The incubation period of a disease is the time between infection and the emergence of symptoms. Although it varies from person to person, understanding the average incubation period of an infectious disease is critical to controlling it.
Authorities use the incubation period to determine the length of quarantine, for instance, as well as to understand how the disease is transmitted and to identify the source of the outbreak.
For COVID-19, current estimates of the incubation period are in the region of 4–5 days. However, there is some uncertainty around these estimates due to the novelty of the disease and the fact that only a limited number of studies have tried to establish the incubation period.
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The National Institutes of Health (NIH) have now led a study that has allowed researchers to formulate a new, longer estimate for the COVID-19 incubation period.
Using a dataset of people who contracted the virus in Wuhan, China, they found that the median incubation period for the disease was 7.76 days.
The study features in Science Advances and is based on the largest number of patients to date.
The researchers used data from 1,084 confirmed cases of COVID-19 in Wuhan, the center of the outbreak. They followed presymptomatic people from the time they left Wuhan until their symptoms developed.
The researchers obtained the data from public databases belonging to health commissions in China and the ministries of health in other countries. Information on confirmed cases included regional information, gender, age, date of symptom onset, date of diagnosis, history of travel, and date of departure from Wuhan.
The researchers estimated the incubation period using a mathematical approach called renewal theory, which makes it possible to study the probability of processes over time. One of the benefits of using this approach is that it reduces recall bias, which is when people make mistakes in remembering past events, such as when they traveled.
The researchers found that the median incubation period (the middle value) was 7.76 days, while the mean incubation period (the statistical average) was 8.29 days. However, in some cases, the incubation period was much longer.
For 10% of people, the incubation period was longer than 2 weeks (14.28 days), and in 1% of cases, the estimated incubation period was more than 20 days.
The youngest person included in the study was a 6-month-old baby, while the oldest person was 86 years old. However, the authors found no evidence that the incubation period for COVID-19 depends on age.
The finding that the incubation period may, in some cases, be more than 2 weeks could have important implications for quarantine periods, which are typically 14 days.
The Centers for Disease Control and Prevention (CDC), for example, state that anyone who has been in contact with somebody with COVID-19 should quarantine for 14 days. The UK Government also advises people to self-isolate for 14 days if they have had exposure to the virus.
If the incubation period is longer than 14 days, there is a chance that someone could still catch the virus from the person at the end of their quarantine period.
However, the authors say it is possible that some people in the study contracted the virus on their way out of Wuhan. Overall, they place the probability that the incubation period is longer than 2 weeks at between 5% and 10%.
Apart from informing quarantine rules, these results could help scientists understand the COVID-19 disease process.
The R0, for instance, which indicates how contagious a disease is, is partially based on the incubation period. Governments around the world are using this value to determine COVID-19 control measures.
As a mathematical modeling study, these findings are based on a number of assumptions. For example, the researchers have had to assume that people contracted the virus in Wuhan or on the way to their destination from Wuhan.
The findings may also not be applicable to later cases of COVID-19 in which the virus had mutated from its original form.
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