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Estimating unobserved SARS-CoV-2 infections in the United States

Our take —

Using a simple stochastic transmission model, authors sought to estimate the unobserved SARS-CoV-2 infections in the United States from January 1 through March 12, 2020, as well as projected deaths after March 13 that would have occurred as a result of these infections. The model estimated that fewer than 10% of locally acquired symptomatic infections were detected during this period when surveillance was limited, and that over 100,000 infections actually occurred. The model did not account for geographic differences in cases or deaths, and assumed exponential growth of the epidemic. The estimates were highly uncertain, due to the unpredictable nature of early transmission, which limits the reader’s ability to draw meaningful conclusions.

Study design

Modeling simulation

Study population and setting

Authors sought to estimate the full extent of community-acquired SARS-CoV-2 in the United States from January 1 through March 12, 2020 when testing and surveillance were limited. Using a stochastic simulation model with separate steps for importation of infections and local transmission, authors incorporated data on imported cases and local deaths in the United States, and estimated the probability of detecting daily infections. Authors estimated the total number of infections that occurred through March 12, and then estimated the number of expected deaths after March 13 that would result from those infections.

Summary of Main Findings

There were 1,514 locally transmitted cases and 39 reported deaths in the United States by March 12, 2020. In the baseline analysis, the model estimated that between January 1 and March 12, 45% (95% Posterior Predictive Interval [PPI] 4%, 97%) of imported symptomatic infections were detected; results from the model simulations estimated that 108,689 (95% PPI 1,023, 14,182,310) cumulative infections actually occurred during this time. During the month of February – when authors argue containment measures would have still been possible – the model estimated that fewer than 10% of locally acquired symptomatic infections were detected. The model further estimated that almost 16% of infections occurred on March 12 alone. Among persons estimated infected prior to or on March 12, the model estimated 827 (95% PPI 6, 115,507) additional deaths after March 13 (resulting from the delay from infection to death)

Study Strengths

Overall, results for cumulative infections, local case detection probability, and ratio of deaths after March 12, 2020 were relatively robust to assumptions about serial interval, case fatality risk, timing and probability of early case importations, and delays in reporting. Estimates from the model regarding detection of local symptomatic infections are consistent with previous serological studies.

Limitations

Posterior predictive intervals were quite wide for several estimates, indicating large uncertainty in the model. Although authors intentionally incorporated uncertainty in eight of the parameters used by the model, the width of the intervals makes many of the estimates challenging to interpret. The branching process in the model also assumed exponential growth of infections and did not take into account the effect that social distancing or other factors would have on transmission.

Value added

This modeling study sought to understand the true extent of SARS-CoV-2 community transmission in the United States compared to the number of cases and deaths reported, particularly during early 2020 when testing and surveillance were limited. Projections indicated that surveillance was able to detect fewer than 10% of locally acquired symptomatic cases.

This review was posted on: 2 November 2020