SARS-CoV-2 has evolved an incubation time more like seasonal coronaviruses


The incubation period for COVID-19 — the time between when SARS-CoV-2 first infects a person and when the resulting COVID-19 symptoms first appear — has progressively shortened as the pandemic lasted longer. and the virus has mutated. That’s according to a new meta-analysis published this week in JAMA Network Open by researchers in Beijing, who collected data on more than 8,000 patients from 142 COVID-19 studies.

When the original version of the new virus from Wuhan, China, mushroomed, the average incubation time was 6.65 days, according to pooled data from 119 studies. But then the incubation period got shorter as the variants evolved. The alpha variant had an average incubation time of 5 days according to one study; beta, 4.5 days, according to another; delta had a mean of 4.41 days, according to pooled data from six studies; and now with omicron, the incubation period has shrunk to 3.42 days, according to data from five studies.

The current reduced incubation period now aligns SARS-CoV-2 more closely with common respiratory viruses, including the four human coronaviruses that circulate seasonally and cause mild infections similar to the common cold. Their incubation period is 3.2 days. Rhinovirus, the most common cause of the common cold, has an average incubation period of 1.4 days. For influenza, it can range from 1.43 to 1.64 days, and parainfluenza has an average of 2.6 days.

Better or worse

In terms of disease severity, the importance of a shorter incubation period is not entirely clear – as was shown by studies included in the meta-analysis that regurgitated specific groups of people, including older adults, children and those who developed severe COVID – 19.

For example, pooled data from eight studies estimating the incubation period in people over 60 years old – people who are relatively high risk of severe COVID-19 – found they tended to have slightly longer incubation times, averaging 7.43 days. This was consistent with previous data showing that older adults also had longer incubation times in the original SARS virus outbreak in the early 2000s. Researchers speculate then and now that the longer incubation time in older adults reflects a slower immune response to the virus. .

But children, who are relatively low risk of severe COVID-19, also tended to have relatively longer incubation times with SARS-CoV-2. Their mean incubation time was 8.82 days, according to pooled data from eight studies. The authors speculate that this may be because symptoms in children are so mild that the detection of COVID-19 symptoms may be delayed.

Further obscuring the picture are pooled data from six studies that looked specifically at incubation periods in people who developed severe disease and those who developed non-severe disease. In this comparison, the patients with severe COVID-19 tended to have shorter incubation times (6.69 days) than those with non-severe cases (6.99 days). The authors of the meta-analysis speculate that this could be related to people with severe disease starting out with more cells initially infected with the virus than those with only mild disease.

“Great significance”

Overall, the complex relationship between incubation period and severity of COVID-19 highlights that disease is dependent on a variety of factors, especially virus-related factors (i.e., virus virulence and infectious dose) and human host-specific factors (i.e., immune system function and previous immunity to infection or vaccination). Omicron, the most recent variant and the one with the shortest incubation time to date, is considered a relatively mild disease. But it also came after widespread vaccination and previous infections, which are generally protective against serious illness.

Despite its complexity, the incubation period is “one of the most important epidemiological parameters of infectious diseases,” the authors write. “Knowledge of the incubation period of the disease is of great importance for the definition of the disease, the management of emerging threats, the estimation of the duration of follow-up for contact tracing and the detection of secondary cases, and of public health programs aimed at reducing local transmission.” such as social distancing, isolation, face mask mandates, and quarantine. This is particularly important for SARS-CoV-2, which has been shown to be highly effective in presymptomatic transmission.

The meta-analysis has a number of limitations. Like all meta-analyses, it collected data from different data sets from studies conducted in many different countries, raising the potential for confusing variables. Some data was also based on human exposure data recall. Finally, most of the studies included in the analysis were during the initial version of SARS-CoV-2. Estimates of the incubation times for the more recent variants were thus based on less data. However, the general finding of the meta-analysis was echoed by others who found that the incubation period has been shortened during the pandemic and, in the era of omicron, is now in the range of three to four days.

The Valley Voice
The Valley Voice
Christopher Brito is a social media producer and trending writer for The Valley Voice, with a focus on sports and stories related to race and culture.


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