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The logic behind isolation: incubation, latent and infectious periods Judith Glynn In October 2014 Nurse Kaci Hickox was placed under quarantine for 21 days when she returned from West Africa to the United States. She protested and eventually got the order overturned. Who was right? And why 21 days? Key time periods for an infection: • incubation period • latent period • infectious period INFECTION ACQUIRED ONSET OF SYMPTOMS RESOLUTION/ DEATH INCUBATION PERIOD DISEASE Incubation period: time from infection to first clinical symptoms INFECTION INFECTIOUS PERIOD LATENT PERIOD INFECTION ACQUIRED ONSET OF INFECTIOUSNESS END OF INFECTIOUSNESS RESOLUTION/ DEATH Latent period: from infection to onset of infectiousness Infectious period: time during which infected person can transmit to others INFECTION ACQUIRED ONSET OF SYMPTOMS RESOLUTION/ DEATH INCUBATION PERIOD DISEASE INFECTION INFECTION ACQUIRED LATENT PERIOD ONSET OF INFECTIOUSNESS INFECTIOUS PERIOD END OF INFECTIOUSNESS Incubation period: time from infection to first clinical symptoms Latent period: from infection to onset of infectiousness Infectious period: time during which infected person can transmit to others ONSET OF SYMPTOMS INFECTION ACQUIRED RESOLUTION/ DEATH INCUBATION PERIOD DISEASE Incubation period Incubation period Characteristic for each disease Variation Distribution usually lognormal (single peak and long right tail) May be influenced by infecting dose Important for: when (and therefore where) infection occurred how long to quarantine for after known exposure Salmonella outbreak, UK 80 70 No. of individuals 60 50 40 30 20 10 0 6 12 18 24 30 36 42 48 54 60 66 72 78 Hours between dinner and onset 84 90 96 102 108 114 120 Latent period INFECTION INFECTIOUS PERIOD LATENT PERIOD INFECTION ACQUIRED ONSET OF INFECTIOUSNESS END OF INFECTIOUSNESS RESOLUTION/ DEATH Latent period Earliest that infection can be transmitted to others INFECTION INFECTIOUS PERIOD LATENT PERIOD INFECTION ACQUIRED ONSET OF INFECTIOUSNESS END OF INFECTIOUSNESS Latent period: from infection to onset of infectiousness RESOLUTION/ DEATH Infectious period Transmission INFECTION INFECTIOUS PERIOD LATENT PERIOD INFECTION ACQUIRED ONSET OF INFECTIOUSNESS END OF INFECTIOUSNESS Determines: - how infection will be transmitted in the population - how long patients should be quarantined for RESOLUTION/ DEATH Disease A (eg measles) INCUBATION PERIOD DISEASE DISEASE INFECTION LATENT PERIOD INFECTIOUS PERIOD Disease B (eg SARS) INCUBATION PERIOD DISEASE DISEASE INFECTION LATENT PERIOD INFECTIOUS PERIOD Ebola Incubation periods from person-person transmission in the first Ebola outbreak Zaire (DRC) 1976 10 9 8 Number of cases 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Days Incubation period: median 8-9 days, minimum 2 days, maximum 21 days 19 20 21 Ebola Symptoms (including fever) start before infectious period INCUBATION PERIOD INCUBATION PERIOD DISEASE INFECTION LATENT PERIOD INFECTIOUS PERIOD Ebola Symptoms (including fever) start before infectious period INCUBATION PERIOD INCUBATION PERIOD DISEASE INFECTION LATENT PERIOD INFECTIOUS PERIOD Infectiousness increases as disease gets more severe Infectious period continues up to (and including) death For survivors, infection in semen up to about 3 months