Study Shows That SARS-CoV-2 Coronavirus Is Evolving To Make Humans Sick Faster As Incubation Periods Are Getting Shorter With Newer Variants!

A new study by researchers from Peking University-China and Tsinghua University-China has found that the SARS-CoV-2 coronavirus is evolving to make humans sick faster as the incubation periods are getting shorter especially with the newer emerging variants.

 
The incubation period is the time elapsed between exposure to a pathogenic organism and when symptoms and signs are first apparent.
 
Lead author, Dr Yu Wu, PhD from the Department of Epidemiology and Biostatics, School of Public Health, Peking University, Beijing told Thailand Medical News, "Our study team conducted a systematic review and meta-analysis of 142 published articles and found that the pooled incubation period of SARS-CoV-2 virus 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.”
 
The study findings suggest that with the evolution of mutant strains, the incubation period of COVID-19 decreased gradually from Alpha variant to Omicron variant.
 
Numerous past 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.
 
In this new study, out of 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 non-severe illness, and 6.69 days (95% CI, 4.53-8.85 days) among patients with severe illness.
 
The study findings 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.
 
The study findings were published in the peer reviewed journal: JAMA Network Open. https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2795489
 
It should be noted that although past studies have estimated the mean SARS-CoV-2 infection incubation period, the reported incubation estimates vary based on the sample size, study design, data extraction period, and nations where the studies were conducted.
 
Furthermore, the incubation periods of SARS-CoV-2 Delta and Omicron VOCs differ from that caused by the ancestral Wuhan-Hu-1 or wild-type (WT) strain.
 
The study team determined the over all pooled COVID-19 incubation period, as well as the incubation periods of infections by different SARS-CoV-2 VOCs. The goal of the study was to obtain information that could be used to guide COVID-19 prevention and control strategy- and policy-making processes, as well as reduce SARS-CoV-2 transmission.
 
Published data were searched from the EMBASE, ScienceDirect, and PubMed databases between December 1, 2019, and February 10, 2022, using keywords such as SARS-CoV-2, novel coronavirus, COVID-19, 2019-nCoV, incubation, or incubation period. There were no language or publication status restrictions, as long as abstracts were available in English.
 
The key study outcome was the average estimates of SARS-CoV-2 incubation periods by different SARS-CoV-2 VOCs. Original studies assessing the COVID-19 incubation period., which is defined as the duration between SARS-CoV-2 infection and symptom onset, were included in the analysis.
 
Data and records were excluded if they were editorials, reviews, letters to editors, perspective articles, commentaries, duplicate articles, and articles with overlapped sample populations.
 
Importantly, the quality of the included studies was assessed independently by two researchers using the Newcastle-Ottawa scale (NOS) and random effects modeling, with the DerSimonian and Laird estimation method used for the meta-analysis.
 
In all, a total of 5,012 records were identified in the databases, of which only 142 studies comprising 8,112 COVID-19 patients were considered for the final quantitative or meta-analysis. Duplicated records, title and abstract records, ineligible full-text articles, records with other prime outcomes, and records with unavailable statistical data were excluded from the analysis.
 
It was quantified that of those included, 45, 82, and 15 studies were of strong, moderate, and weak quality, respectively. About 76% of the included studies were conducted in China, with 66% published between January and March 2020. Six, four, and three studies were conducted in South Korea, France, and Japan, respectively, whereas two studies were conducted in Singapore, India, Vietnam, and Australia. About 84% of the studies comprised patients the ancestral WT strain infections, 3.5% included patients with different strain infections, and 7.7% included patients with unknown strain SARS-CoV-2 infections.
 
All the studies included were substantially heterogeneous (I2 = 99%); however, the researchers did not report any publication bias in the studies. The standard error was also very low for all except one of the included studies.
 
The study findings showed that the overall pooled COVID-19 incubation period was 6.6 days and ranged from 1.8 and 18.9 days. Comparatively, the incubation periods of SARS-CoV-2 infections caused by the Alpha, Beta, Delta, and Omicron VOCs were documented in one, one, six, and five studies, respectively.
 
Most importantly, the average incubation periods of SARS-CoV-2 infections were five, 4.5, 4.4, and 3.4 days for those caused by the Alpha, Beta, Delta, and Omicron VOCs, respectively.
 
It was found that the average COVID-19 incubation periods were 7.4 days among individuals over the age of 60 years and 8.8 days among children younger than 18 years, 7.0 days among individuals with non-severe COVID-19, and 6.7 days among severe COVID-19 patients.
 
Interestingly, COVID-19 incubation periods among elder individuals may be longer than the pooled incubation period due to slower and weaker type 1 interferon (IFN) immunological responses among elders. In addition, the lack of fever responses, non-specificity of illness presentations, and the presence of multiple comorbidities may contribute to delayed COVID-19 detection among older adults.
 
However, COVID-19 incubation periods were shorter than the pooled period of COVID-19 incubation in children. This may be attributed to the fact that SARS-CoV-2-infected children typically present with mild COVID-19 symptoms without classical pulmonary pneumonia phenotype. Thus, COVID-19 symptoms could be confused with other diseases, thereby making COVID-19 difficult to detect in children.
 
However, it should be noted that children can still transmit SARS-CoV-2 during the incubation period and may not accurately express COVID-19 symptoms. COVID-19 incubation periods were shorter than the pooled incubation period in severe SARS-CoV-2 infections, which may be related to the number of cells infected with SARS-CoV-2 in the initial stages.
 
The study team concluded that the research findings highlight SARS-CoV-2 evolution and the emergence of SARS-CoV-2 variants with differently enhanced virulence and transmissibility. Importantly, COVID-19 incubation periods were gradually reduced from the Alpha VOC to the Omicron VOC. Most importantly, evaluating the incubation periods of COVID-19 caused by different SARS-CoV-2 VOCs helps to determine appropriate quarantine periods. 
 
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