In late May 2009, the Australian Quarantine and Inspection Service called the Communicable Disease Branch at NSW Health to report an outbreak of respiratory illness on a cruise ship. The ship was returning to Sydney after a 10-day cruise in the Pacific Ocean. NSW Health arranged rapid testing on a sample of ill passengers which detected both pandemic (H1N1) influenza virus and seasonal (H3N2) influenza viruses in different passengers. Investigators reviewed the cruise ship’s medical records and interviewed passengers find anyone on the cruise who had developed influenza like illness, and arranged testing of their respiratory samples to confirm the diagnosis.
Of 1970 passengers and 734 crew members, 82 (3.0%) were infected with pandemic (H1N1) 2009 virus, 98 (3.6%) with influenza A (H3N2) virus, and 2 (0.1%) with both. In reviewing the data they found that among cases of H1N1 pandemic influenza, 1 case reported onset on 17 May, 3 on 21 May, 7 on 22 May, 11 on 23 May, 6 on 24 May and 31 on the last day of the cruise, 25 May. Among cases of H3N2 seasonal influenza, 1 case reported onset on 18 May, 9 on 21 May, 13 on 22 May, 9 on 23 May, 11 on 24 May and 22 on 25 May.
How might you display these data in a form that is easily digested?
You can construct an epicurve. An epicurve is a simple graph that shows number of cases in the Y axis by date in the X axis. It is a histogram, so that there should be no spaces between the vertical columns, and the area under the curve equals the total number of cases. Epicurves are great. They can tell you a lot about an outbreak. Short sharp curves with a sudden increase in case numbers over a short period suggests a point source outbreak. Slow rising epicurves, starting with only one case with growing cases with each incubation period (as in the curve below) suggests person to person spread.
Ward KA, Armstrong P, McAnulty JM, Iwansenko JM, Dwyer DE. Outbreaks of pandemic (H1N1) 2009 and seasonal influenza A (H3N2) on cruise ship. Emerg Infect Dis [serial on the Internet]. 2010 Nov [cited 5 June 2014]. http://wwwnc.cdc.gov/eid/article/16/11/10-0477.htm