COVID-19 Incubation Periods Becomes Shorter With Each New Variant
The time between exposure to COVID-19 and becoming ill/testing positive appears to have become shorter and shorter since the pandemic began, according to new research from China. The time period from Alpha to Omicron has shrunk from 5 days to 3.42, researchers report. Here are more details.
PEKING, CHINA – Incubation periods of COVID-19 appear to have shortened with the appearance of new variants.
A Chinese study of different SARS-CoV-2 strains determined that the pooled incubation period was 6.57 days. The systematic review and meta-analysis of 141 articles found, however, that the incubation periods of COVID-19 caused by the Alpha, Beta, Delta, and Omicron variants were 5.00, 4.50, 4.41, and 3.42 days, respectively.
“These results suggest that with the evolution of mutant strains, the incubation period of COVID-19 decreased gradually from Alpha variant to Omicron variant,” the Peking University-led researchers write in JAMA Network Open.
The study team used articles from PubMed, EMBASE, and ScienceDirect, searched between Dec. 1, 2019, and Feb. 10, 2022, to reach their conclusions. The focus was on original studies of the incubation period of COVID-19, defined as the time from infection to the onset of signs and symptoms.
Included were 142 studies with 8112 patients. Results indicate that the pooled incubation period was 6.57 days (95% CI, 6.26-6.88) and ranged from 1.80 to 18.87 days.
Researchers point out that the incubation period of COVID-19 caused by the Alpha, Beta, Delta, and Omicron variants was reported in 1 study (with 6,374 patients), 1 study (10 patients), 6 studies (2368 patients) and 5 studies (829 patients), respectively.
Based on those studies, the meta-analysis calculates that the mean incubation period of COVID-19 was:
- 00 days (95% CI, 4.94-5.06 days) for cases caused by the Alpha variant,
- 50 days (95% CI, 1.83-7.17 days) for the Beta variant,
- 41 days (95% CI, 3.76-5.05 days) for the Delta variant, and
- 42 days (95% CI, 2.88-3.96 days) for the Omicron variant.
Factors other than variants affected the mean incubation period. The authors note that mean incubation was 7.43 days (95% CI, 5.75-9.11 days) among patients older than 60), 8.82 days (95% CI, 8.19-9.45 days) among infected children (ages 18 years or younger), 6.99 days (95% CI, 6.07-7.92 days) among patients with non-severe illness, and 6.69 days (95% CI, 4.53-8.85 days) among patients with severe illness.
“The findings of this study suggest that SARS-CoV-2 has evolved and mutated continuously throughout the COVID-19 pandemic, producing variants with different enhanced transmission and virulence,” the researchers explain. “Identifying the incubation period of different variants is a key factor in determining the isolation period.”
The authors point out that the Incubation period “is one of the most important epidemiological parameters of infectious diseases. Knowledge of the disease’s incubation period is of great significance for case definition, management of emerging threats, estimation of the duration of follow-up for contact tracing and secondary case detection, and the establishment of public health programs aimed at reducing local transmission.”
The study advises that COVID-19 seems to have a longer incubation period than that of other acute respiratory viral infections such as human coronavirus (3.2 days), influenza A (1.43-1.64 days), parainfluenza (2.6 days), respiratory syncytial virus (4.4 days), and rhinovirus (1.4 days), adding that the median incubation period for SARS in 2009 had been estimated as 4.0 days, which was lower than COVID-19. “In this study, the shortest mean incubation reported was 1.8 days and the longest incubation was 18.87 days,” the researchers write. “At present, based on the assumption that the incubation period of COVID-19 is 1 to 14 days, the [World Health Organization] still recommends that COVID-19 close contacts be isolated for 14 days.”