# 8336
Although individual host responses can vary considerably, public health officials are constantly looking to define the `outside’ duration of viral incubation, as to know when to sound the `all clear’ when someone who has been exposed is out of danger.
The incubation period of novel Influenza – which can be carried sub-clinically, yet potentially still passed on to others – is of particular concern, as those exposed may be quarantined to protect the public.
And someone with travel history to an area where they might have been exposed to a specific pathogen will more likely be tested if they fell ill within its known incubation period.
The incubation period of viruses varies widely, with seasonal influenza running 1 to 4 days (avg. 2 days), measles running 7 to 21 days (avg. 14 days), MERS-CoV up to 15 days. In late April 2013, just over a month after the first H7N9 cases were announced from China, Hong Kong’s CHP sent a letter to doctors warning that the incubation period could be up to 10 days.
In view of the latest best available evidence, the longest incubation period of human infection with avian influenza A(H7N9) virus has been revised from 7 days to 10 days and the epidemiological criteria of the reporting criteria has been revised accordingly (see attached). Please also note that Taiwan has not been included as an affected area as the case recorded was classified as an imported infection.
Hong Kong has continued to use 10 days as an `outside’ incubation period, for testing and quarantine purposes.
Just last month, however, in a major release from China’s NHFPC: H7N9 Avian Flu Guidance Update, the incubation period for H7N9 was described as:
According to the incubation period of influenza and human infection with H7N9 avian influenza existing case findings, the incubation period is generally 3 to 4 days.
It is axiomatic that the more cases that are examined, the more studies that are published, the better becomes our knowledge. Which brings me to a new study, appearing this week in the journal Epidemiology & Infection (h/t Crof) , that seems to bear out Hong Kong’s early - more conservative stance - finding the H7N9’s incubation period can range up to 10 days.
Epidemiol Infect. 2014 Feb 24:1-7. [Epub ahead of print]
Huang Y1, Xu K2, Ren DF3, Ai J2, Ji H2, Ge AH2, Bao CJ2, Shi GQ4, Shen T4, Tang FY2, Zhu YF2, Zhou MH2, Wang H2.
Abstract
SUMMARY Human infection with the emerging avian influenza A(H7N9) virus in China in 2013 has raised global concerns. We conducted a retrospective descriptive study of 27 confirmed human influenza A(H7N9) cases in Jiangsu Province, to elaborate poultry-related exposures and to provide a more precise estimate of the incubation periods of the illness.
The median incubation period was 6 days (range 2-10 days) in cases with single known exposure and was 7·5 days (range 6·5-12·5 days) in cases with exposures on multiple days, difference between the two groups was not significant (Z = -1·895, P = 0·058). The overall median incubation period for all patients was estimated to be 7·5 days (range 2-12·5 days).
Our findings further highlight the necessity for public health authorities to extend the period of medical surveillance from 7 days to 10 days.
