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How to calculate the Influenza or Influenza-Like Illness (ILI) Case Outbreak Threshold for Cumulative Reports  

CDC requests reporting of total influenza/influenza-like illness cases (including zero)—not individual case information—for each voyage by using the Maritime Conveyance Cumulative Influenza/Influenza-Like Illness (ILI) Form [PDF - 1 page] .

Immediately report the following situations to the CDC Quarantine Station with jurisdiction over the U.S. seaport where the ship is expected to arrive:

  • Changes in the clinical profile and severity of illnesses reported or severe complications
    in at least two epidemiologically linked influenza cases.
  • Death caused by or suspected to be associated with influenza or ILI onboard the vessel.
  • Outbreaks of influenza or ILI (exceeding 1.380 cases per 1,000 traveler days) among passengers or crew members.

An outbreak is occurring if the total number of cases of influenza or ILI on a cruise ship among passengers OR crew exceed the threshold of 1.380 cases per 1000 person-days, as calculated below.  This threshold is based on Alaska regional data from 1998.*

Threshold case rate:

  • The burden of illness indicating a possible onboard ILI outbreak
  • Equivalent to 1.38 cases per 1,000 passenger-days

Variables (Passengers and crew are calculated separately):

  • Total passengers or crew members = (p)
  • Number of days onboard for the voyage = (d)
  • Passenger or crew days = p times d = (p X d)
  • Cumulative cases to reach threshold = (n)


  • n = (1.38)(p X d) ÷ 1,000
  • The number of actual cases reported should be greater than n to be considered an outbreak.


Example 1: How do you calculate the passenger threshold?

A ship with 1,200 passengers onboard for 21 days reports 30 passenger cases

  • p = 1200, d = 21
  • n = (1.38)(p X d)/1000 = (1.38)(1200 X 21)/1000 = 34.8 ≈ 35 cases
  • Since 35 is greater than 30, the threshold (n) has not been reached

Example 2: How do you calculate the crew member threshold?

The same ship with 800 crew members onboard for 21 days reports 25 crew member cases

  • p = 800, d = 21
  • n = (1.38)(p X d)/1000 = (1.38)(800 X 21)/1000 = 23.2 ≈ 23 cases
  • Since 23 is less than 25, the threshold (n) has been reached

*Bodnar UR, Maloney SM, Fielding KL, et al. Preliminary guidelines for the prevention and control of influenza-like illness among passengers and crew members on cruise ships. Atlanta: US Department of Health and Human Services, 1999.