Age groups that sustain resurging COVID-19 epidemics in the United States

Abstract

After initial declines, in mid-2020 a resurgence in transmission of novel coronavirus disease (COVID-19) occurred in the United States and Europe. As efforts to control COVID-19 disease are reintensified, understanding the age demographics driving transmission and how these affect the loosening of interventions is crucial. We analyze aggregated, age-specific mobility trends from more than 10 million individuals in the United States and link these mechanistically to age-specific COVID-19 mortality data. We estimate that as of October 2020, individuals aged 20 to 49 are the only age groups sustaining resurgent SARS-CoV-2 transmission with reproduction numbers well above one and that at least 65 of 100 COVID-19 infections originate from individuals aged 20 to 49 in the United States. Targeting interventions-including transmission-blocking vaccines-to adults aged 20 to 49 is an important consideration in halting resurgent epidemics and preventing COVID-19-attributable deaths.

Fig. 1. Mobility trends, and estimated time evolution of contact intensities in the United States.

(A) National, longitudinal mobility trends for individuals aged 18-24, 25-34, 35-44, 45-54, 55-64, 65+, relative to the baseline period February 3 to February 9, 2020. Projected per capita visits standardised daily visit volumes by the population size in each location and age group. The vertical dashed lines show the dip and rebound dates since when mobility trends began to decrease and increase, which were estimated from the time series data. (B) 1-week average of age-specific mobility trends between October 22, 2020 - October 28, 2020 across the United States. (C) Inferred time evolution of contact intensities in California, calculated as per Eq. 4.

Fig. 2. Model fits and key generated quantities for New York City, California, Florida and Arizona.

(Left) Observed cumulative COVID-19 mortality data (dots) versus posterior median estimates (line) and 95% credible intervals (ribbon). The vertical line indicates the collection start date of age-specific death counts. (Middle) Estimated number of infectious individuals by age (posterior median). (Right) Estimated age-specific effective reproduction number, posterior median (line) and 95% credible intervals (ribbon).

Fig. 3. Time evolution of estimated age-specific SARS-CoV-2 reproduction numbers across the US.

Each panel shows for the corresponding location (state or metropolitan area) the estimated posterior probability that the daily effective reproduction number from individuals stratified in 7 age groups were below one. Darker colors indicate low probability that reproduction numbers were below one.

Fig. 4. Estimated spatial variation in the share of young adults aged 20-34 and adults aged 35-49 to COVID-19 spread until mid August, 2020.

Posterior median estimates of the contribution to cumulated SARS-CoV-2 infections until August 17, 2020, prior to school reopening in the first states in the model. State-level COVID-19 epidemics not considered in this study are in grey.

Fig. 5. Share of age groups among COVID-19 attributable deaths and infections in the United States.

(Top) Proportion of monthly observed deaths attributed to COVID-19 by age group. Age-specific COVID-19 attributable deaths were recorded from state or city Departments of Health. Departments of Health used their own age stratification, and the observed data were re-estimated into common age groups across states with a Dirichlet-Multinomial model (see supplementary materials). A star (^∗) next to a location’s name indicates that there was a statistically significant shift in the share of individuals aged 80+ among deaths in the corresponding location. (Middle) Proportion of monthly reported cases among 20-49 year olds. Monthly cases were back-calculated using the meta-analysis infection fatality rate estimates of (20). The figure shows the estimated share of individuals aged 20-49 among monthly cases (posterior median: line, 95% credible interval: ribbon). (Bottom) New daily estimated infections by age group for New York City, Florida, California and Arizona (posterior median).

Fig. 6

Retrospective counterfactual modelling scenarios exploring the impact of school reopening on COVID-19-attributable cases. Shown in blue and red are estimated, daily COVID-19 cases (posterior median: line, 95% credible interval: ribbon) under the model until October 29, 2020, assuming reported cases among school-aged children from Florida and Texas under-report actual cases by a factor of 6 or less. In counterfactual modelling scenarios, the retrospective impact of continued school closures was explored until October 29, 2020, and the predicted case trajectories are shown (posterior median: black line, 95% credible interval: black ribbon).