When the original version of the new virus emerged from Wuhan, China, the average incubation period was 6.65 days, according to pooled data from 119 studies. But then the incubation period shortened as the variants evolved. The alpha variant had an average incubation of 5 days, according to one study. beta, 4.5 days, according to another; The delta averaged 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 shortened 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 the flu it can range from 1.43 to 1.64 days and the para-influenza averages 2.6 days.

Better or worse

In terms of disease severity, the significance of a shorter incubation period is not entirely clear—as evidenced by studies included in the meta-analysis of outbreaks in specific groups of people, including the elderly, children, and people who developed severe COVID-19. For example, pooled data from eight studies that calculated the incubation period only in people over the age of 60—people at relatively high risk of severe COVID-19—found that they tended to have slightly longer incubation periods, averaging 7.43 days . This matched previous data, which found that older adults also had longer incubation periods in the original SARS outbreak in the early 2000s. Researchers then and now speculate that the longer incubation period in older adults reflects slower immune responses to virus. Advertising
But children, who are at relatively low risk of severe COVID-19, also tended to have relatively longer incubation periods with SARS-CoV-2. The mean incubation period was 8.82 days, based on pooled data from eight studies. The authors speculate that this may be because symptoms in children are so mild that detection of COVID-19 symptoms may be delayed. Further confusing the picture are data from six studies that looked specifically at incubation periods in people who developed severe disease and in people who developed non-severe disease. In this comparison, patients with severe COVID-19 tended to have shorter incubation periods (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 importance”

Overall, the complex relationship between incubation time and severity of COVID-19 highlights that disease depends on several factors, particularly virus-related factors (i.e. virus virulence and infectious dose) and human-specific factors ( eg immune system function and previous immunity from infection or vaccination). Omicron, the most recent variant and the one with the shortest incubation period to date, is thought to cause relatively mild disease. But it also occurred after extensive vaccination and previous infections, which are generally protective against serious disease. However, despite the 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 defining outbreaks, managing emerging threats, estimating the duration of surveillance for contact tracing and secondary case detection, and establishing public health programs aimed at reducing local transmission” . such as social distancing, isolation, face mask orders and quarantine. This is particularly important for SARS-CoV-2, which has been shown to be highly efficient at presymptomatic transmission. The meta-analysis has several limitations. Like all meta-analyses, it pooled data from different datasets from studies conducted in many different countries, creating the potential for confounding variables. Some of the data was also based on people’s recall of exposure dates. Finally, the majority of studies included in the analysis were during the original release of SARS-CoV-2. Thus, estimates of incubation periods for more recent variants were based on less data. However, the general finding of the meta-analysis was echoed by others who found that the incubation period has shortened during the pandemic and, in the micron era, now ranges from three to four days.