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Does incubation period of COVID-19 vary with age? A study of epidemiologically linked cases in Singapore

Our take —

This study provides preliminary evidence on differences in incubation period by age group of COVID-19. While the study found a statistically significant difference in incubation by age, with older individuals having longer incubation periods, the relationship is not linear (those who are 60-69 in this study had the shortest incubation period), and should not be over-interpreted. There is the possibility of selection bias and recall bias, with patients asked to not only know, but also retrospectively report, the timing of exposure.

Study design

Case Series

Study population and setting

In this study, all positive COVID-19 cases from restructured hospitals (run by a private company but owned by the government) in Singapore with known exposure between January 23 and April 2, 2020 were included. A positive case was defined as an individual with evidence of SARS-CoV-2 using a PCR test. The goal of the analysis was to estimate incubation period and its association with age. Incubation period was defined as the date of exposure to the date of onset of infection (presence of any symptom indicative of respiratory infection) plus one. A total of 164 cases, or about 16% of all patients, were included in the study, as it was possible to both ascertain their definitive date of exposure (177/1049) and their date of symptom onset (164/177).

Summary of Main Findings

The mean age of included individuals was 44.2 years (standard deviation: 15.8). Notably, excluded cases were younger, more likely to be male, and more likely to be imported cases. Overall, the median incubation period was 5 days (range: 1-12 days). The median incubation period for those <30, 30-39, and 40-40 years old was 5.0 days, while the median incubation period for those 50-59 years was 6.0 days, for those 60-69 years was 4.5 days, and for those 70 years and above was 8.0 days. When comparing the median incubation period of those 70 years and older to those younger than 70 years old, it was found that the incubation period was longer among older individuals (8 days vs. 5 days). This difference was statistically significant (p=0.04).

Study Strengths

In Singapore, every positive case is followed up with a detailed epidemiological investigation and forward and backward mapping of patient activity. This process facilitates the ability to study key epidemiologic features of COVID-19, such as the incubation period.

Limitations

The sample size is relatively small, and participants self-reported timing of exposure to a confirmed COVID-19 case. There was no adjustment for other variables that may be related to both age and the incubation period (confounding), and selection bias is likely given the high proportion of individuals excluded because it was not possible to ascertain their date of exposure and/or date of symptom onset.

Value added

While differences in morbidity, mortality, and clinical progression of COVID-19 by age has been well-documented, less is known about differences by age in key epidemiologic features (e.g. incubation period).

This review was posted on: 30 September 2020