#16,959
It is almost axiomatic that - due to viral evolution - anything we say about a virus today may not hold true next week, next month or next year. Some viruses - like influenza and coronaviruses - are more mutable than others - like measles or whooping cough - but over time, all viruses can mutate and evolve.
The SARS-CoV-2 virus we are dealing with today has changed markedly, and repeatedly, since it first emerged in late 2019, greatly increasing its transmissibility while losing (at least, since Omicron) some of its virulence.
Today's study from JAMA also finds that COVID's incubation period has decreased by nearly 50% over the first two years of the pandemic.
Original Investigation
Infectious Diseases
August 22, 2022
Incubation Period of COVID-19 Caused by Unique SARS-CoV-2 StrainsA Systematic Review and Meta-analysis
Yu Wu, PhD1; Liangyu Kang, MD1; Zirui Guo, MD1; et alJue Liu, PhD1; Min Liu, PhD1; Wannian Liang, PhD2
Author Affiliations Article Information
JAMA Netw Open. 2022;5(8):e2228008. doi:10.1001/jamanetworkopen.2022.28008
Key Points
Question What are the incubation periods of COVID-19 caused by different SARS-CoV-2 strains?
Findings In this systematic review and meta-analysis of 141 articles, the pooled incubation period was 6.57 days. 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.
Meaning These results suggest that with the evolution of mutant strains, the incubation period of COVID-19 decreased gradually from Alpha variant to Omicron variant.
Abstract
Importance Several studies were conducted to estimate the average incubation period of COVID-19; however, the incubation period of COVID-19 caused by different SARS-CoV-2 variants is not well described.
Objective To systematically assess the incubation period of COVID-19 and the incubation periods of COVID-19 caused by different SARS-CoV-2 variants in published studies.
Data Sources PubMed, EMBASE, and ScienceDirect were searched between December 1, 2019, and February 10, 2022.
Study Selection Original studies of the incubation period of COVID-19, defined as the time from infection to the onset of signs and symptoms.
Data Extraction and Synthesis Following the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) reporting guideline, 3 reviewers independently extracted the data from the eligible studies in March 2022. The parameters, or sufficient information to facilitate calculation of those values, were derived from random-effects meta-analysis.
Main Outcomes and Measures The mean estimate of the incubation period and different SARS-CoV-2 strains.
Results A total of 142 studies with 8112 patients were included. The pooled incubation period was 6.57 days (95% CI, 6.26-6.88) and ranged from 1.80 to 18.87 days. The incubation period of COVID-19 caused by the Alpha, Beta, Delta, and Omicron variants were reported in 1 study (with 6374 patients), 1 study (10 patients), 6 studies (2368 patients) and 5 studies (829 patients), respectively. The mean incubation period of COVID-19 was 5.00 days (95% CI, 4.94-5.06 days) for cases caused by the Alpha variant, 4.50 days (95% CI, 1.83-7.17 days) for the Beta variant, 4.41 days (95% CI, 3.76-5.05 days) for the Delta variant, and 3.42 days (95% CI, 2.88-3.96 days) for the Omicron variant. The mean incubation was 7.43 days (95% CI, 5.75-9.11 days) among older patients (ie, aged over 60 years old), 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 nonsevere illness, and 6.69 days (95% CI, 4.53-8.85 days) among patients with severe illness.
Conclusions and Relevance 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. Identifying the incubation period of different variants is a key factor in determining the isolation period.
A shorter incubation period has previously been associated with more severe illness with both SARS and MERS, and that may have been a contributing factor in Delta's impact in 2021. But Omicron replicates even faster, making it an exception to the rule.
The authors write:
Previous studies on SARS indicated that the incubation period of patients was related to the severity of the disease, and the incubation period of fatal cases was shorter.166 Virlogeux et al167 also found that Middle East Respiratory Syndrome patients with a shorter incubation period proceeded to have more severe disease. However, there are few studies on the association between the length of COVID-19 incubation period and the severity of infection. Our study found that the incubation period of COVID-19 in patients with severe illness was shorter (6.69 days) than patients with nonsevere illness. Studies have indicated that shorter incubation periods are associated with more serious disease, and this is related to the number of cells initially infected by the virus.123
Of course, COVID has hardly been hampered by its carriage in humans.
Hopefully COVID will continue to attenuate it virulence and eventually become just one of a plethora of background respiratory viruses that plague mankind. But evolution is random, and abrupt changes in the virus - not unlike the one that allowed it to jump species and spark a pandemic - are always possible.
