When the original version of the novel virus mushroomed out of Wuhan, China, the mean incubation period 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 a mean incubation 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 shortened incubation period now puts SARS-CoV-2 more in line with commonplace 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 a mean incubation period of 1.4 days. For influenza, it can range from 1.43 to 1.64 days, and parainfluenza has a mean of 2.6 days. //

But children, who are at relatively low risk of severe COVID-19, also tended to have relatively longer incubation periods with SARS-CoV-2. Their mean incubation period 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 detection of COVID-19 symptoms can be delayed.