 variant, with the assumption that it was 50% more transmissible than were previously circulating SARS-CoV-2 variants.[3,4] Individual modeling teams provided probabilistic projections for each future week, characterizing uncertainty with quantiles. These were combined into an ensemble for each scenario, outcome, week, and location by using the median across teams for each quantile.[4,6] The individual models differed substantially in structure and design,[4] but all accounted for age groups, enabling prioritization of vaccination based on federal and state guidelines.

In all four scenarios, COVID-19 cases were projected to increase through May 2021 at the national level because of increased prevalence of the B.1.1.7 variant and decreased NPI mandates and compliance (Figure 1). A sharp decline in cases was projected by July 2021, with a faster decline in the high-vaccination scenarios. Increases in hospitalizations and deaths (Figure 1), although more moderate, were also projected. A peak of 7,000–11,100 weekly deaths nationwide was projected in May (range = 5,382–15,677, which includes the central 50% of the projected distributions for all scenarios in the ensemble). The larger increases in cases relative to hospitalizations and deaths were attributable to higher vaccination coverage among groups with higher risk for severe COVID-19.

Figure 1.

Weekly projections of reported numbers of cases (A), hospitalizations (B), and deaths (C)* under four scenarios representing different levels of vaccination and nonpharmaceutical intervention adherence — United States, March 27–September 25, 2021
Abbreviation: NPI = nonpharmaceutical intervention.
*Historical data are shown as filled points, curves represent ensemble projections based on six models, and the grey area represents the maximum and minimum of the 50% projection intervals among all four scenarios. Vertical arrows represent the last date of observations used in the projections. Observations available after projections were made are shown as open points. Projection intervals are based on the 25th percentile of the more optimistic scenario (high vaccination and moderate NPI use) and the 75th percentile of the more pessimistic scenario (low vaccination and low NPI use). Ensemble projection curves represent the median of six median model projections, so they might not always appear smooth; the discontinuity in low vaccination scenario ensembles arises as two models project a late summer resurgence.

Moderate NPI use reduced cases and deaths in both the high and low vaccination scenarios, compared with low NPI use. The effect of maintaining moderate levels of NPI adherence was larger in the low vaccination scenarios, illustrating the counterbalance between and complementary effects of the two strategies (Figure 2). When low vaccination coverage was combined with low NPI adherence, cumulative cases, hospitalizations, and deaths were substantially higher compared with other scenarios. The largest differences among scenarios was in the cumulative excess percentage of hospitalizations. Differences in deaths were lower because many of the groups at highest risk were already vaccinated at the beginning of the projection window. Differences in cases were relatively small because in all scenarios a substantial number of new cases occurred.

Figure 2.

Excess percentage of reported cases, hospitalizations, and deaths projected to occur under scenarios with reduced vaccination coverage, nonpharmaceutical intervention adherence, or both, compared with the more optimistic scenario (high vaccination and moderate nonpharmaceutical intervention adherence),* nationally (A) and by state (B)§ — United States, March 27–September 25, 2021
Abbreviation: NPI = nonpharmaceutical intervention.
*Cumulative estimates for the projection period March 27–September 25, 2021, are compared with the more optimistic scenario (high vaccination and moderate NPI).
National estimates represent the range of projections generated by the six contributing teams (symbols = individual models, dash = ensemble median). Individual models have been developed by six academic teams and are named JHU_IDD-CovidSP (A); JHUAPL-Bucky (B); Karlen-pypm (C); MOBS_NEU-GLEAM_COVID (D); USC-SIkJalpha (E); and UVA-adaptive (F). Details on model structure and assumptions are available at MIDAS Network COVID-19 Scenario Modeling Hub. Accessed April 19, 2021. https://github.com/midas-network/covid19-scenario-modeling-hub
§Box plots represent the distribution of ensemble estimates in the 50 U.S. states and the District of Columbia. Boxes represent the interquartile range and the horizontal lines within each box represent the median. The whiskers extend to the most extreme data point that is no further from the box than 1.5 times the interquartile range.

Whereas the benefits of increased control measures varied substantially between models, the largest excess percentages in estimated effects for each model were consistently found in scenarios with the lowest NPI use and vaccination levels (Figure 2). Considerable range in state-specific projections was observed (Figure 2), suggesting that some states could reach levels of disease similar to those observed in late 2020 in scenarios with lower use of NPIs.

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