A modelling study of COVID-19 patients in China in 2020 published in BMC Public Health estimates that some 10% had incubation periods that were longer than a fortnight.
A team led by researchers at Fudan University in Shanghai determined that of 11,425 patients who tested positive from COVID-19 from January to August 2020, 268 (10, 2%) had incubation periods longer than 14 days.
These patients were at lower risk for severe disease (adjusted odds ratio [aOR], 0,39) and were less likely to transmit the virus to others (aOR, 50,4) than those with shorter incubation periods.
Most patients with prolonged incubation times were older than 60 and were exposed to the virus in public places (median, 7,1 days) or workplaces (6,7 days) than at home (5, 5 days). Imported COVID-19 cases also had longer incubation times than local cases (6,9 vs 6, 4 days).
Patients 60 and older and those diagnosed as having diabetes, however, were at greater risk for severe illness, and the former group was also more likely than those 45 and younger to spread the virus. Patients whose time from symptom onset to hospital release was longer than three weeks had a shorter incubation than those with an interval of under three weeks (median incubation, 5,6 vs 7,3 days).
The study authors noted that most previous studies had estimated COVID-19 incubation periods at two to 12 days. In December 2020, the US Centers for Disease Control and Prevention shortened the recommended quarantine after COVID-19 exposure from 14 to 7 to 10 days.
“Isolation and medical quarantine policies of two weeks currently in place may miss the patients with longer incubation period,” the authors wrote.
Study details
Epidemiological features of COVID-19 patients with prolonged incubation period and its implications for controlling the epidemics in China
Zhi-Jie Zhang, Tian-Le Che, Tao Wang, Han Zhao, Jie Hong, Qing Su, Hai-Yang Zhang, Shi-Xia Zhou, Ai-Ying Teng, Yuan-Yuan Zhang, Yang Yang, Li-Qun Fang & Wei Liu.
Published in BMC Public Health on 9 December 2021
Abstract
Background
COVID-19 patients with long incubation period were reported in clinical practice and tracing of close contacts, but their epidemiological or clinical features remained vague.
Methods
We analysed 11,425 COVID-19 cases reported between January and August 2020 in China. The accelerated failure time model, Logistic and modified Poisson regression models were used to investigate the determinants of prolonged incubation period, as well as their association with clinical severity and transmissibility, respectively.
Result
Among local cases, 268 (10.2%) had a prolonged incubation period of > 14 days, which was more frequently seen among elderly patients, those residing in South China, with disease onset after Level I response measures administration, or being exposed in public places. Patients with prolonged incubation period had lower risk of severe illness (ORadjusted = 0.386, 95% CI: 0.203–0.677). A reduced transmissibility was observed for the primary patients with prolonged incubation period (50.4, 95% CI: 32.3–78.6%) than those with an incubation period of ≤14 days.
Conclusions
The study provides evidence supporting a prolonged incubation period that exceeded two weeks in more than 10% for COVID-19. Longer monitoring periods than 14 days for quarantine or persons potentially exposed to SARS-CoV-2 should be justified in extreme cases, especially for those elderly.
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