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. 2022 Jun 21:11:e73584.
doi: 10.7554/eLife.73584.

Projected resurgence of COVID-19 in the United States in July-December 2021 resulting from the increased transmissibility of the Delta variant and faltering vaccination

Affiliations

Projected resurgence of COVID-19 in the United States in July-December 2021 resulting from the increased transmissibility of the Delta variant and faltering vaccination

Shaun Truelove et al. Elife. .

Abstract

In Spring 2021, the highly transmissible SARS-CoV-2 Delta variant began to cause increases in cases, hospitalizations, and deaths in parts of the United States. At the time, with slowed vaccination uptake, this novel variant was expected to increase the risk of pandemic resurgence in the US in summer and fall 2021. As part of the COVID-19 Scenario Modeling Hub, an ensemble of nine mechanistic models produced 6-month scenario projections for July-December 2021 for the United States. These projections estimated substantial resurgences of COVID-19 across the US resulting from the more transmissible Delta variant, projected to occur across most of the US, coinciding with school and business reopening. The scenarios revealed that reaching higher vaccine coverage in July-December 2021 reduced the size and duration of the projected resurgence substantially, with the expected impacts was largely concentrated in a subset of states with lower vaccination coverage. Despite accurate projection of COVID-19 surges occurring and timing, the magnitude was substantially underestimated 2021 by the models compared with the of the reported cases, hospitalizations, and deaths occurring during July-December, highlighting the continued challenges to predict the evolving COVID-19 pandemic. Vaccination uptake remains critical to limiting transmission and disease, particularly in states with lower vaccination coverage. Higher vaccination goals at the onset of the surge of the new variant were estimated to avert over 1.5 million cases and 21,000 deaths, although may have had even greater impacts, considering the underestimated resurgence magnitude from the model.

Keywords: COVID-19; Delta variant; SARS-CoV-2; disease modeling; epidemiology; global health; human; pandemic; scenario projection.

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Conflict of interest statement

ST, CS, MQ, LM, RB, KS, EH, LC, JL, JK, HH, MK, KT, SW, LS, KR, JL, JD, JK, EL, JP, AH, DK, MC, JD, KM, XX, AP, AV, AS, PP, SV, AA, BL, BK, JO, MO, GH, BH, JC, AV, MM, SH, PB, DM, SC, RP, DJ, JT, MG, TY, SP, JH, RS, MB, MJ, CV No competing interests declared, JL has served as an expert witness on cases where the likely length of the pandemic was of issue, JS and Columbia University disclose partial ownership of SK Analytics. Discloses consulting for BNI, MR reports stock ownership in Becton Dickinson & Co, which manufactures medical equipment used in COVID testing, vaccination, and treatment

Figures

Figure 1.
Figure 1.. Historical data and weekly ensemble projections of reported numbers of COVID-19 cases.
(A) Hospitalizations (B) and deaths (C) under four scenarios representing different levels of vaccination and Delta variant transmissibility increase — United States, October, 2020–December, 2021.Projections are ensemble estimates of 9 models projecting four 6-month scenarios with 95% prediction intervals (the grey shading encompasses the prediction intervals from all four scenarios). Projections used empirical data from up to July 3, 2021, to calibrate models (black filled dots). The vertical lines indicate the beginning of each projection, with only data available prior to that point used to fit the projections. Observations available after the projection start are displayed as open dots.
Figure 1—figure supplement 1.
Figure 1—figure supplement 1.. Historical data and weekly individual projections of reported numbers of COVID-19 cases under four scenarios representing different levels of vaccination and Delta variant transmissibility increase — United States, October 2020–December 2021.
Solid color lines and shaded areas represent median projections and 95% prediction intervals of reported cases for the entire US from each of the 9 models for each of the four scenarios. Projections used empirical data from up to July 3, 2021, to calibrate models (black filled dots). The vertical lines indicate the beginning of each projection, with only data available prior to that point used to fit the projections. Observations available after the projection start are displayed as open dots.
Figure 1—figure supplement 2.
Figure 1—figure supplement 2.. Historical data and weekly individual projections of reported numbers of COVID-19 hospitalizations under four scenarios representing different levels of vaccination and Delta variant transmissibility increase — United States, October 2020–December 2021.
Solid color lines and shaded areas represent median projections and 95% prediction intervals of reported hospitalizations for the entire US from each of the 9 models for each of the four scenarios. Projections used empirical data from up to July 3, 2021, to calibrate models (black filled dots). The vertical lines indicate the beginning of each projection, with only data available prior to that point used to fit the projections. Observations available after the projection start are displayed as open dots.
Figure 1—figure supplement 3.
Figure 1—figure supplement 3.. Historical data and weekly individual projections of reported numbers of COVID-19 deaths under four scenarios representing different levels of vaccination and Delta variant transmissibility increase — United States, October 2020–December 2021.
Solid color lines and shaded areas represent median projections and 95% prediction intervals of reported deaths for the entire US from each of the 9 models for each of the four scenarios. Projections used empirical data from up to July 3, 2021, to calibrate models (black filled dots). The vertical lines indicate the beginning of each projection, with only data available prior to that point used to fit the projections. Observations available after the projection start are displayed as open dots.
Figure 2.
Figure 2.. Projected cumulative cases and mortality in the most pessimistic scenario (low vaccination, high variant transmissibility) and current vaccination coverage by state — United States, July 4, 2021–January 1, 2022.
(A) Correlation between cumulative projected cases per 10,000 population during the 6-month period and proportion of the eligible population vaccinated with at least one COVID-19 vaccine dose by July 3, 2021, by state. Circle sizes represent population size. Single dose coverage was used as data reporting were most reliable for the first dose at the time of this analysis; yet second dose coverage is highly correlated with first dose coverage (Pearson rho = 0.92 on July 3, 2021, p<10–15). (B) Cumulative projected cases per 10,000 population during the 6-month period, by state. (C) Correlation between cumulative projected deaths per 10,000 population during the 6-month period and proportion of the eligible population vaccinated with at least one COVID-19 vaccine dose by July 3, 2021, by state. Circle sizes represent population size. (D) Cumulative projected deaths per 10,000 population during the 6-month period, by state.
Figure 3.
Figure 3.. Comparison of the median projected and observed state-level total COVID-19 case incidences occurring during July 4–31, 2021, United States.
Comparison is based on ranking of incidence per capita in 50 states + DC (Spearman’s rank correlation = 0.867). The grey solid line represents perfect agreement between ranks (y=x), which overlays a regression line fitted to the data (dashed line) and 95% confidence intervals (grey shaded area).

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