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coronavirus

How long it takes for Coronavirus (COVID-19) symptoms to appear?

COVID-19 Incubation Period

Possible outliers: 0 – 27 days

The importance of knowing COVID-19 incubation period

Understanding the incubation period is very important for health authorities as it allows them to introduce more effective quarantine systems for people suspected of carrying the virus, as a way of controlling and hopefully preventing the spread of the virus.

The incubation period (time from exposure to the development of symptoms) of the virus is estimated to be between 2 and 14 days based on the following sources:

  • The United States’ CDC estimates the incubation period for COVID-19 to be between 2 and 14 days.
  • The World Health Organization (WHO) reported an incubation period for COVID-19 between 2 and 10 days
  • China’s National Health Commission (NHC) had initially estimated an incubation period from 10 to 14 days.
  • DXY.cn, a leading Chinese online community for physicians and health care professionals, is reporting an incubation period of “3 to 7 days, up to 14 days”.

The estimated range will be most likely narrowed down as more data becomes available.

Incubation period of up to 24 days?

The incubation period has been found to be as long as 24 days (range: 0-24 days; median: 3.0 days) in a study published on February 9.

The WHO said in a press conference on February 10 that:

  • a very long incubation period could reflect a double exposure.
  • 24 days represented an outlier observation that must be taken into consideration in the context of the main finding of the study.
  • WHO is not considering changing recommendations regarding incubation periods.

More recently, however, a case with an incubation period of 19 days was observed in a JAMA study published on Feb. 21., and another case with an incubation period of 27 days was reported by Hubei Province on Feb. 22.

Incubation period of 5.2 days on average

A Chinese study published in the New England Journal of Medicine on Jan. 30[7], has found the incubation period to be 5.2 days on average, but it varies greatly among patients. The Chinese team conducting the study said their findings support a 14-day medical observation period for people exposed to the pathogen.

Below is an extract of the study findings;

Among the first 425 patients with confirmed NCIP, the median age was 59 years and 56% were male. The majority of cases (55%) with onset before January 1, 2020, were linked to the Huanan Seafood Wholesale Market, as compared with 8.6% of the subsequent cases.

The mean incubation period was 5.2 days (95% confidence interval [CI], 4.1 to 7.0), with the 95th percentile of the distribution at 12.5 days.

In its early stages, the epidemic doubled in size every 7.4 days. With a mean serial interval of 7.5 days (95% CI, 5.3 to 19), the basic reproductive number was estimated to be 2.2 (95% CI, 1.4 to 3.9).

Conclusions On the basis of this information, there is evidence that human-to-human transmission has occurred among close contacts since the middle of December 2019. Considerable efforts to reduce transmission will be required to control outbreaks if similar dynamics apply elsewhere. Measures to prevent or reduce transmission should be implemented in populations at risk.

Incubation Period in Travelers from Wuhan

A study financed by the Netherlands Ministry of Health and published on Eurosurveillance, analyzed data on 88 cases with known travel history (to and) from Wuhan which were detected between 20 and 28 January as being infected with COVID-19.

The mean incubation period was estimated to be 6.4 days. The incubation period ranges from 2.1 to 11.1 days. The upper limit of 11.1 days could be considered conservative.

Comparison with other viruses

For comparison, the incubation period for the common flu (seasonal influenza) is typically around 2 days. Incubation period for other coronaviruses: SARS 2-7 days; MERS 5 days typically (range 2-14 days).

VirusIncubation Period
(typical cases)
Novel Coronavirus
(COVID-19)
2-14 or 0-24 days *
SARS2-7 days,
as long as 10 days
MERS5 days (range: 2-14)
Swine Flu1-4 days,
as long as 7 days
Seasonal Flu2 days (1-4 range)

Summary

  • 2-14 days represents the current official estimated range for the novel coronavirus COVID-19.
  • However, a case with an incubation period of 27 days has been reported by Hubei Province local government on Feb. 22.
  • In addition, a case with an incubation period of 19 days was observed in a JAMA study of 5 cases published on Feb. 21. 
  • An outlier of a 24 days incubation period had been for the first time observed in a Feb. 9 study.[11] WHO said at the time that this could actually reflect a second exposure rather than a long incubation period, and that it wasn’t going to change its recommendations.
  • Period can vary greatly among patients.
  • Mean incubation period observed:
    3.0 days
     (0 – 24 days range, study based on 1,324 cases)
    5.2 days (4.1 – 7.0 days range, based on 425 cases).
  • Mean incubation period observed in travelers from Wuhan:
    6.4 days
     (range from 2.1 to 11.1 days).

Source

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