Using the COVID-19 to influenza ratio to estimate early pandemic spread in Wuhan, China and Seattle, US

Abstract

Background: Pandemic SARS-CoV-2 was first reported in Wuhan, China on December 31, 2019. Twenty-one days later, the US identified its first case--a man who had traveled from Wuhan to the state of Washington. Recent studies in the Wuhan and Seattle metropolitan areas retrospectively tested samples taken from patients with COVID-like symptoms. In the Wuhan study, there were 4 SARS-CoV-2 positives and 7 influenza positives out of 26 adults outpatients who sought care for influenza-like-illness at two central hospitals prior to January 12, 2020. The Seattle study reported 25 SARS-CoV-2 positives and 442 influenza positives out of 2353 children and adults who reported acute respiratory illness prior to March 9, 2020. Here, we use these findings to extrapolate the early prevalence of symptomatic COVID-19 in Wuhan and Seattle.

Methods: For each city, we estimate the ratio of COVID-19 to influenza infections from the retrospective testing data and estimate the age-specific prevalence of influenza from surveillance reports during the same time period. Combining these, we approximate the total number of symptomatic COVID-19 infections.

Findings: In Wuhan, there were an estimated 1386 [95% CrI: 420-3793] symptomatic cases over 30 of COVID-19 between December 30, 2019 and January 12, 2020. In Seattle, we estimate that 2268 [95% CrI: 498, 6069] children under 18 and 4367 [95% CrI: 2776, 6526] adults were symptomatically infected between February 24 and March 9, 2020. We also find that the initial pandemic wave in Wuhan likely originated with a single infected case who developed symptoms sometime between October 26 and December 13, 2019; in Seattle, the seeding likely occurred between December 25, 2019 and January 15, 2020.

Interpretation: The spread of COVID-19 in Wuhan and Seattle was far more extensive than initially reported. The virus likely spread for months in Wuhan before the lockdown. Given that COVID-19 appears to be overwhelmingly mild in children, our high estimate for symptomatic pediatric cases in Seattle suggests that there may have been thousands more mild cases at the time.

Keywords: Adult infections; COVID-19; Influenza; Pediatric infections; Seattle; Wuhan.

Fig. 1

Estimated symptomatic COVID-19 infections of people over 30 years in the 13 districts of Wuhan from December 30, 2019 to January 12, 2020. A retrospective study identified four ILI cases of COVID-19 from two hospitals in central Wuhan . We estimate that there were a total of 1386 [95% CrI: 420–3793] adult cases of COVID-19 during that 14-day period across the 13 central districts of Wuhan, ranging from 19 cases [95% CrI: 6–51] in suburban Hannan to 177 cases [95% CrI: 54–485] in central Wuchang, as indicated by shading (Table S3).

Fig. 2

Estimated symptomatic COVID-19 infections of people over all ages in the 22 studied Public Use Microdata Areas of Seattle from February 24, 2020 to March 9, 2020. A retrospective study identified 2 children and 23 adult ILI cases associated with COVID-19 from the Seattle Flu pandemic surveillance platform . We estimate that there were a total of 6748 [95% CrI: 4133, 11,020] (2268 [95% CrI: 498, 6069] and 4367 [95% CrI: 2776, 6526] for people under and over 18 years, respectively) cases of COVID-19 cases during that 15-day period across the 22 PUMAs of Seattle, ranging from 231 cases [95% CrI:199,265] in PUMA 11,614 to 410 cases [95% CrI:364,459] in PUMA 11,601 (Table S2).

Fig. 3

Estimating adult COVID-19 infections based on the ratio between patients retrospectively testing positive for COVID-19 and influenza in two hospitals in Central Wuhan from December 30, 2019 to January 12, 2020. First we use influenza surveillance data (number of outpatients, percent positive influenza tests, and number of ILI outpatients reported for the Wuhan region by the Chinese CDC) to estimate the proportion of adult outpatients (all cause) testing positive for influenza from December 30, 2019 to January 12, 2020 (left graphs). Second, we estimate the ratio of COVID-19 positive to influenza positive patients among adult outpatients with ILI, based on a recent retrospective study in two Wuhan hospitals (0.61 [95% CrI: 0.20–1.64]) . We then estimate the number of symptomatic COVID-19 infections among adults across Wuhan during this time period based on the proportion of influenza positive outpatients and the ratio of COVID-19 to influenza positive outpatients, using Monte Carlo sampling to incorporate uncertainty in our estimates of both quantities (upper right). Finally, we estimate the age-specific COVID-19 adult infections for the 13 central districts in Wuhan based on the district level population sizes for each age group. Given that the four detected COVID-19 cases lived in central Wuhan in ref. , we assumed that risk was uniform across all 13 districts during the 14-day time period. .

Fig. 4

Estimating the number of symptomatic COVID-19 cases among all age groups in Wuhan prior to January 23, 2020 and all age groups in Seattle prior to March 9, 2020. (A) For Wuhan, we assume an epidemic doubling time of either 7.3 [95% CrI: 6.3–9.7] days (red) or 5.2 [95% CrI: 4.6–6.1] days (blue). We further assume the numbers of COVID-19 infections estimated for the 13 central districts (Table S3) are equal to the sum of the daily number of incident infections from December 30, 2019 to January 12, 2020. Using an exponential model of epidemic growth we estimate that the first COVID-19 infection occurred on (red) November 17, 2019 [95% CrI: October 26-December 3, 2019] or (blue) December 2, 2019 [95% CrI: November 20-December 13, 2019], and then project the daily COVID-19 infections until January 23, 2020. (B) For Seattle, we assume an epidemic doubling time of 6.1 [90% uncertainty interval of 5.1 to 8.2] days and that the numbers of COVID-19 infections estimated across the 22 PUMA's are equal to the sum of the daily number of incident infections from February 24th to March 9th, 2020. Using an exponential model of epidemic growth we estimate the initial pandemic wave in Seattle originated with a single infected case who developed symptoms on January 6, 2020 [95% CrI: December 25, 2019 - January 15, 2020] and then project the daily COVID-19 infections until March 9, 2020. In both graphs, lines and bars indicate the median and 95% CrI estimates, respectively. Gray shading indicates the time period of our initial estimates.