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 icon[PDF – 1 page]

CDC requests that cruise lines immediately report any of the following events to the CDC Quarantine Station having jurisdiction over the next U.S. port of entry:

  • Outbreaks of influenza or ILI (exceeding 1.380 cases per 1,000 traveler days) among passengers or crew members.
  • Hospitalization (ashore or at sea) caused by, or suspected to be associated with, influenza or ILI onboard the vessel.

In addition, CDC is taking this opportunity to emphasize that any death, including those caused by, or suspected to be associated with, influenza or ILI, that occurs aboard a cruise ship destined for a U.S. port must be reported to CDC immediately. Report ILI hospitalizations or deaths by submitting an individual Maritime Conveyance Illness or Death Investigation Form pdf icon[PDF – 4 pages] (Updated as of 11/10/2021) for each hospitalization or death, or report by phone.

An outbreak is occurring if the total number of cases of influenza or ILI on a cruise ship among passengers OR crew exceeds the threshold of 1.380 cases per 1,000 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):

  • n = number of cumulative ill cases needed to reach threshold (i.e. meet outbreak criteria)
  • p = total number of passengers or crew members
  • d = number of days onboard the voyage at the time of reporting to Q Station,
    • where equation for d = [Report to Q Station Date – Departure Date (1st day of voyage)]

Theshold Calculation:

  • n = (1.38)(p X d) ÷ 1,000
  • The actual number of ill 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=1,200
  • d=21
  • n=(1.38)(p X d)/1,000=(1.38)(1,200 X 21)/1,000 = 34.8
  • Since 30 (the number of cases), is less than 34.8 (the ILI threshold), 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)/1,000 = 23.2
  • Since 25 (the number of cases), is greater than 23.2 (the ILI threshold) 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.