The incubation period of a pathogen/disease describes the duration between initial exposure and the onset of disease symptoms. In the case of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) or COVID-19, the incubation period is between 2 - 14 days according to the CDC in December 2020.
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What is coronavirus?
Coronaviruses are a group of viruses that cause infection in animals and humans. In humans, coronaviruses cause a variety of respiratory infections, ranging from a mild common cold to more severe Middle East Respiratory Syndrome (MERS), Severe Acute Respiratory Syndrome (SARS), and recently evolved coronavirus disease 2019 (COVID-19). The coronavirus responsible for COVID-19 is named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
What is an incubation period?
An incubation period measures the time from pathogen exposure to disease onset. This provides valuable information about disease transmission dynamics, as well as facilitating the implementation of appropriate non-pharmaceutical control measures, such as contact tracing and estimating the duration of home quarantine for people who are infected by the pathogen or came in contact with an infected person.
Moreover, the incubation period can help health professionals effectively diagnose a disease based on observed symptoms and the patient’s clinical history.
There are several determining factors for the incubation period, including the route of pathogen transmission, pathogen load (amount of pathogen entered the body), and functionality of the host immune system.
The most reliable way of estimating the incubation period is to methodically study case reports that involve patients with single pathogen exposure. In the case of patients who are exposed to the pathogen multiple times, it is impossible to determine which exposure triggered the disease onset.
What is the incubation period of SARS-CoV-2?
Since its emergence in December 2019 in China, the rapid spread of deadly SARS-CoV-2 has warned the need to search epidemiological characteristics of the virus, including the incubation period.
According to the World Health Organization and the Centers for Disease Control and Prevention, the estimated range of incubation periods of SARS-CoV-2 was 1 – 14 days. The average incubation period was estimated to be 5 – 6 days.
In the early stages of the pandemic, a study analyzing confirmed cases of COVID-19 (January 2020 – February 2020) obtained from 50 provinces, regions, and countries outside Wuhan, China (a total of 181 cases), estimated that the average incubation period of SARS-CoV-2 was 5.1 days. About 97.5% of COVID-19 patients are estimated to develop symptoms within 11.5 days of infection, and about 2.5% of patients are estimated to develop symptoms within 2.2 days.
Regarding specific symptoms, the study findings reveal that the average incubation period to fever onset is 5.7 days. The study has also assumed that out of 10,000 confirmed cases, 101 will develop symptoms after 14 days of isolation.
One important thing to be remembered is that the incubation period may vary from person to person. One study analyzing 1099 confirmed COVID-19 patients has shown that the average incubation period is estimated to be 3 days, with an incubation period range of 0 to 24 days. There is also evidence claiming that the incubation period can be extended up to 27 days. There is one piece of evidence regarding asymptomatic cases claiming that the incubation period for an asymptomatic patient is 19 days.
However, regarding an incubation period as long as 24 days, the World Health Organization mentioned in a press release that such a long incubation period is more likely due to a second infection rather than a continuation of the first infection.
Do new variants affect the incubation period?
It is important to note that as of 2021, many variants of SARS-CoV-2 have developed, some of them which have been a cause for concern due to problematic mutations causing changes such as an increased transmissibility.
Research on the incubation period has not been widely carried out with regard to individual variants. More information is needed to make any conclusions on whether the incubation period could have changed. For now health authorities such as the CDC and WHO still recommend a quarantine period of 14 days.
What are the incubation periods of other respiratory viruses?
According to scientific evidence, the estimated incubation period of SARS CoV is 5 to 7 days, extending up to 10 days. In the case of MERS CoV, the average incubation period is 5 days; however, the incubation period may vary from 2 to 14 days.
In seasonal influenza, the average incubation period is 2 days, with a range of 1 to 4 days. In the case of swine flu (H1N1 influenza virus), the incubation period varies between 1 and 4 days, which can be extended up to 7 days.
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