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The incubation period from infection to symptoms or a first positive COVID-19 test has decreased as the coronavirus has evolved, dropping from 5 days to 3.5 days, according to a new study published in JAMA Network Open.
With the Alpha variant, the incubation period was about 5 days, which fell to 4.5 for Beta, 4.41 for Delta, and 3.42 for Omicron.
"Knowledge of the disease's incubation period is of great significance for case definition, management of emerging threats, estimation of the duration of follow-up for contact tracing and secondary case detection, and the establishment of public health programs aimed at reducing local transmission," the study authors wrote.
Researchers at Peking University and Tsinghua University in China analyzed 142 studies published between December 2019 and February 2022. The studies included data about more than 8,100 COVID-19 patients.
Most of the studies (65.5%) were done between January and March 2020, and 76.1% were done in China. The remaining studies came from multiple countries or were based in Australia, France, India, Japan, Singapore, South Korea, and Vietnam.
About 84% of the studies included patients infected with the wild-type (or non-mutated) COVID strain, while 3.5% included patients infected with multiple strains, and 7.7% involved unknown strains.
Among the studies, 45 were considered to have strong evidence, 82 were moderate, and 15 were weak.
Overall, the average incubation period of all the variants was 6.57 days, ranging from 1.8 to 18.87 days. Among people ages 60 and older, the average incubation was 7.43 days. Among children, the average incubation period was 8.82 days.
The incubation period was about 7 days among those with mild or moderate illness and 6.7 days among those with severe illness.
The average incubation periods fell over time with each new variant, the study authors found. For Alpha, which had one study with 6,374 patients, the incubation period was about 5 days. For Beta, which had one study with 10 patients, the incubation period was 4.5 days. For Delta, which had six studies with 2,368 patients, the incubation period was 4.41 days. For Omicron, which had five studies with 829 patients, the incubation period was 3.42 days.
"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," the study authors wrote. "Identifying the incubation period of different variants is a key factor in determining the isolation period."
COVID-19 appears to have a longer incubation time than other viral respiratory infections, they noted, including human coronavirus or upper respiratory tract infection (3.2 days); influenza A (1.43 to 1.64 days); parainfluenza or bronchitis (2.6 days); respiratory syncytial virus, or RSV (4.4 days); rhinovirus, or the common cold (1.4 days); and severe acute respiratory syndrome, or SARS (4 days).
Two weeks ago, the CDC released guidance that said close contacts of COVID-19 patients no longer need to quarantine. Instead, they should wear a high-quality mask for 10 days and get tested 5 days after exposure. Infected people should isolate for at least 5 days after a positive test.
But based on the assumption that the COVID-19 incubation period ranges from 1 to 14 days, some countries and the World Health Organization still recommend close contacts to isolate for 14 days, the study authors wrote.
"With the shortening of the incubation period of new variants, the isolation period can be adjusted appropriately to reduce the pressure on the health system," they said.
JAMA Network Open: "Incubation Period of COVID-19 Caused by Unique SARS-CoV-2 Strains."
CDC: "CDC streamlines COVID-19 guidance to help the public better protect themselves and understand their risk."
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Cite this: COVID Incubation Periods Have Fallen Over Time - Medscape - Aug 23, 2022.