Probable Longer Incubation Period for Elderly COVID-19 Cases: Analysis of 180 Contact Tracing Data in Hubei Province, China

Figures & data

Figure 1 Exposure to symptom onset timeline for Wuhan-imported (upper panel) or locally infected (lower panel) COVID-19 cases sorted by earliest exposure date.

Table 1 Estimated Incubation Period for COVID-19 Cases Using Different Parametric Models

Distribution	Log-Likelihood*	Parameter 1	Parameter 2	Mean	5^th Percentile	50^th Percentile	95^th Percentile	99^th Percentile
Weibull	−449.5	1.6 (1.4–1.9)	7.3 (6.6–7.9)	6.5 (5.9–7.1)	1.2 (0.9–1.5)	5.8 (5.2–6.4)	14.3 (13.0–15.7)	18.7 (16.7–20.9)
gamma	−452.7	2.1 (1.5–3.0)	3.0 (2.2–4.2)	6.5 (5.9–7.1)	1.3 (0.7–1.8)	5.5 (4.8–6.2)	15.1 (13.4–16.9)	21.0 (18.1–24.1)
Log-normal	−479.3	1.6 (1.5–1.8)	0.9 (0.6–1.2)	7.2 (6.2–8.9)	1.2 (0.6–2.1)	5.0 (4.3–5.9)	20.5 (14.7–29.5)	36.6 (22.2–58.8)

Note: *Larger value of log-likelihood indicates a better fit.

Table 2 Parameter Estimates and 95% Confidence Intervals of Weibull Distributions Used to Estimate the Incubation Period for COVID-19 Cases

Parameter	All (n = 180)	Age (Years)
		<30 (n = 27)	30–59 (n = 123)	≥60 (n = 30)
Shape	1.6 (1.4–1.9)	1.3 (0.8–1.8)	1.6 (1.4–1.9)	2.4 (1.6–3.2)
Scale	6.5 (5.9–7.1)	5.3 (3.5–7.1)	7.3 (6.4–8.2)	8.9 (7.4–10.4)
5th percentile	1.2 (0.9–1.5)	0.5 (0.2–0.9)	1.1 (1.0–1.3)	2.6 (2.0–3.2)
50th percentile	5.8 (5.2–6.4)	4.0 (3.5–4.4)	5.8 (5.6–6.0)	7.7 (6.9–8.4)
95th percentile	14.3 (13.0–15.7)	12.3 (11.7–12.8)	14.4 (14.2–14.7)	14.1 (13.2–15.0)
99th percentile	18.7 (16.7–20.9)	17.1 (16.5–17.6)	18.9 (18.6–19.2)	16.9 (15.9–17.8)

Figure 2 Probability density function (left panel) and cumulative distribution function (right panel) of the Weibull distribution used to estimate the distribution of incubation periods by age group for 180 COVID-19 cases in Hubei, China.

Table 3 Association Between Incubation Periods and Characteristics of COVID-19 Cases in Multivariate Weibull Regression Model

Variables	HR (95% CI)	P value (Wald Test)	P value (LRT)
Sex			0.549
Female	reference
Male	1.06 (0.88, 1.27)	0.549
Age (years)			0.048
<30	reference
30–59	1.27 (0.99, 1.64)	0.065
≥60	1.49 (1.09, 2.05)	0.013
Source of infection			0.050
Wuhan-imported	reference
Locally infected	0.82 (0.68, 1.00)	0.052

Abbreviations: HR, hazard ratio; CI, confidence interval; LRT, log-likelihood ratio test.